Obtuse-Leaved Tick Trefoil
Desmodium obtusum
Bean family (Fabaceae)

Description: This perennial wildflower is 2–3' tall, branching occasionally. The stems are medium green to brown and sparsely to densely covered with both hooked and straight hairs. Alternate compound leaves occur at intervals along these stems; the structure of the leaves is trifoliate. Each compound leaf has a hairy petiole up to " long; at the base of each petiole, there is a pair of tiny deciduous stipules that are linear-lanceolate in shape. Individual leaflets are up to 2" long and 1" across; they are oval to narrowly oval in shape, smooth and slightly ciliate along their margins, and rough-textured. Their upper surfaces are medium green and sparsely covered with short stiff hairs, while their lower surfaces are light green and covered with similar hairs. The terminal leaflet is larger in size than the lateral leaflets; the lateral leaflets have very short petiolules (secondary petioles), while the petiolule of the terminal leaflet is up to " long.

The upper stems terminate in either panicles or racemes of flowers about –1' long and about one-third as much across or less. The central stalk of a panicle branches either oppositely or in whorls of 3. Both the central stalk and its lateral branches (if any) are covered with hooked hairs. Individual flowers occur on short pedicels up to " long. Each flower is about " long, consisting of 5 pink to rosy pink petals, a very short tubular calyx that is toothed, an ovary with a single style, and several stamens. The flower has a pea-like structure, consisting of an upper banner and two lateral wings that enclose a lower keel. At the base of the banner, there is a small patch of yellow. The blooming period occurs from mid-summer to early fall and lasts about 1–1 months. Fertile flowers are replaced by small loments (segmented flat seedpods) about –" long. Each loment usually consists of 2-3 segments; the lower side of each segment is more rounded than the upper side. Loments have very short stipes and sometimes terminal beaks; their sides are covered with hooked hairs. Each segment of a loment contains a single seed. The root system consists of a narrow caudex with fibrous roots. This wildflower reproduces by reseeding itself, sometimes forming small colonies.

Cultivation: The preference is partial sun, dry-mesic conditions, and sandy or rocky soil. Nitrogen is added to the soil through a symbiotic association between the root system and mycorrhizal bacteria.

Range & Habitat: The native Obtuse-Leaved Tick Trefoil occurs in scattered locations in southern, western, and a few other areas in Illinois; it is uncommon within the state. Illinois lies along the NW range limit of this species. Habitats include open sandy woodlands, upland rocky woodlands, sandy savannas, woodland edges, rocky glades, and powerline clearances in sandy or rocky woodlands. Occasional wildfires or other disturbance tends to increase populations of this wildflower in the preceding habitats.

Faunal Associations: The flowers offer only pollen as a reward to insect visitors. These floral visitors consist primarily of bumblebees (Bombus spp.), long-horned bees (Melissodes spp.), and leaf-cutting bees (Megachile spp.). Other insects feed on the foliage, flowers, seedpods, or plant juices of Desmodium spp. (Tick Trefoil species). These insect feeders include such skipper caterpillars as Achalarus lyciades (Hoary Edge), Thorybes bathyllus (Southern Cloudywing), Thorybes pylades (Northern Cloudywing), and Epargyreus clarus (Silver-Spotted Skipper); the caterpillars of the butterflies Everes comyntas (Eastern Tailed Blue) and Strymon melinus (Gray Hairstreak) also feed on these plants. Other insects feeders include various beetles, stink bugs, aphids, thrips, and moths (see Insect Table for a listing of these species). Among vertebrate animals, the seeds are eaten by some upland gamebirds (Bobwhite, Wild Turkey) and small rodents (White-Footed Mouse, Deer Mouse), while the foliage is browsed by White-Tailed Deer and the Cottontail Rabbit. The foliage is also palatable to cattle, horses, and sheep. The seedpods cling readily to the fur of animals and the clothing of humans; as a result, the seeds are distributed to new locations.

Photographic Location: A sandy savanna at the Hooper Branch Savanna Nature Preserve in Iroquois County, Illinois.

Comments: This is one of several Desmodium spp. (Tick Trefoil species) in Illinois. Many of these species are found in savannas and open woodlands; their small pinkish flowers are very similar to each other. Obtuse-Leaved Tick Trefoil has wider leaflets than most other species in this genus (about 2-3 times as long as they are wide). Other critical features for identification include the following: 1) the stipules are tiny and deciduous, 2) the loments usually consist of only 2-3 segments, 3) the segments of the loments are rounded on both sides, although more so on their lower sides than their upper sides, and 4) the leaflets are relatively large (up to 2" long and 1" across) and rough-textured. Another common name of this species is Stiff Tick Trefoil, which is derived from an obsolete scientific name, Desmodium rigidum.