Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [82,98,101,191,235,267,268]).
Aboveground description: Yellow and white sweetclover have very similar growth habits and morphology [11,117]. In their vegetative state, yellow and white sweetclover are difficult to distinguish [98,110]. The most obvious distinction between the two species is flower color, which is yellow for yellow sweetclover and white for white sweetclover [101,107,201,268]. When flowers are dry, however, both may appear cream colored [93].
Sweetclover is a biennial [126,127,158,191], although in rare instances annual and perennial growth forms were observed. At the Matanuska Research Farm in south-central Alaska, sweetclover seed from mid-latitudes and from Alaskan roadsides produced plants that flowered in the first year. Artificial light experiments revealed that annual growth was triggered by long light periods and a lack of darkness [124,125]. In London, Ontario, some sweetclover plants clipped "rigorously" during the growing season flowered in the 3rd growing season. All plants died after flowering (Cavers unpublished data cited in [251]). Studies by Smith [217] showed that annual and biennial white sweetclover growth is controlled by a single gene, which can be altered through a single mutation.
In the 1st year of growth, sweetclover produces a single stem with many branches. Near the end of the 1st growing season, nutrients are allocated below ground to the taproot, and root crown buds are formed. In the 2nd year of growth, sweetclover stem number increases, plants are much larger, and flowers and seeds are produced (review by [220]). More details about phenology and the characteristics of 1st and 2nd year plants are provided below in the Belowground description and Seasonal development sections.
Sweetclover is generally an erect, freely branched plant up to 10 feet (3 m) tall [82,235]. Stems are coarse with alternate, 3-pinnate leaves and axillary flowers [93,158,282]. Leaflets are small, 0.4 to 1 inch (1-2.5 cm), and the pea-like, perfect flowers occur in 30- to 70-flowered racemes that measure 1.5 to 4.7 inches (4-12 cm) long [101,158,191,241]. Legumes are up to 4 mm long, scarcely dehiscent, and typically produce just 1 seed, but may produce 2 [82,98,169,191].
Although yellow and white sweetclover are more alike than different, the following morphological differences are common:
Sweetclover growth form and morphology are variable, not only because many different cultivars, forms, and ecotypes were introduced in North America but also because growth characteristics can be influenced by environmental conditions. Plant height increases with increasing day length, and low temperatures can limit flower production (review by [251]). For descriptions of some sweetclover cultivars, see the following references: [219,273,285].
Belowground description: Sweetclover produces a taproot with secondary fibrous roots and bacterial nodules [11,101,268]. Taproots are semiwoody, and lateral roots can be extensive (reviews by [45,254]). Lateral roots may extend 6 to 8 inches (15-20 cm) from the taproot (review by [251]). In experimental fields in Columbus, Ohio, 1st-year sweetclover taproots penetrated over 4 feet (1.2 m) deep. Second-year plants had roots up to 5.5 feet (1.7 m) deep in July. Bacterial nodules occurred on roots as deep as 4 feet (1.2 m). Increased root length between these 1st- and 2nd-year plants may not reflect plant age but site differences. Most sweetclover plants attained their maximum root length in the 1st year. Root systems were shallower in dry than moist soils [289].
Bare [10] reports that yellow sweetclover roots are generally shorter but spread farther than white sweetclover roots. First-year white sweetclover dug from a field in Columbus, Ohio, in early May produced taproots that averaged 9.3 inches (23.6 cm) long and 0.3 inches (0.8 cm) in diameter [289]. On a sandy site near Central City, Nebraska, researchers described the root system of a 1st-year white sweetclover plant that was 3 feet (1 m) tall with 15 stems. Near the root crown, the taproot measured 1.5 inches (4 cm) in diameter. Diameter tapered to about 1 cm by about 1.5 feet (0.5 m) deep and remained that size to the maximum taproot depth of 5 feet (1.5 m). Most lateral roots also reached 5 feet (1.5 m) deep, but rarely did they extend more than 2 feet (0.6 m) from the taproot. Sublateral roots were abundant, and generally the soil up to 5 feet (1.5 m) deep beneath white sweetclover was "well filled with roots" [275].
Sweetclover is nonnative throughout North America. Eurasia [10,98,101] and, more specifically, the Mediterranean region from central Europe to Tibet [55,188], is the native range for sweetclover.
Although widely and similarly distributed in the United States, yellow and white sweetclover are considered most common in the upper Midwest and Great Plains regions [45,115]. In the West, yellow sweetclover is rare west of the Cascades [101], and in the East, white sweetclover occurs slightly farther north and south of yellow sweetclover [75,286,298]. In Hawaii, only white sweetclover is reported [268]. In Alaska and eastern Canada, white sweetclover occurs farther north than yellow sweetclover [37,251]. White sweetclover is also more common than yellow sweetclover in the Canadian Shield region [251]. Plants Database provides a distribution map for sweetclover. This map does not report sweetclover in Nunavut, Canada, but Turkington and others [251] indicate that white sweetclover has been collected from every Canadian province and territory.
Introduction and spread in North America: A sweetclover was reported in North America by 1664, but species was not identified [251]. Early spread of sweetclover was likely facilitated by beekeepers and agriculturalists [96]. Below is a sporadic timeline that provides information about early (pre-1920) introductions and spread of sweetclover in the United States:
By the early 1900s, sweetclover was recognized and promoted for soil reclamation. Sweetclover was cultivated extensively after it was found stabilizing abandoned tobacco fields on severely eroded slopes and had improved soils enough to support tobacco agriculture again [33,219]. Even as sweetclover was being hailed as a soil-building crop, some farmers hesitated to plant sweetclover fearing it might interfere with future crop production. In a 1917 USDA publication, successful use of sweetclover in crop rotation was highlighted to provide farmers with "sufficient proof" that their fears were unfounded [33]. In Illinois, there were 48,000 acres (19,000 ha) of sweetclover growing by 1910 and 757,000 acres (310,000 ha) by 1929. In Nebraska, there were 30,000 acres (12,000 ha) of sweetclover in cultivation in 1920 and 1.1 million acres (450,000 ha) by 1930 [219]. By 1919, nearly every US state had at least 50 acres (20 ha) of sweetclover in production [33]. Cultivation was most extensive in Montana, North Dakota, Minnesota, Iowa, and Wisconsin [55].
