Senna family (Caesalpiniaceae)
Description: This herbaceous perennial plant is largely unbranched and up to 6' tall. The stout central stem is light green, and slightly hairy in the upper stem and at the base of the upper compound leaves. The compound leaves are evenly pinnate with about 5-10 pairs of leaflets. The gray-green to medium green leaflets are individually up to 2½" long and ¾" across. Each leaflet is oblong or elliptic-oblong and smooth along the margins; it has a small pointed tip and a short petiolule (basal stalklet) that is 1/8" in length or less. On the upper side of a petiole near its base is a small club-shaped gland; it is ovoid or dome-shaped above a short stalk. This gland secretes nectar to attract certain kinds of insects (see below for more information). At the base of each petiole, there is a pair of small linear-lanceolate stipules; they are tardily deciduous.
The central stem terminates in either a raceme or panicle of flowers about ½-1½' long. In addition, smaller racemes of flowers often develop from the axils of upper compound leaves. Each flower is about ¾" across; it has 5 pale yellow sepals, 5 yellow petals, 10 stamens with dark brown anthers, and a pistil with spreading white hairs. The petals have a tendency to turn white as they age, while the hairy pistil eventually develops into a seedpod. In each flower, the stamens are divided into 3 groups: the lower 3 stamens have long anthers and long filaments, the middle 4 stamens have long anthers and short filaments, while the upper 3 anthers have short anthers and short filaments. Of these, the lower and middle stamens are fertile, while the upper stamens are sterile. The flowers have no nectaries. The blooming period is mid- to late summer, which lasts about a month. There is no noticeable floral scent. The flattened seedpods are about 3-4" long, ½" across, and dark brown when they are fully mature; each seedpod has 10-18 segments, and each segment contains a single seed that is less than than ¼" long. The seeds are oval-rhombic in shape and flattened; they are nearly as wide as they are long. The root system consists of fibrous roots and rhizomes. This plant often forms vegetative colonies from the rhizomes.
Cultivation: The preference is partial to full sun, and moist to mesic conditions. A rich loamy soil is preferred, although sandy and rocky soil are also tolerated. This plant can become quite tall when the soil is fertile and moist; it may flop over while the flowers and seedpods are developing.
Range & Habitat: The native Wild Senna occurs in scattered counties throughout Illinois; it is perhaps a little more common in the east than the west (see Distribution Map). This species is occasional in some areas, and uncommon or absent in others. Populations in the wild are probably declining as a result of modern development. Habitats include moist meadows near rivers, savannas, fens, pastures, and roadsides. Some disturbance is beneficial when it reduces competition from shrubs and trees. Occasionally, Wild Senna is found in flower gardens because of the showy flowers.
Faunal Associations: The flowers atttract bumblebees primarily, which seek pollen. Halictid bees also visit the flowers for pollen, but are less likely to achieve cross-pollination. The extra-floral nectaries, on the other hand, attract primarily ants and a few other insects, including ladybird beetles and flies. It is possible that some of these insects protect the plant from other insects that would attack the foliage. The caterpillars of some Sulfur butterflies rely on the foliage of Senna spp. (Sennas) as a source of food. In Illinois, this includes Eurema nicippe (Sleepy Orange), Phoebis philea (Orange-Barred Sulfur), and Phoebis sennae eubule (Cloudless Sulfur). Other insects that feed on Sennas include caterpillars of the moths Ascalapha odorata (Black Witch) and Pleuroprucha insularia (Common Tan Wave); caterpillars of the last species feed on the flowers. Mammalian herbivores usually avoid consumption of the foliage, which has purgative properties. The seeds may be eaten by some upland gamebirds, particularly the Bobwhite.
Photographic Location: The photographs were taken of plants growing in an herbal garden at Meadowbrook Park, Urbana, Illinois. The photograph of the stem and petiole reveals one of the club-shaped nectary glands.
Comments: This is a striking plant while in bloom and it has attractive foliage. It is difficult to distinguish Wild Senna from Senna marilandica (Maryland Senna), which has a very similar appearance. Generally, Wild Senna has a more northern distribution than Maryland Senna, but in Illinois their ranges overlap. On Wild Senna, the pistils of the flowers have long white hairs, while in Maryland Senna these hairs are shorter and more appressed. Wild Senna has oval-rhombic seeds that are almost as wide as they are long, while Maryland Senna has more narrow seeds that are almost twice as long as they are across. Another scientific name for Wild Senna is Cassia hebecarpa.