White-nymph (Trepocarpus aethusae) of the Carrot (Apiaceae) family is the only species worldwide of the genus Trepocarpus––the genus is “monotypic.” Etymology of the generic name is uncertain. The specific epithet is a reference to Arethusa, a water nymph of Greek mythology. The species occurs in the central and southwestern portions of the Southeastern U.S., primarily from eastern Texas and Oklahoma to Alabama and Tennessee and north to southeastern Missouri, southern Illinois, and eastern Kentucky, with scattered occurrences in Georgia, Florida, and South Carolina. In Arkansas, it grows throughout most of the state except for portions of the Ozark Highlands. Habitats are moist to wet soils of lowland woods, floodplains, and swamp margins.
The herbaceous annual has a taproot to 2 inches long. First growth of a new plant appears during winter as a pair of long, grass-like cotyledons (seed leaves). The 2-3 foot tall mature plants bear alternate, highly dissected leaves on light green, finely ribbed, smooth and glabrous stems. Stem nodes are purple. Internodes zigzag from node to node. Crushed plant parts have a strong carrot odor.
The compound leaves are triangular in outline, the leaflets and sub-leaflets dissected into delicate lobes. Lower and mid-plant leaves are 3-times pinnate and uppermost leaves are 2-times pinnate. Petioles, about ¼ inch long, have winged upper edges and clasping bases. A large, mid-stem leaf may be 3½ inches long (including petiole) and 4 inches wide. Mid-stem leaves have several to eight or more opposite pairs of leaflets.
The inflorescence consists of compound umbels atop straight, leafless floral stems that are 2-4 inches long. Umbels consist of 3-4 umbellets that terminate straight ¼-½ inch long peduncles. Umbels and umbellets are subtended by several irregularly sized and positioned acicular bracts up to 1 inch long. Umbellets comprise 4-9 fertile and infertile flowers; pedicels of fertile flowers are very short (less than 1/16 inch) to absent while pedicels of the smaller infertile flowers may be ¼ inch long.
Fertile flowers, with spreading corollas slightly wider than 1/16 inch, have 5 minute acicular sepals, 5 petals, 5 stamens, and 1 pistil (consisting of an inferior ovary bearing 2 styles and stigmas). Floral parts are attached at the summit of a prominent naked ovary about ¼ inch long and 1/16 inch wide.
Umbellets produce several to half a dozen or more elongate fruits. As fruits mature in mid-summer, the entire plant quickly fades. Fruits, remaining as paired nutlets, persist on the dead branches as the plant disintegrates. Seed dispersal may be birds dropping a nutlet while removing chaff or by water-transport of the buoyant nutlets.
White-nymph, with lacy foliage and tiny white (nymph-like?) flowers, can be a lovely specimen plant, but can get lost among other plants. It can be an aggressive self-seeder and spreads easily in its preferred habitat. Spread can be controlled by removal of entire plants before seed dispersal. It may be suitable for a “controlled” area of a native plant garden or in a natural area. In midsummer, after seed-set, the plant quickly disintegrates. It is a host plant of the Black Swallowtail (Papilio polyxenes). It is not eaten by deer.
Although white-nymph is the only species of its genus, its structure is somewhat similar to that of several other species in the carrot family in Arkansas. It can be recognized by its intricately dissected, flattened leaves and white-petaled flowers atop large, glabrous ovaries.
Article and photographs by ANPS member Sid Vogelpohl