Although first planted for bees and soil improvement, soon sweetclover was recommended for a variety of uses. Sweetclover was planted extensively for livestock and wildlife forage [5,188] and to stabilize roadside cuts [90]. During the droughts of the 1930s, sweetclover cultivation was again actively promoted, and cultivation reached peak acreages [96]. In the 1960s and 70s, yellow sweetclover was seeded on US Fish and Wildlife land to provide nesting cover for waterfowl on abandoned fields and other degraded habitats in North Dakota, South Dakota, Minnesota, and Montana [51]. Sweetclover was seeded as recently as 1996 on Forest Service land in Montana [154] and as of 1998 on a burned US military site in Utah [112].
The use of sweetclover in revegetation likely increased as studies of its success in accomplishing management goals were publicized. In western South Dakota researchers reported that perennial grass production increased by 50% within 4 years of seeding yellow sweetclover on a rangeland severely depleted by droughts and heavy grazing [172]. The abundance of sweetclover introductions has likely facilitated its spread. In Alaska, starts and stops in the distribution of sweetclover roadside populations are common, suggesting that sweetclover was introduced in multiple roadside revegetation efforts. River floodplains have likely been invaded by seed produced by sweetclover populations at road-river intersections. Once established along the river, sweetclover is likely dispersed down river during flood events [37]. Additional discussions of sweetclover dispersal and invasiveness are presented in later sections.
Sweetclover generally produces both readily germinable and water-impermeable or "hard" seeds. Percentages of hard seed produced vary. Of the sweetclover seeds collected from the Royal Botanic Gardens in Cambridge, England, 32% of yellow sweetclover and 85% of white sweetclover seeds were hard [274]. Over 90% of white sweetclover seeds collected in July and August from roads and grasslands near Leuven, Belgium, were hard [260]. The seed collected from sweetclover that had germinated from seed stored in the soil for up to 14 years in North Dakota was nearly 100% hard [233]. Factors controlling the proportion of hard seed produced were not described in the available literature (2010).
Germination of hard sweetclover seeds can be encouraged by heat treatments and fluctuating temperatures around freezing. Light is not required for sweetclover germination, and high temperatures (95 °F (35 °C)) discourage germination [248]. One study found that germination of white sweetclover seeds was significantly lower (P=0.0069) in the field (6.7%) than in the laboratory (11.8%) [181], suggesting that germination results from greenhouse studies may not be fully realized in field conditions.
Heat: Researchers report that fire can stimulate germination of soil-stored sweetclover seed ([35,128], review by [45]). In the laboratory, heat treatments have increased the germination of hard sweetclover seed. After soaking hard white sweetclover seeds in 180 °F (80 °C) water, most became permeable to water (Martin 1922 cited in [190]). Dry heat treatments of 150 °F (66 °C) for 5 days produced a maximum germination increase of 10% for hard sweetclover seed. One minute at 220 °F (100 °C) produced only a 2.5% increase in hard seed germination, but 4 minutes at 220 °F (100 °) significantly (P<0.05) increased the germination of hard seeds, by 9.1% [200]. For more information on sweetclover and fire, see Fire Effects and Management.
Chilling: Alternating temperatures that include near freezing temperatures may increase germination of hard sweetclover seed more than constant freezing or chilling temperatures [155,260]. After conducting several experiments, Martin [155] found that moisture content did not affect softening or germination of hard seed but that 2 or more months of alternating temperatures around freezing produced high germination percentages. Seeds buried outdoors from October to late April at 1 to 3 inches (2.5-7.5 cm) deep germinated better than seeds buried deeper, where minmum temperatures and temperature fluctuations were reduced [155]. After conducting field and laboratory experiments on white sweetclover seed collected in Leuven, Belgium, researchers concluded that chilling and exposure to alternating temperatures increased germination [260]. Fluctuating cold temperatures may not be sufficient for germination of all hard sweetclover seed, however. In experimental field plots at the University of Saskatchewan, nearly all white sweetclover seeds required scarification to germinate even after 17 years in the soil [13].
Other vegetation: Presence of other vegetation may affect sweetclover germination. In northern Arizona, yellow sweetclover formed dense stands on sites lacking bunchgrass cover, but as bunchgrasses increased yellow sweetclover decreased. Experiments revealed that live foliage extracts from Arizona fescue (Festuca arizonica) and mountain muhly (Muhlenbergia montana) significantly reduced the germination percentage, germination rate, and initial root development of yellow sweetclover (P<0.05 for germination percentage and rate) [199].
Throughout its nonnative range, sweetclover is described on open, disturbed sites that include roadsides, railways, fields, and waterways [10,37,48,98,151,188].
Climate: The wide distribution of sweetclover implies wide climatic tolerance. Moisture is important for sweetclover seedling establishment, but once established, plants tolerate extremely dry conditions. In the fall, contractile roots pull sweetclover root crowns beneath the soil surface (≥2 inches (5 cm)), protecting plants from freezing temperatures (review by [251]). Yellow sweetclover is considered more heat and drought tolerant than white sweetclover (reviews by [220,254]). Although yellow sweetclover has also been described as more cold hardy than white sweetclover (review by [254]), current distributions suggest this may not be true (see General Distribution). In Alaska, sweetclover occupies habitats with extreme weather. In Ketchikan, annual precipitation averages 160 inches (3,940 mm) and temperatures average 45 °F (7.2 °C). In interior Alaska, annual precipitation can be as low as 6 inches (170 mm), and the average annual temperature can be as low as 26 °F (-3.3 °C) [37]. During growth chamber experiments, researchers found that 1- to 4-week-old yellow sweetclover seedling survival was high at 21 °F (-6 °C). Survival was much lower at 18 °F (-8 °C) [165].
Elevation: Range of elevations reported for sweetclover in western North America Area Elevation ArizonaSoils: Sweetclover grows on a variety of alkaline or slightly acidic soils ([33,37], review by [220]). Very low nutrient levels and fine- and coarse-textured soils are tolerated ([37,167,245,279], review by [233]). Several reviews indicate that yellow sweetclover tolerates nutrient-poor and dry soils better than white sweetclover [49,89,219,254].
