Delta Maidenhair

raddianum: named after Guiseppe Raddi (1770-1829), Italian botanist who collected in Brazil from where the type originates.

Rhizome short, creeping; rhizome scales brown, lanceolate, entire. Fronds closely spaced to tufted, thinly herbaceous, arching. Stipe up to 25 cm long, thin, shiny, dark brown to black, glabrous. Lamina 3-4 pinnate, triangular to broadly ovate in outline, 10-25 × 8-15 cm, glabrous. Rhachis and stalks black, glabrous. Pinnules persistent, 1.2 × 1 cm, obcuneate to trapeziform with straight sides, outer margin irregularly incised into rounded lobes with toothed margins, veins ending in the sinuses of the marginal serrations. Sori on the outer margins of ultimate segments, indusial flaps broadly overlapping the sori, kidney shaped to circular, c. 1.5 mm in diameter.

Native to South America, widely escaped throughout the moister regions of Africa.

Adiantum raddianum Presl

Adianthum raddianum Presl, Tent. Pterid. 158, 1836.—Tryon, Contr. Gray Herb. 144:169, 1964.—Hoshizaki, Baileya 17:134, 1970.

Adiantum cuneatum Langsdorff & Fischer, Ic. Fil. 23, t. 26, 1810.—Fosberg, Bull. Torr. Bot. Cl. 70:387, 1943; Occ. Pap. Bishop Mus. 24:11, 1969 [non A. cuneatum Forster, Prodr. 84, 1786].

According to Hoshizaki this is the plant commonly known in cultivation as A. cuneatum, and undoubtedly the plant widely escaped and naturalized in the Hawaiian Islands. Her illustration shows precisely the pinnule shape and the veins terminating in the sinuses between the lobes on sterile portions of the distal margins of the pinnules shown by the plant introduced in Hawaii. There are numerous horticultural varieties of this species, some of which are illustrated by Mrs. Hoshizaki.

Tryon was apparently the first to indicate that A. raddianum Presl is the earliest available name for this species, since A. cuneatum is antedated by A. cuneatum Forster. We may further point out that A. cuneatum Langsdorff & Fischer and A. raddianum Presl are nomenclaturally equivalent, as the latter is based on a supposed A. cuneatum Raddi (1825: [59, 100] pl. 78, 2a, b). Raddi was merely using Langsdorff’s and Fischer’s name, though ascribing it to Willdenow (1810:450). Willdenow merely included A. cuneatum Langsdorff & Fischer in his account of the genus. Raddi’s illustration, though not showing venation, seems clearly to apply to the plant described by Langsdorff and Fischer.

Carex L.

Adiantum raddianum, the Delta maidenhair fern, is one of the most popular ferns to grow indoors. It is native to South America and its common name comes from its shiny, dark leafstalks that resemble human hair. It typically grows about 17–19 in (43–48 cm) tall and up to 22 in (56 cm) wide. In the wild, it is found on forest floors, rock crevices, river banks, coastal cliffs, and basalt banks along trails and streams. The triangular fronds are semi-erect in the beginning then droop gracefully as they age and can be up to 12 inches (30 cm) long by 6 inches (15 cm) wide. The genus name Adiantum comes from the Greek word "adiantos", meaning "unwetted" (in reference to the leaves).

This plant is hardy down to 5 °C (41 °F), so must be grown indoors in temperate regions. However it may be placed outside in a sheltered spot during the summer months. It requires high humidity, well-drained soil, bright indirect light, and a fairly constant temperature. It prefers neutral or slightly alkaline soil and is suitable for USDA hardiness zones 10 and 11. It can be susceptible to scale and mealybug. The cultivars ‘Brilliantelse’ and ‘Kensington Gem’ have won the Royal Horticultural Society's Award of Garden Merit.

A. raddianum is sometimes considered invasive in Hawaii and French Polynesia.