(Wild Senna) American Senna is one of the plants formerly in the genus Cassia. It is a large erect herbaceous perennial that can can grow 3 to 6 feet high on green stems that are largely unbranched, slightly angled, and may be smooth or may have fine hair, especially on the upper part.
The leaves are even-pinnate with 6 to 10 pairs of oblong to elliptic leaflets that each have a short stalk, a midrib vein, fine hair on the otherwise smooth margins and very fine surface hair giving a gray-green appearance. Each leaflet is mucronate - that is, there is a projection of the mid-vein beyond the tip of the of leaflet forming a thin point. The leaflet base is asymmetrical. Each leaf is long-stalked with a pair of very thin linear stipules at the base where there is also a fairly large ovoid gland on the upper side which produces nectar that attracts certain insects.
The inflorescence consists of several many-flowered racemes rising from the leaf axils and usually a terminal panicle. Flower buds are nodding.
The flowers are unlike most pea family flowers. Each flower is stalked, The corolla is about 3/4 inch wide and has 5 unequal yellow petals which group themselves with three above and two widely spaced below allowing the stamens and the pistil to be completely exposed. There are ten stamens placed into three groups with unequal length yellow filaments and dark brown anthers. The lower and middle group of stamens are sterile, only the upper three are fertile. The pistil is green and very hairy and leads to an ovary with 10 to 15 ovules dense with fine hair. The calyx has 5 unequal lobes which are a paler yellow. The tips of these are more pointed and reflex completely when the flower opens.
Seeds: Fertile flowers produced a linear flattened seed pod that has joints that appear almost square, each jointed section containing a flat seed almost as wide as long, that is depressed in the center. The pod is covered with fine hair and does not open immediately upon ripening but will split explosively after it has dried out. Seeds number about 1,400 to the ounce. For germination, seeds need scarification and at least 10 days of cold stratification.
Habitat: American Senna grows best in rich soils with good moisture, full to partial sun. The root system is rhizomatous and the thick spreading rhizomes form clonal colonies. It is a late flowering species and seed is often not set until October.
Names: An older scientific name for the species is Cassia hebecarpa Fernald. The plant was formerly in the Senna family (Caesalpiniaceae). The genus Senna, is derived from the Arabic 'sana', a name for the type plant of this genus. The species name, hebecarpa is from two Greek words, hebe, for 'youth, or dawn of puberty' and carpos for 'fruit', the combined meaning of which is a bit obscure but refers to the fuzzy seed pods.
The authorship of the plant classification is a bit complex. The first person listed who did work in the genus and the former genus Cassia was ‘Fernald’ which refers to Merritt Lyndon Fernald (1873-1950) American botanist, Harvard Professor, scholar of taxonomy, author of over 850 papers, editor of the 7th & 8th editions of Gray’s Manual of Botany. His work has been updated by Irwin & Barneby. 'Barneby' is Rupert Charles Barneby (1911-2000) British born botanist whose specialty was the Fabaceae family, worked at the New York Botanical Garden and described over 1000 new species and published The American Cassilinae with "Irwin' who is Howard Samuel Irwin, (1928-) American botanist at the New York Botanic Garden, eventually becoming its president.
Comparisons: The most confusing species will be S. marilandica but there the seed pod joints are twice as long as wide, the flower has 20 to 25 ovules and and the leaf stalk gland is round or dome shaped but this gland shape is not always distinctive but the number of ovules and pod shape are distinctive.
Above: Wild Senna flowers have a different shape from the normal pea family flower: Five unequal petals group into 3 above and two below, opening outward, exposing 10 stamens split into 3 groups of unequal length with only the upper group of 3 fertile. The pistil is green and hairy.
Below: 1st photo - The leaves are evenly pinnate with 6 to 10 pairs of leaflets. Note the small projection at the tip of each leaflet. The leaf margin is entire but with fine hair (2nd photo) the the underside has a paler color.
Below: 1st photo - Small hairy seed pods form from the fertilized flowers. 2nd photo - The calyx has five yellow-green pointed lobes that reflex completely when the flower opens.
Below: 1st photo - The maturing segmented pod has almost square segments containing one seed each. When dry, these pods split open explosively to release the seeds. 2nd photo - At the base of each leafstalk are a pair of very narrow green stipules (better seen in the 3rd photo below) and an upright ovoid gland that produces an insect attracting nectar.
Below: 1st photo - The plant is quite tall and wide as the leaves spread horizontally. 2nd photo - Stems are mostly smooth, stout and greenish with slight angles. 3rd photo - The inflorescence is a cluster rising from the leaf axils, buds are nodding. Note the linear leaf stipules.
American Senna is not native to Minnesota but is found in the U.S. from the Wisconsin and Illinois side of the Mississippi River eastward to the coast, excepting the states bordering on the Gulf Coast. In Canada it has been reported in Ontario. Maryland Senna was once in the Wildflower Garden, brought in by Eloise Butler in 1911 but there is no record of American Senna having been planted.
Several Senna species are purgatives or mild laxatives depending on the dose. S. hebecarpa leaves and seed pods contain a very powerful laxative compound called an anthroquinone. The active principal in sennas is cathartic acid which seems to be eliminated by digestion. Cattle sense enough of these chemicals to avoid the plant. If plants get out of control it is a difficult task to eradicate them short of using herbicides. Ada George (Ref. #6b) once wrote: "If the plants are few they may be grubbed out, but if plentiful this would be a task for Hercules. Cutting close to the ground at the time of bloom, repeating the operation as the roots send up more stalks, will finally exhaust their vitality; but the treatment must be so persistent as to allow no opportunity for storing fresh nutriment."
American Senna is a larval host or nectar source for the Cloudless Sulphur mid-size butterfly (Phoebis sennae)
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"