Sweetclover occupies a variety of soil types and textures but growth and productivity can vary by soil type and region. Residents of Fort Smith near Canada's Wood Buffalo National Park reported that sweetclover expanded its range on fine-textured soils but nevertheless was primarily restricted to disturbed sites [279]. In meadows in Michigan's Oakland County, white sweetclover was "plentiful" on sites with "considerable clay" [245]. In southwestern North Dakota, yellow sweetclover occupied a variety of habitats with textural classes ranging from loams to clays and pH ranging from 7.9 to 8.8 [73]. A review reports that sweetclover is most productive on silt loams to clay loams with neutral to alkaline pH [89]. Wasser [273] reports a minimum pH tolerance of 5.5 for yellow sweetclover. Seeding of yellow sweetclover was successful on a South Dakota rangeland where soils were up to 65% clay [172]. On riverbanks in Quebec, white sweetclover was most common on alkaline, sandy soils with very rapid drainage and low to high degrees of stoniness [167]. On Alaska rivers and roads, white sweetclover density was lower on cobbly than on sandy surfaces [37].
Many studies report an association between white sweetclover and calcareous soils. White sweetclover was especially common on calcareous soils in Michigan [267], the northeastern United States and southeastern Canada [75], the Gulf and Atlantic Coasts [52], and eastern Texas [272]. During a study conducted in Canada, researchers found that white sweetclover plants grown from seeds collected on calcareous soils grew well only on calcareous soils. White sweetclover plants grown from seed collected on acidic soils grew well on acidic and calcareous soils but grew best on calcareous soils [192].
Salinity: Sweetclover tolerates moderate salinity [119,257]. A review reports that salinity levels of 0.2 to 0.4% or 2 to 4 ppt (3-5 mS/cm) are tolerated (review by [233]). In Alberta, Saskatchewan, and Manitoba, white sweetclover occurred on soils where salt crystals were visible on the surface [17,50]. Yellow sweetclover was reported in a marsh near Lincoln, Nebraska, where the salinity averaged 0.2% [257]; white sweetclover grew along the South River in Anne Arundel County, Maryland, where salinity levels ranged from 0.2 to 3.6% [185].
Moisture and flooding: Sweetclover is common in riparian areas and typically tolerates short-duration flooding early in the growing season [11,238]. A review indicates that white sweetclover is slightly more flood tolerant than yellow sweetclover. In southern Ontario, white sweetclover is occasional along rivers with several weeks of winter and spring flooding (review by [251]). Along Alaska's Nenana River, white sweetclover survived shallow flooding that lasted only a few days [37]. In southern Idaho, Rosentreter [202] reported that yellow and white sweetclover abundance increased in periodically flooded stream banks, but yellow sweetclover is typically killed by high water during the growing season (review by [89]). During field experiments in London, Ontario, fewer than 10% of sweetclover plants survived 5 days of immersion in the Thames River when the temperature was 68 °F (20 °C) (Weekes and Cavers unpublished data cited in [251]).
Sweetclover is consumed by a variety of large and small, native and domestic herbivores. Sweetclover seeds and insect visitors can be important to birds (reviews by [84,240,241]).
Large mammals: Deer, antelope, elk, and livestock feed on sweetclover. When diets of co-occurring elk, deer, and livestock were compared, sweetclover was often a larger component of native ungulate diets than cattle diets.
Studies in the Great Lakes and western regions of the United States indicate that deer use of sweetclover can be heavy but may vary seasonally. In a prairie restoration area in Bloomington, Minnesota, white-tailed deer fed heavily on yellow sweetclover [58]. On the Mud Lake National Wildlife Refuge in Holt, Minnesota, the average use of sweetclover by white-tailed deer was 18.5% [109]. In a review of mule deer feeding habitats in the western United States, heavy use of yellow sweetclover in the summer was reported several times and heavy fall use was reported once. Several studies also reported moderate use throughout the year [134]. Within the Beaver Creek Watershed south of Flagstaff, Arizona, yellow sweetclover made up 0.6% of tame mule deer diets in June and 41.3% in July. Yellow sweetclover was not eaten in August or September [259]. On the Kaibab Plateau in northern Arizona, 7.7% of mule deer feeding observations were on white sweetclover in the first half of June. Observed feeding on white sweetclover was much lower from 16 June to 30 August [108]. In south-central New Mexico, fecal analyses indicated that yellow sweetclover averaged 2.7% of mule deer diets in September and 2.1% in March [152].
Utilization of sweetclover by pronghorn has been reported in Wyoming [97] and Montana [283]. In central Montana, yellow sweetclover was a greater proportion of summer pronghorn diets than white sweetclover. White sweetclover averaged 1% to 2% and yellow sweetclover averaged 12% to 29% of pronghorn summer diets. Yellow sweetclover was much more frequent than white sweetclover in the study area [283], which likely affected pronghorn diets more than preference.
In the majority of studies comparing the diets of native ungulates and livestock, sweetclover was more common in native ungulate than livestock diets. In grasslands and big sagebrush habitat types in north-central Montana, yellow sweetclover made up 27% of summer and a trace of winter mule deer diets based on rumen analyses. Yellow sweetclover made up 8% of summer cattle diets based on feeding site observations [53]. Based on 4 years of feeding habit observations in Montana's Missouri River Breaks, yellow sweetclover was 53% of elk, 44.5% of mule deer, and 26% of cattle total summer feeding use. Use of yellow sweetclover was much lower (12-14%) in winter and spring [149]. In north-central New Mexico, yellow sweetclover was 12% of pronghorn and cattle winter diets in a drought year but was not utilized in a year with above-normal precipitation. Yellow sweetclover was not a part of either species' spring diets in any year [226].
Livestock feed on sweetclover, but bloating is possible from consuming only sweetclover, and poisoning can occur if sweetclover hay is not properly cured. On the Blodgett Forest Research Station in central California, white sweetclover was most important in summer cattle diets. Based on fecal analyses, white sweetclover was 1.9% to 4.3% of July diets [121]. Although livestock bloating can occur with a diet primarily of sweetclover, this risk is reduced when dry roughage is available [117,220]. Sweetclover contains coumarin, which can break down into compounds that prevent blood clotting. These chemical changes occur when hay molds or is improperly cured. Internal hemorrhaging or excessive bleeding from small wounds can lead to death [10,48,188]. Death from internal hemorrhaging is more common in cattle than in sheep or horses [10].
Small mammals: Sweetclover has been reported in black-tailed jackrabbit, eastern cottontail, and prairie dog diets. In grasslands in Washington County, Colorado, sweetclover was important to jackrabbits in November and February [224]. In the winter, eastern cottontails fed extensively on small white sweetclover branches and shoots around the Green Pond near Syracuse, New York [249]. In the Conata Basin in western South Dakota, cover of yellow sweetclover was low and made up a small proportion of prairie dog diets. Cover of yellow sweetclover in the study area was 0.1% or less. Based on fecal analyses, yellow sweetclover was 0.3% of March prairie dog diet composition [258].
