Southern wild senna is an erect perennial, normally with a single unbranched stem. Flowers are yellow and occur in short racemes that arise on stems from the leaf axils. The flowers appear to be regular (not pea flower-shaped) and consist of 5 spreading petals of which 2 are larger than the other 3. Blooms July–August. Leaves are compound with 8–10 pairs of oblong to oblong-elliptic leaflets, to 2 inches long, abruptly tapered to a sharply pointed tip. Fruits are legumes (bean pods), flattened, 2½ to 3½ inches long, dark brown or black at maturity.
Similar species: Missouri’s other two Senna species are weedy, less common, and less widespread. Both produce a disagreeable odor when crushed. Sicklepod (S. obtusifolia) is found mostly in our extreme southeast and southwest counties and sporadically north to St. Louis. It has usually 3 pairs of broadly obovate leaflets. Coffee senna (S. occidentalis) occurs mostly in our southeastern lowlands and has 3–5 pairs of ovate to broadly lanceolate leaflets.
Height: 3–8 feet.
Statewide. Scattered south of the Missouri River; less common farther north.
Habitat and Conservation
Grows on banks of streams and rivers, soughs, bottomland and upland prairies, bottomland forests, rich upland forests, bases, ledges, and tops of bluffs, glades, and savannas; also pastures, old fields, fallow fields, roadsides, and open, disturbed areas. It is sometimes grown as a landscape ornamental.
Southern wild senna is sometimes cultivated in gardens as a low-maintenance ornamental for its attractive foliage and flowers and its interesting fruits. Several members of this large genus have been used medicinally or as coffee adulterants; others are pasture weeds implicated in livestock poisoning.
There are about 300 species in genus Senna. These include the sources for cassia gum, a thickening agent in foods, and for neutral henna, a yellow hair dye. Senna leaves have been used as a cathartic or laxative since ancient times. Our word "senna" is virtually unchanged from the Arabic word for these plants. Medieval Europeans looked to Arab doctors for medical knowledge, including the idea of using sweet liquids for taking medicines (which is how we got the words "syrup" and "julep").
Bumblebees pollinate wild sennas.
A wide variety of moth and butterfly larvae eat the leaves, but grazing mammals tend to avoid eating this toxic, cathartic plant.
A small gland at the base of the leaf stalks provides nectar to ants, which apparently protect the leaves from herbivorous insects.