Illinois Tick Trefoil
Desmodium illinoense
Bean family (Fabaceae)

Description: This native perennial plant is 2–4' tall, branching sparingly. The stout central stem is round and covered with fine hooked hairs. The alternate compound leaves are trifoliate and have petioles about 1–2" long. The leaflets are up to 3" long and 1" across. A typical leaflet is lanceolate-oblong or lanceolate-ovate, with smooth margins, and a leathery texture. The base of a leaflet is rounded, while the tip is usually blunt. The lower surface is light or whitish green and has prominent veins; there are hooked hairs along the major veins. The central stem terminates in an elongated raceme of flowers about –1' long. Usually, only a few flowers are in bloom at the same time. Each flower is about 1/3" long (up and down), and either pale purple or white. The two upper petals are larger than the others and well-rounded; when these petals are pale purple, there is a small patch of white near the throat of the flower, which is surrounded by a narrow border of burgundy. There are also two side petals that enclose a lower petal, which project outward. Together, they form a typical pea-shaped flower. The hairy green calyx is divided into 5 lobes of unequal length, while the pedicels of the flowers are slender and hairy. The blooming period usually occurs during mid-summer and lasts about 3 weeks. There is no noticeable floral scent. The flowers are replaced by flat seedpods about 1-3" long that are called 'loments.' These loments have 3-9 segments that are well-rounded on both the upper and lower sides. The surface of these loments are covered with hooked hairs that can cling to clothing or fur. Individual segments of the loments can break off and cling to more than one passing carrier. The root system consists of a stout taproot. Vegetative colonies are not formed.

Cultivation: The preference is full or partial sun, mesic conditions, and a fertile loam or clay loam soil. This plant has good drought tolerance, and appears to have fewer problems with powdery mildew than some other Desmodium spp., such as Desmodium canadense (Showy Tick Trefoil). If it is grown in full sun, Illinois Tick Trefoil remains quite erect because of its stout central stem. Its root system adds nitrogen to the soil.

Range & Habitat: Illinois Tick Trefoil occurs occasionally in most counties of central and northern Illinois, but it is uncommon or absent in many areas of southern Illinois (see Distribution Map). Habitats include mesic to slightly dry black soil prairies, sand prairies (less typical), oak savannas, scrubby barrens, and areas along railroads and roadsides, especially where prairie remnants occur. Usually, this species occurs as scattered plants, and doesn't form the dense colonies of some Desmodium spp., such as Showy Tick Trefoil.

Faunal Associations: The flowers attract long-tongued bees primarily, which collect pollen. This includes bumblebees and large Leaf-Cutting bees. The caterpillars of two skippers, Thorybes pylades (Northern Cloudywing) and Thorybes bathyllus (Southern Cloudywing), feed on the foliage of this and other Tick Trefoils. The seeds are eaten by some upland gamebirds and small rodents, including the Bobwhite, Wild Turkey, and White-Footed Mouse. The foliage is palatable to mammalian herbivores, and consumed by deer, rabbits, groundhogs, and livestock. These same animals help to distribute the seedpods, which cling to fur. People also help to distribute the seedpods, which readily cling to clothing.

Photographic Location: The photographs were taken at the Shortline Railroad Prairie in Champaign County, Illinois. The plants were growing in a mesic black soil prairie not far from a corn field.

Comments: Illinois Tick Trefoil is not very showy because only a few flowers are in bloom at the same time. Unlike other Desmodium spp. (Tick Trefoils), it has high fidelity to prairies, and is less often observed around woodlands. Illinois Tick Trefoil can be readily distinguished from Desmodium canadense (Showy Tick Trefoil), another species that often occurs in prairies, by considering the following features: 1) the former species usually has fewer flowers in bloom at the same time as the latter, 2) the compound leaves of the former have much longer petioles (over 1" long) than the latter, 3) the stipules at the base of the petioles of the former are larger, broader, and more persistant than the stipules of the latter, and 4) the undersides of the leaflets of the former have more prominent veins than the leaflets of the latter. In general, the form and number of segments in the loments are important in distinguishing the different species of Tick Trefoil, while their flowers are very similar in appearance. The loments of other Tick Trefoils usually have fewer segments than Illinois Tick Trefoil, and often the upper sides of their loments are more flat.