Birds: Sweetclover seeds and/or insect visitors are important forage for waterfowl, game birds, and song birds (reviews by [156,230,240]). As with mammals, bird use of sweetclover may vary by season. Sweetclover seeds were recovered from the crops of winter-killed gray partridge, ring-necked pheasants, and California quail. The largest volume (19.5%) of sweetclover was recovered from California quail crops. Much less sweetclover was recovered from summer- or fall-collected crops of any bird species [130]. In Idaho, sweetclover is important forage for sage grouse broods [4]. In Nevada, white-crowned sparrows, house finches, and mourning doves are commonly flushed from sweetclover vegetation when seeds are ripening. Sweetclover seeds are also utilized by Gambel's quail, California quail, and ring-necked pheasants in the Intermountain West (review by [84]). Up to 10% of Gambel's quail diets in Nevada were sweetclover (review by [156]). In the Nebraska Sandhills, yellow sweetclover was recovered from crops or gizzards of adult sharp-tailed grouse collected in the summer. Yellow sweetclover occurred in 18% of samples. Yellow sweetclover was not recovered from crops or gizzards of young sharp-tailed grouse [132]. In the Great Plains, yellow sweetclover attracts insects that are consumed by songbirds and upland game birds (review by [240]). In Minnesota, sweetclover occurred in gray partridge diets (review by [156]).
While sweetclover seed is eaten by many bird species, it may not be preferred. When dark-eyed juncos and Harris's sparrows were offered yellow foxtail (Setaria pumila ssp. pumila) and yellow sweetclover seed in captivity, daily consumption of yellow foxtail was 5 to 6 g/day and yellow sweetclover was less than 1 g/day. Hatchery-raised northern bobwhites consumed more than 12 g of yellow foxtail/day and less than 3 g of yellow sweetclover/day [175].
Insects: Sweetclover attracts a variety of insects. In a review, Turkington and others [251] suggest that white sweetclover attracts a wider variety of insects than yellow sweetclover. Bees commonly visit sweetclover plants [188,251] and are important to sweetclover pollination [55,144,285]. Early beekeepers were responsible for some of the early spread of sweetclover in North America [96]. In the Denver-Boulder metropolitan area of Colorado, researchers commonly found sweetclover pollen on native bees collected from native grasslands. Most collected bees were generalists from the Halictidae family. It is possible that native plants were experiencing decreased pollination because of high nonnative plant visitation rates, but this was not studied [100].
Insect diversity on sweetclover can be high. On eastern Minnesota prairies, 15 insect species were collected from yellow sweetclover and 19 from white sweetclover plants. Two insect species were unique to yellow sweetclover [195]. Along riparian areas of the Colorado River in the Grand Canyon, invertebrate densities were greatest from sweeps of white sweetclover and saltcedar. Researchers did 50 insect sweeps on 10 common native and 3 nonnative species (Stevens 1976 cited in [227]).
In Indiana and Wisconsin, sweetclover is a nectar source for 1st- and 2nd-generation adult Karner blue butterflies, which are endangered (review by [85]). White sweetclover was one of the most frequently selected nectar sources in the Indiana Dunes area. When another plant was within 7 feet (2 m) of white sweetclover, Karner blue butterflies selected white sweetclover over the other plant a significant number of times (P<0.01) [83].
Palatability and/or nutritional value: Sweetclover is considered high-quality, palatable wildlife and livestock forage [168,230], but if sweetclover is the only diet component or is improperly cured as hay, it can cause problems for livestock. Some indicate that sweetclover is higher quality forage than alfalfa [230], but forage quality declines as plants mature and become woody [220].
The nutrient content of sweetclover has been reported in the summer in Arizona [259] and throughout the growing season in Ohio [289]. White sweetclover seeds collected near Champaign, Illinois, averaged 4,687 cal/g [120].
Cover value: In the Great Plains, yellow sweetclover is used by game birds for nesting, brood rearing, and winter cover [240]). In parts of Canada, ring-necked pheasants nest in sweetclover stands [251].
Sweetclover has several medicinal and household uses. Sweetclover produces a coumarin compound that can be converted to dicoumarin, which is used medicinally as an anticoagulant [48]. Yellow sweetclover has also been used medicinally to treat external and internal inflammation and stomach and intestinal ulcers [10]. Sweetclover inflorescences have been used in eye lotions [151].
As its common name suggests, sweetclover has a pleasant smell. Northeastern "woodland" natives used dried white sweetclover leaves and flowers in teas [207]. Sweetclover leaves were also used to scent linens and sleeping quarters by early settlers [10] and members of Omaha and Dakota tribes [72].
Nurse crop: Several sweetclover growth characteristics make it attractive as a nurse crop in revegetation. Sweetclover produces strong and deep penetrating taproots that can loosen and aerate compacted soils. Roots are also the site for nitrogen fixation, and as roots decay, nitrogen availability is increased. These processes result in improved soil conditions for succeeding plants (review by [220]). Yellow sweetclover has been used successfully as a nurse crop revegetation of sagebrush ecosystems. Yellow sweetclover establishes and develops faster than seeded grasses and minimizes invasion by other, less desirable invasive species. In sagebrush habitats, yellow sweetclover generally decreases in importance as grasses increase in size and abundance [159].
Use of sweetclover as a nurse crop has been important to its spread throughout North America (see Introduction and spread in North America), and in many habitats, sweetclover can persist and delay development and success of native species (see Impacts on vegetation).
Sweetclover flowers April to October throughout North America [11,52,82,117,158,191,235,241,292]. Generally yellow sweetclover flowers 1 to 3 weeks earlier than white sweetclover [39,55,220,229,254,267,289]. The flowering rate in individual white sweetclover racemes is usually about twice as fast as that for yellow sweetclover [251], which may explain why Willard [289] suggests that yellow sweetclover flowering is more uniform than that of white sweetclover. Moisture conditions, elevation, and likely other site factors can affect flowering. Following a dry spring and an especially dry June in Ohio, sweetclover flowered for a second time in mid- to late July. There were several moderate rains in July [289]. In ponderosa pine forests in north-central Arizona, the flowering period for yellow sweetclover was much shorter at a site that was 600 feet (180 m) higher than its comparison site [31].
