Sicklepod
Senna obtusifolia
Caesalpinia family (Caesalpiniaceae)

Description: This annual native plant is 1-2' tall, branching occasionally. The stems are light green and somewhat ridged. The lower stems often sprawl along the ground in open areas, otherwise this plant is erect. The compound leaves alternate along the stems. They are evenly pinnate, usually with 3 pairs of leaflets, and have long leaf stems (primary petioles). The obovate leaflets are individually 1" long and 1" across. They are devoid of hairs and pale green on their undersides. There is an extra-floral nectary close to the lowest pair of leaflets on the upper side of each compound leaf. This nectary resembles a small brown spike. The foliage has a slightly rank odor.

One or two flowers develop from the upper axils of the compound leaves. These flowers have pedicels about 1" long and tend to nod slightly downward. They are about 1" across, and consist of 5 rounded yellow petals and 10 stamens; the petals are often slightly unequal in size. The flowers are subtended by a light green calyx that is divided into 5 folded sepals; these sepals are shorter than the petals and hairless. In Illinois, the blooming period occurs from late summer through the fall, and lasts about 1-2 months. There is no floral scent. A long seedpod develops from each flower that is 4-6" long, but only 1/3" across. This seedpod curves downward and resembles a sickle in its overall shape. The root system consists of taproot. This plant spreads by reseeding itself.

Cultivation: The preference is full sun and moist to slightly dry conditions. This plant tolerates different kinds of soil, from fertile loam to gravelly soil; in fact, it often thrives in railroad ballast. Drought tolerance is good, and the foliage is rarely bothered by foliar disease. It is somewhat slow to develop for an annual plant, and tends to bloom late in the year in Illinois.

Range & Habitat: Sicklepod has been reported from only a few counties in southern Illinois and in the Chicago area; it has been observed recently along a railroad track in Champaign County, where it may be adventive from the south (see Distribution Map). This is a rare plant in Illinois, although it is more common in many south-central and southeastern states of the US; it also occurs in the American tropics. Habitat information is scanty, but it has been observed in moist meadows along rivers, open areas along railroads, and in waste areas. It could probably thrive in disturbed areas of prairies.

Faunal Associations: The extra-floral nectaries attract ants primarily; they may also attract some wasps, flies, or small bees. The pollen of the flowers attract long-tongued bees, such as bumblebees, Miner bees, and large Leaf-Cutting bees. The caterpillars of several species of Sulfur butterflies are known to feed on the foliage of Cassia spp., including Eurema lisa (Little Sulfur), Eurema nicippe (Sleepy Orange), and Phoebis sennae cubule (Cloudless Sulfur). Some upland gamebirds occasionally eat the seeds of Senna spp., especially the Bobwhite and Greater Prairie Chicken. Mammalian herbivores are unlikely to utilize this plant as a food source to any significant extent because of the unpleasant taste and toxic nature of the foliage, which has been found capable of poisoning livestock.

Photographic Location: Photographs were taken along a railroad in Savoy, Illinois.

Comments: This rare plant has attractive foliage, but blooms sparingly. In the photograph of the flowering plant on the lower right, one of the sickle-shaped seedpods is evident; it is still green. Sicklepod resembles Senna marilandica (Maryland Senna) and Senna hebecarpa (Wild Senna) somewhat in appearance, but it is a shorter annual plant that has 1) fewer leaflets per compound leaf, 2) leaflets with a more obovate shape, and 3) seedpods that are more curved and slender. In the past, Sicklepod was often assigned to the Bean family (Fabaceae), rather than the Caesalpinia family.

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