Bean family (Fabaceae)
Description: This wildflower is a summer annual about 6-12" tall that branches occasionally; it is ascending to erect. The stems are light green, terete, and hairy. The alternate leaves are 1–2½" long and about one-third as much across; they are elliptic to oblong, flat, and smooth along their margins. Both the upper and lower sides are hairy. The upper surface is medium green, while the lower surface is gray-green. Each leaf has a single central vein that is prominent. The petioles are hairy and short (about 1/8" long). Occasionally, racemes of 1-4 flowers are produced oppositely from some of the leaves. The peduncle (central stalk) of each raceme is 1½–4" long, while the short pedicels of the flowers are about 1/8" long (except the pedicel of the terminal flower, which is often longer). Both peduncles and pedicels are hairy. At the base of each raceme, there is a pair of large persistent stipules about ½" long. These stipules are joined together, becoming gradually wider toward their pointed tips. Individual flowers are about 1/3" long; they have a typical pea-like flower structure: the yellow petals form an upright banner and a projecting keel that is enclosed by a pair of lateral wings. Each calyx is light green and covered with long hairs; it has five long teeth. The blooming period occurs from mid-summer to early fall for about 2-3 months. Usually, only a few (if any) flowers are in bloom at the same time. The flowers are replaced by inflated seedpods that are short-oblongoid in shape. The seedpods are initially light green, but they later turn dark brown or black at maturity. Full-sized seedpods are 1–1½" long and about 1/3 as much across; each seedpod contains several seeds, which can rattle if it is shook. The seeds are distributed in part by the wind, which can blow the inflated seedpods across open ground. The root system consists of a taproot. This wildflower reproduces by reseeding itself.
Cultivation: The preference is full or partial sun, dry-mesic to dry conditions, and barren soil containing sand, gravel, or clay. This wildflower is intolerant of competition from taller ground vegetation.
Range & Habitat: The native Rattlebox is occasional in the southern half of Illinois, while in the northern half of the state it is uncommon or absent (see Distribution Map). Habitats include sand prairies, gravel prairies, clay prairies, sandy and rocky savannas, upland savannas, rocky glades, openings and small meadows in upland woodlands, fallow fields, and areas along railroads. Rattlebox prefers dry open areas with a history of disturbance.
Faunal Associations: The flowers are pollinated primarily by long-tongued bees, including bumblebees (Bombus spp.), Carder bees (Anthidium spp.), Cuckoo bees (Coelioxys spp.), and large Leaf-Cutting bees (Megachile spp.). Sometimes the flowers are visited by small butterflies or skippers, but they are less effective at cross-pollination. The caterpillars of Utetheisa bella (Bella Moth) feed on the foliage of Rattlebox. In addition, there have been reports of the butterfly caterpillars of Callophrys irus (Frosted Elfin) and the skipper caterpillars of Erynnis baptisiae (Wild Indigo Duskywing) feeding on the foliage. The foliage of Rattlebox is toxic to herbivorous mammals, particularly horses, and it is generally avoided by them as a food source. However, sometimes White-Tailed Deer chomp off the tops of individual plants.
Photographic Location: An upland woodland opening at Busey Woods in Urbana, Illinois.
Comments: This is the only Crotalaria sp. (Rattlebox) in Illinois. There are other species in this genus that are located in the southern and western states; they are often perennials with more showy flowers. Rattlebox is easy to identify when its inflated seedpods are present; they are large in size in relation to the rest of the plant and tend to stand out. There are other legumes that produce inflated seedpods (e.g., Baptisia spp.), but they usually have compound leaves.