Sweetclover green-up, fruit development, and seed maturation were described in a few locations. In north-central Arizona, vegetative growth of yellow sweetclover began in mid-April. Vegetative growth was delayed 2 to 3 weeks at a high-elevation site compared to a low-elevation site. Mature yellow sweetclover fruits were present by late August at the low-elevation site, and fruit maturation continued into November at the high-elevation site [31]. In southern Ontario, there were usually ripe yellow sweetclover seeds by late July, but it was early August before ripe white sweetclover seeds occurred. Often sweetclover seeds remained on the plant through the winter (Rempel unpublished data cited in [251]).
Root crown buds and carbohydrate storage: In late summer and early fall, 1st-year sweetclover plants increase their taproot size, root crown bud number and size, and underground carbohydrate storage. In fields near Ames, Iowa, stems are 90% or more of the total weight of 2- to 3-month-old white sweetclover. By late September of the 1st growing season, roots provide up to 80% of total plant weight [157]. In fields in Wisconsin, the largest increases in root size and carbohydrate storage for 1st-year sweetclover occurred between 18 September and 18 October [216]. Additional field studies conducted in Ames, Iowa showed that 1st-year sweetclover taproot weight and root crown bud abundance increased from late summer to early fall. When particularly "vigorous" 1st-year yellow sweetclover plants were excavated on 20 August, taproots averaged 2.6 g, and there were 2.2 root crown buds/plant. Taproot weight averaged 11.7 g, and there were 29.4 crown buds/plant on 20 November. Greenhouse experiments revealed that increases in taproot size and root crown bud production occurred with decreasing photoperiod [118]. In experimental fields in Columbus, Ohio, sweetclover produced large root crown buds in August. Buds became larger and more numerous until November [289]. At the Agronomy Research Center in West Lafayette, Indiana, researchers found that total nonstructural root carbohydrates (TNCs) for 1st-year yellow sweetclover were highest from November to December. TNCs were lowest in May after the emergence of 2nd-year plants [142]. For more on how these changes in root development and storage may affect management of sweetclover, see Control.
Sweetclover flowers are perfect [55,158], and although experiments have shown that seed can be produced by self-fertilized flowers, this rarely occurs in natural conditions [34], especially for yellow sweetclover [122]. Sweetclover flower structure encourages cross pollination by insects. When insects land on lower flower petals, stigma and anthers bend and contact the insect body [10]. Bees are the most common sweetclover pollinators; honey bees, bumble bees, and leaf-cutter bees were reported as important pollinators ([144,285], review by [55]). Successful pollination by insects can be affected by season and weather. Cloudy, wet weather decreases bee activity [10,285], and Bare [10] reports that the honey production capacity of sweetclover is greater in early summer than late summer and greater for areas west of the Mississippi River than areas east of the River.
Fertilization: After reviewing published studies and conducting original studies on many yellow and white sweetclover strains and varieties, Kirk and Stevenson [122] concluded that self fertilization does occur naturally for some white sweetclover strains or varieties, but seed production from naturally self-fertilized yellow sweetclover is rare. Studies that followed the Kirk and Stevenson review (published in 1931) generally support their conclusions. In the greenhouse, researchers used genetic markers to determine that cross-fertilization in annual and biennial white sweetclover populations averaged 67% and 58%, respectively [79]. When yellow sweetclover flowers were artificially self pollinated, fruit set ranged from 0 to 69.2% and averaged 19.3% [206]. In a field experiment along the Rio Grande in Albuquerque, New Mexico, fruit set was low when yellow sweetclover racemes were protected from insect visitors. Fruit set by protected racemes (6%) was significantly (P<0.001) lower than that set by unprotected racemes (44%) [163]. For more on this study, see the Seed production section below.
Studies clearly indicate that sweetclover produces a seed bank; however, estimates of the longevity of seed in the soil vary from >2.5 [126,260] to 81 years (review by [204]). Sweetclover seeds have germinated after 81 years of storage (Becquerel 1934 cited in [41]), but field studies involving the recovery and germination of buried seed over time are lacking.
Sweetclover produces a percentage of hard seeds (see Germination) that germinate only after scarification. Hard seeds likely make up the majority of the seed bank [115,233]. When white sweetclover seed was buried in pots on an abandoned rock quarry near Syracuse, New York, 43% and 26% of the seed on sparsely and densely vegetated sites, respectively, failed to germinate but was still viable a year later [126]. In other studies, sweetclover emerged from soil samples although plants were absent from the aboveground community [29,198].
Storage conditions: Sweetclover seeds have survived and germinated after decades in storage, but storage conditions were rarely described, making it difficult to assess their relevance to field situations (Becquerel 1934 cited in [41], Ewart 1908 cited in [274], Munn 1954 cited in [219], Crocker 1938 cited in [251]). Thirty percent of white sweetclover seeds germinated after 19 years in an unheated shed in Twin Falls, Idaho. Maximum and minimum temperatures for Twin Falls can be 105 °F (41 °C) and -26 °F (-32 °C), and in the shed, temperatures were slightly higher [106]. After 81 years of storage in unknown conditions, 0.6% of white sweetclover seed was viable (Crocker 1938 cited in [251]).
Field conditions: Field studies suggest that sweetclover seed remains viable after 14 to 17 years in the soil but may survive over 50 years in the soil. Several researchers indicate that sweetclover can be abundant even after "several years" without mature plants on a site ([115], review by [187]). After 5 years of underwater storage in Prosser, Washington, a small proportion of white sweetclover seed germinated, but 42% of seeds were still firm [36]. In North Dakota, sweetclover remained viable in the soil for at least 14 years. Sweetclover was planted and allowed to produce seed on 2 agricultural plots. In the following years, plots were cultivated and planted to other crops. Sweetclover seedlings emerged almost every spring for 14 years, even though 1st-year plants were killed each year [233]. On experimental plots at the University of Saskatchewan, white sweetclover seed survived 17 years in the soil. Crop history records and the distribution and quantity of seed led the researcher to conclude that white sweetclover germinated from soil-stored seed, not dispersed seed [13]. In another field study, a researcher visited several areas where circumstances would indicate long-lived, soil-stored seed. In Copenhagen, Denmark, a pork market that was built in 1910 was torn down in 1961. Some archaeological digging occurred, and by 1963, yellow sweetclover was growing on site. Because wind-dispersal is unlikely, the researcher speculated that yellow sweetclover germinated from soil-stored seed [176].
Because sweetclover lacks appendages for wind dispersal, most seed falls near the parent plant (review by [49]), but observations and experiments indicate that long-distance dispersal by animals and water is possible. Long-distance dispersal may also occur through the transport of contaminated seed or animal feed ([14], review by [49]).
Water: Several sources suggest that sweetclover seed is dispersed by water. Based on plant distributions along waterways in Alaska [37,225], Montana [14], Arizona [237], and New Jersey [221], sweetclover seed dispersal by water seems likely. Experiments conducted in London, Ontario, showed that over 65% of white sweetclover seeds were still floating after 15 minutes in violently agitated water (unpublished experiments described in [251]).
Animal: Observations and experiments leave little doubt that sweetclover seed is transported by animals. In the Intermountain West, sweetclover spread along cattle trails was reported [11], and in the Missouri Ozarks, sweetclover was restricted to horse trails [236]. While collecting sweetclover seeds for later experiments, students found that sweetclover seeds with fruit layers attached were transported on human clothing (unpublished experiments described in [251]).
Experiments show that viable sweetclover seed can be recovered from animal feces. When white-tailed deer pellets were collected from mixed-deciduous forests in Ithaca, New York, a maximum of 13 white sweetclover seeds germinated/pellet group [171]. Three white sweetclover seeds were collected from crops of mourning doves, and 1 seed germinated. A seed recovered from the gizzard did not germinate [8]. When calves, horses, sheep, hogs, and chickens were fed a known quantity of white sweetclover seed, 17.7%, 10%, 17.1%, 11%, and 0% of the seed germinated from collected feces, respectively. When recovered seeds were treated with sulfuric acid, germination rates increased by 40% or more, indicating that a large portion of white sweetclover seeds were still hard after passing through these animals (see Germination for more about hard sweetclover seed). Five percent of white sweetclover seeds that remained inside calves for up to 48 hours germinated. Ten percent of seeds recovered after 48 to 80 hours inside calves germinated. Sweetclover seeds may also be transported in partially composted manure. Two percent of white sweetclover seeds germinated after 2 months of burial in manure [92].
Although wind dispersal is relatively unimportant for sweetclover, a study found that the weight of white sweetclover seeds depended on time of production. Fifty seeds produced early in the growing season and late in the growing season averaged 78.9 mg and 59.3 mg, respectively [28]. Whether or not lighter seeds could be dispersed longer distances is unknown, and germination percentages were not reported.
High levels of seed production are reported for sweetclover ([228], Rempel and Cavers unpublished data cited in [251]), and available reports indicate that white sweetclover generally produces more seed than yellow sweetclover (Rempel and Cavers unpublished data cited in [251]). However, seed production estimates using calculations that assume all flowers produce fruits and all fruits produce 1 seed can largely overestimate production [126]. Methods used to determine seed production in the following studies were not reported. In London, Ontario, large white sweetclover growing in open conditions produced 200,000 to 350,000 seeds/plant. Large yellow sweetclover growing under similar conditions rarely produced more than 100,000 seeds/plant (Rempel and Cavers unpublished data cited in [251]). In North Dakota, an average-sized white sweetclover with 5 stems produced 14,235 seeds [228].
Klemow [126] found that estimates of seed production were exaggerated when estimation calculations did not factor in flower abortion and empty fruits. In a white sweetclover population ecology study in an abandoned rock quarry in Syracuse, New York, Klemow [126,127] estimated that white sweetclover produced an average of 4,380 fruits/plant and 11,640 fruits/plant in sparsely vegetated (cover 8%) and densely vegetated (cover 41%) sites, respectively. About 80% of fruits contained a seed, so the average seed production per plant was 3,530 on the sparse site and 9,710 on the dense site. Fruit and seed production estimates, however, assumed that all flower buds counted in August would develop into flowers, form fruits, and produce seed. A later visit to these sites showed that flower buds were often aborted or failed to produce fruit. When abortion and empty fruits were factored into calculations, the most and least seed produced by white sweetclover over the 5-year study period was 5,000 and 171 seeds/plant, respectively [126,127].
Sweetclover seed production is often reduced if plants are damaged, grow on infertile sites, or if insect visitation is limited by weather, insectivorous birds, or other means. In Ontario, Canada, researchers observed that damaged sweetclover plants or plants in extremely infertile soils sometimes produced less than 100 seeds (Rempel and Cavers unpublished data cited in [251]). After many field observations and studies, Coe and Martin [34] found that seed production was greater in dense than in sparse sweetclover stands, reduced in drought conditions, and lower when cloudy, rainy weather limited insect visitation. In Arlington, Virginia, a 3-foot (0.9 m)-tall sweetclover produced 196 racemes, and racemes produced an average of 20.4 fruits each. This plant grew in a stand density of 4 sweetclover/ft². When plants were protected from insects, racemes averaged 0.63 fruits each [34]. Along the Rio Grande in Albuquerque, New Mexico, yellow sweetclover fruit set increased with increasing distance from cliff swallow colonies. When plants were within 660 feet (200 m) of the colonies, fruit set was reduced by about half. Once cliff swallow chicks fledged, however, the relationship between fruit set and colony proximity was lost. Cliff swallows were consuming insects that visited yellow sweetclover. Researchers estimated that a cliff swallow colony of 150 nests could consume over 500,000 insects per day, based on an average insect size of 5 mm [163].
Generally sweetclover is an early to mid-seral species common on open, disturbed sites. Sweetclover rarely persists in dense shade and often appears early in the succession of recently disturbed or bare sites. It is important to note that year-to-year sweetclover cover can vary a lot; "boom" growth years are common [262]. In South Dakota, times when areas are covered with white and/or yellow sweetclover flowers are described as "sweetclover years" [115]. In big sagebrush/grasslands in central Montana, researchers reported 10% to 12% cover of yellow sweetclover in one year and less than 1% cover the next [270]. Large fluctuations in sweetclover cover make interpretation of seral change along a chronosequence difficult.
Shade: Most reviews and studies indicate that sweetclover grows best in full sun or partial shade. A review of Upper Midwest habitats indicates that sweetclover is most frequent in open, disturbed upland prairies, savannas, and dunes (review by [45]). Other reviews report that sweetclover is less "vigorous" and produces fewer seeds in shade than in full sun [251]; however, shade tolerance may be greater in hot, dry climates [222].
Most studies and observations indicate that although sweetclover is common on open sites, some degree of shade tolerance also exists. At the Mammoth campground in Yellowstone National Park, yellow sweetclover was positively associated with open canopy conditions (P<0.02), and 75% of yellow sweetclover occurrences were beneath canopies of 10% or less [1]. In the Swan Valley of northwestern Montana, yellow sweetclover cover was much greater on logged (14%) than unlogged (<1%) coniferous forests. Logging occurred up to 30 years earlier [67]. Along rivers in Alaska, white sweetclover did not occur beneath dense alder (Alnus spp.) or balsam poplar (Populus balsamifera) canopies [37]. In quaking aspen (P. tremuloides) woodlands that dominated about 90 years after deforestation of the boreal mixed-wood forest in Alberta, white sweetclover was restricted to within 49 feet (15 m) of the deforested edge [71]. Some studies indicate mild to substantial shade tolerance in sweetclovers. In a savanna in the University of Wisconsin-Madison Arboretum, white sweetclover cover was 7.5% beneath oak (Quercus spp.) canopies and 6.7% outside of the canopies. Photosynthetically active radiation was 52% to 83% lower beneath than outside oak canopies [131]. Along portions of the Rio Grande in New Mexico, sweetclover often dominated beneath a dense overstory of Fremont cottonwood (P. fremontii), Goodding willow (Salix gooddingii), and Russian-olive (Elaeagnus angustifolia). In these habitat types, litter was 1 to 6 inches (2.5-20 cm) deep [25], suggesting that sweetclover seedlings as well as mature plants were tolerating heavy shade.
Bare site succession: Bare soil is rapidly colonized by sweetclover, but rarely does sweetclover persist as a dominant. On calcareous soils deposited during construction in central Germany, white sweetclover dominated (60-75% cover) in the 2nd and 4th years of succession. White sweetclover populations collapsed in the 5th year, but in the 7th and 10th years of succession, white sweetclover cover exceeded 10% [208]. In Plzen, Czech Republic, white sweetclover dominated a nutrient-poor site 6 years after bare soil was left by a human-caused disturbance. White sweetclover did not dominate in any other year [189]. Yellow sweetclover production was greatest 4 years after disturbance in a sagebrush habitat in northwestern Colorado. After all vegetation and the top 2 inches (5 cm) of soil were removed and the remaining 14 inches (35 cm) of soil was mixed, yellow sweetclover production was 1 g/m² in first postdisturbance year, 13 g/m² in the 2nd, 5 g/m² in the 3rd, 32 g/m² in the 4th, and less than 1 g/m² in the 5th, 6th, and 7th years [161]. Near Duluth, Minnesota, yellow sweetclover appeared 4 years after bare sand was deposited in a high-water year. Persistence beyond this time was not reported [136]. On fly-ash mine pits in Tennessee, the importance of white sweetclover was greatest on 8-year-old pits when 6-month, 3-year, and 8-year-old pits were compared. Fly ash that is deposited into the pits is "essentially sterile", free of seeds and other reproductive plant material [76]. Sweetclover occurred in the early succession of sand flats in eastern New York created by deposition of material dredged from the Hudson Estuary channel. Dredging began in 1929, and species composition was first evaluated in 1935 [162]. In vacant lots in Montreal, Quebec, white sweetclover-dominated sites had a large amount (10.8-26.5%) of bare ground [265].
Floodplain succession: On floodplains, sweetclover is common in early- and mid-seral stages of succession. In southwestern North Dakota, yellow sweetclover occurs in early-seral eastern cottonwood-Rocky Mountain juniper (P. deltoides-Juniperus scopulorum) stands, and mid-seral eastern cottonwood-green ash (Fraxinus pennsylvanica) stands on more stable floodplains [73]. In eastern cottonwood stands along the Missouri River in southeastern South Dakota, white sweetclover cover generally decreased with increasing eastern cottonwood stand age. In stands estimated to be 10, 14, 23, 35, and 55 years old, yellow sweetclover cover averaged 8%, 12%, 2%, 2%, and 0%, respectively [291].
Old field succession: Abandoned agricultural fields are common sweetclover habitat. Typically sweetclover abundance is lowest in the most successionally advanced old fields. Likely the composition and density of associated vegetation affects sweetclover persistence. In the Black Forest of central Colorado, yellow sweetclover occurred on a 4-year-old field but not on 1-, 9-, or 22-year-old fields [145]. When a previously cultivated site at the Fermi National Accelerator Laboratory in Illinois was seeded with native tallgrass prairie species, white sweetclover frequency was greater after 1 to 6 years after seeding than 12 years after seeding [215]. At the WW Kellogg Biological Station in Kalamazoo County, Michigan, large white sweetclover patches occurred in fields last plowed 10 to 16 years previously [284]. When different-aged stands were sampled along a chronosequence of old field to deciduous forest in southwestern Ohio, sweetclover was present but not common on 2-, 10-, and 50-year-old fields. Sweetclover did not occur in stands 90 or 200 years old. Two-year-old fields were dominated by red clover (Trifolium pratense); 10-year-old stands were dominated by Canada goldenrod (Solidago canadensis) and meadow fescue (Schedonorus pratensis); 50-year-old stands were dominated by Canada goldenrod and had 30% cover of white ash (F. americana) and black cherry (Prunus serotina). Stands 90 years and older were dominated by deciduous forest species [264].
Disturbances: Throughout its nonnative range, sweetclover is described on disturbed sites [10,89,201,267,282,298]. Generally sweetclover occurs on recently disturbed sites, but without further disturbance, sweetclover fades from the community. In mixed- and shortgrass prairie near Cheyenne, Wyoming, yellow sweetclover occurred and was sometimes frequent on sites disturbed about 3 to 25 years earlier. On undisturbed sites or sites disturbed more than 25 years ago, yellow sweetclover was rare or absent [205].
In prairies, sweetclover often occurs on soil mounds created by wildlife. In the prairie potholes of Montana, North Dakota, South Dakota, and western Minnesota, sweetclover occurred on earth mounds created by pocket gophers and badgers [99]. In Billings, North Dakota, yellow sweetclover occurred in all 4 active prairie dog towns visited [234]. On Cayler Prairie Preserve in Dickinson County, Iowa, sweetclover was consistently associated with excavation mounds created by badgers that were hunting ground squirrels. Sweetclover was uncommon in undisturbed tallgrass prairie [186]. In Wisconsin prairies, soil mounds created by ants or other animals are the first establishment site for white sweetclover [43].
Roads and waterways are common sweetclover habitats and have been important to sweetclover spread (see Introduction and spread). In Alaska, sweetclover was restricted to roadsides and floodplains and did not occur in roadless areas [37]. In the Northern Rocky Mountains, yellow sweetclover occurred in disturbed areas (roads, ditch banks, or logged sites) above and below timberline but did not occur in little-disturbed or undisturbed vegetation [276]. In mixed-grass prairie and open ponderosa pine woodlands in Wind Cave National Park, South Dakota, logistical regression analyses revealed that sweetclover was associated with roads and trails. Yellow sweetclover was also common in prairie dog towns, and white sweetclover was most common on low-elevation, recently burned sites [177]. In the wet to mesic Chiwaukee tallgrass prairie in Wisconsin, density of white sweetclover was significantly greater on past disturbed than undisturbed transects [164]. In the late 1990s, researchers surveyed 1,940 miles (3,120 km) of county and state roads in western Adirondack Park, New York. White sweetclover occurred at more than 100 sites in the survey area [20].
Grazing: In the studies that evaluated sweetclover on grazed and ungrazed sites, typically cover on grazed sites was similar to or greater than cover on ungrazed sites. Yellow sweetclover persisted on grazed and ungrazed sites during drought conditions in semiarid grassland in north-central Arizona. Moderate- and high-impact, short-duration grazing rotations were evaluated. During the 8-year study, precipitation levels for the 8 months prior to July were 2.5 to 12 inches (62.4-312.8 mm) lower than the 20-year average [146]. In western Colorado, yellow sweetclover cover within long-term (41-51 years) ungulate exclosures in sagebrush and mountain shrubland did not differ much from cover outside the exclosures [153]. When grazed and ungrazed portions of Fults Prairie, Illinois, were compared, yellow sweetclover was absent from the ungrazed portion and had 36% frequency on the grazed portion. Researchers suggested that decreased abundance of climax vegetation on grazed sites increased the success of yellow sweetclover and other “weedy” species [174]. While aboveground cover of yellow sweetclover may not differ on grazed and ungrazed sites, seed density differed in soils collected from grazed and ungrazed mixed-grass prairie sites in western North Dakota. Fewer than 5 yellow sweetclover seedlings emerged from soil samples taken from grazed sites, and more than 20 seedlings emerged from soil samples from ungrazed sites [111].
For information about sweetclover and fire-disturbed sites, see the discussion on Long-term fire effects and postfire succession.
Sweetclovers are part of the Melilotus (Fabaceae) genus. While some systematists treat
white sweetclover (Melilotus alba) Medik. and yellow sweetclover
(Melilotus officinalis) (L.) Lam as distinct species ([116,231], Isely 1990 cited in [48]),
others suggest they are not distinct and recognize only one species, Melilotus officinalis ([256], van der Meyden personal communication cited in [268]).
Other systematists suggest recognizing both species, since they have been identified as such
for over 200 years [268], and Barneby [11] reports that the
2 species are genetically incompatible.
This review treats yellow and white sweetclover as individual species. "Sweetclover" is used when citing information common to both species.
There are many sweetclover cultivars. Information about cultivars is available in the following references: [219,273,285].
Melilotus, known as melilot, sweet clover, and kumoniga (from the Cumans),[3] is a genus in the family Fabaceae (the same family that also includes the Trifolium clovers). Members are known as common grassland plants and as weeds of cultivated ground. Originally from Europe and Asia, it is now found worldwide.
This legume is commonly named for its sweet smell, which is due to the presence of coumarin in its tissues. Coumarin, though responsible for the sweet smell of hay and newly mowed grass, has a bitter taste, and, as such, possibly acts as a means for the plant to discourage consumption by animals.[4] Fungi (including Penicillium, Aspergillus, Fusarium, and Mucor[5]) can convert coumarin into dicoumarol, a toxic anticoagulant. Consequently, dicoumarol may be found in decaying sweet-clover, and was the cause of the so-called sweet-clover disease, recognized in cattle in the 1920s.[6] A few varieties of sweet clover have been developed with low coumarin content and are safer for forage and silage.[7]
The name sweet clover varies orthographically (sweet-clover, sweetclover).
Melilotus species are eaten by the larvae of some Lepidoptera species, such as those of the genus Coleophora, including C. frischella and C. trifolii.
Melilotus, often used as a green manure, can be turned into the soil to increase its nitrogen and organic matter content. It is especially valuable in heavy soils because of its deep rooting. However, it may fail if the soil is too acidic. It should be turned into the soil when 8 to 10 inches tall. Unscarified seed is best sown in spring when the ground is not too dry; scarified seed is better sown in late fall or even in the snow, so it will germinate before competing weeds the following spring.[8]
Melilotus siculus (messina) is notable for its high combined tolerance to salinity and water logging. As of 2019, the "Neptune" variety has the highest tolerance and persistence under salinity among all pasture legumes, according to the Australian Department of Primary Industries and Regional Development. The salt-tolerant symbioant Ensifer medicae SRDI554 is recommended.[9]
Blue melilot (Trigonella caerulea) is not a member of the genus, despite the name.
The genus Melilotus currently has nineteen recognized species:[10]
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Melilotus albus Melilotus, known as melilot, sweet clover, and kumoniga (from the Cumans), is a genus in the family Fabaceae (the same family that also includes the Trifolium clovers). Members are known as common grassland plants and as weeds of cultivated ground. Originally from Europe and Asia, it is now found worldwide.
This legume is commonly named for its sweet smell, which is due to the presence of coumarin in its tissues. Coumarin, though responsible for the sweet smell of hay and newly mowed grass, has a bitter taste, and, as such, possibly acts as a means for the plant to discourage consumption by animals. Fungi (including Penicillium, Aspergillus, Fusarium, and Mucor) can convert coumarin into dicoumarol, a toxic anticoagulant. Consequently, dicoumarol may be found in decaying sweet-clover, and was the cause of the so-called sweet-clover disease, recognized in cattle in the 1920s. A few varieties of sweet clover have been developed with low coumarin content and are safer for forage and silage.
The name sweet clover varies orthographically (sweet-clover, sweetclover).
