Gardeners who include senna in their landscapes aren't just doing it to provide a lively gathering spot for a diverse range of beneficial insects. Let's face it; even the most environmentally-concerned gardener will screech "NIMBY" if an ecologically-important specimen detracts from the garden's beauty. These gorgeous, graceful perennial shrubs soften up garden backgrounds with their delicate, deep-green foliage and tropical, almost alien-looking yellow flowers, adding feathery volume to planting schemes in which other mid-sized shrubs might be cumbersome and heavy-handed.
Native range, history, and medicinal uses
There are as many as 350 species in the Senna genus, with only about 50 of these being cultivated for ornamental or medicinal use. Senna thrives primarily in tropical regions, with a small percentage being native to temperate climates. Senna plants are legumes and members of the pea (Fabaceae) family. They collect nitrogen from the air, and "fix" it into the soil.
S. alexandrina is regarded as the "typical" senna species. Native to northern Egypt but cultivated in Africa and western Asia, this is the species most used for its medicinal properties. Teas made from its seed pods contain Senna glycoside, a compound named for the plant that has a laxative effect. Apparently, constipation's a pretty big issue over there, since the senna trade is a relatively sustainable source of income for some nomadic tribes in the region between the Nile and the Red Sea.
American senna (Senna hebecarpa), also known as "wild senna" or "wild cassia," is native to eastern North America. It's found as far north as Ontario and Maine, and as far south as Georgia and the southern Appalachians. Native Americans used crushed S. hebecarpa in teas and poultices to treat fevers, aches, and skin conditions as well as constipation.
We want to warn our readers that concentrations of this plant are poisonous if used incorrectly. Always consult with a physician before using medicinal herbs.
"Hebe" is the Greek goddess of youth, and "carpa" is a reference to the wrist or hand. Senna is an adaptation of an Arabic word that means "shining" or "brightness." You'll see the genus name Senna interchanged with Cassia, though the former became its own group to differentiate the smaller shrub-like plants from the larger, tree-like specimens belonging to the latter. A very close (and nearly identical relative) also referred to as "wild senna" is classified as Senna marilandica.
Senna in the garden
Wild senna is the species most often grown in North American landscapes, so we're going to leave S. alexandrina in the office bathroom with a stack of old Sunset magazines and use S. hebecarpa as the type standard for this post. Like other sennas, wild senna (also known as "American senna") is adaptable and very easy to grow, so our recommendations can serve as a rough guide to other species.
Wild senna is a shrubby plant with a somewhat woody base. Its long, minimally-branching, hairy and light green stalks grow from a compact base. Its dark green leaves are pinnately-compound, meaning they have oblong segments growing in 5 to 10 pairs from a single axil. Each leaflet is up to 3" long and 3/4" wide. Seed-shaped glands at the juncture of leaf stems and stalks secrete a sticky substance that is the prime attractant for visiting insects. The plants can grow up to an impressive 6' tall and 4' wide. Wild senna's leafy branches tend to grow horizontally, providing a gently layered look to the plant's silhouette.
Wild senna's 3/4" yellow, clustering flowers, which grow upward from terminal racemes, vaguely resemble those of its sweet pea cousins. Five light-colored sepals point backward from five yellow petals, which loosely enclose 10 stamens and a fuzzy pistil; the latter develops into long, flat seed pods that turn brown in the fall, and from a distance, the stamens and anthers give the flower centers a rusty brown appearance.
S. hebecarpa is a late-summer bloomer, flowering from July through August. As its blooms age, the yellow petals often fade to a creamy white. In spite of its prolific but somewhat short bloom period, the plant is valued just as much for its foliage as it is for its flowers.
Important food for beneficial bugs
Senna hebecarpa flowers lack noticeable fragrance and don't secrete nectar. Like most sennas, this species is an important nursery for a particular type of butterfly caterpillar within its biome; in this case, it's the lemon-colored larvae of the cloudless sulfur butterfly (Phoebis sennae eubule). You'll find many other butterfly and moth species hanging around your wild sennas, though deer, rabbits, and other mammals tend to leave it alone. The sap also attracts aphids and other sap-loving bugs, including bumblebees. Aphids and the sap itself lure in ladybugs and certain wasp and ant species that enjoy snacking on fat, juicy insects. These beneficial predators help protect other plants in your garden.
As mentioned above, wild senna's attractiveness to predatory insects allows itself to fend off most disease-carrying critters, but caterpillars will definitely munch on the leaves. Since senna has plenty of foliage—and butterflies are in decline—it's a worthwhile sacrifice that typically doesn't detract from the shrub's overall beauty.
Wild senna isn't particularly prone to diseases, though as with most plants, overwatering and poor air circulation can cause root and crown rot.
Ideal soil, moisture, and sunshine
Well-drained rich, loamy soil with some sand mixed in is the best substrate for Senna hebecarpa, which has no tolerance for clay soil. It prefers a pH between 6.5 and 7.2.
While it tolerates the occasional short drought, we highly recommend keeping the soil around your wild senna consistently moist for the most prolific blooms. Partial shade is acceptable in the late afternoon if you're in a region with above average summer temperatures, but it grows best in full sun.
Wild senna is a perennial in USDA Hardiness Zones 4 through 9.
Maintenance and harvesting
This is an easy-care shrub, though you might want to prune it in early spring and after flowering to keep it tidy. Pick up any fallen seed pods to prevent re-seeding; wild senna propagates by seed and root rhizomes and can get out of hand. You'll want to cut it back to six inches above soil level when it dies back in late fall for a denser, bushier plant the following spring.
Landscaping with wild senna
Senna hebecarpa is a nice consolation prize for gardeners who don't have the space for the much-larger but similar-looking Siberian pea-shrub (Caragana arborescens). Mature senna makes a good privacy screen or windbreak in the growing season.
Permaculture buffs who want to develop "food forests" like to inter-plant nitrogen-fixing legumes with edible berries, nuts, fruits, and veggies, and the sennas fit in well among their gardening schemes. While they don't like to be crowded too snugly, they get along with almost any species that share similar environmental requirements.
Growing senna from seed
All shrub-type senna plants are easy to grow from seed, whether you start them indoors (our preference) or sow them outside. They're temperate and semi-tropical plants, so they require warm temperatures to germinate. The earlier you get them established, the better.
Senna seeds are tough. We recommend scarification by scoring or lightly sanding the hulls, and/or soaking them in warm water for 24 hours. Some gardeners like to subject the seeds to a hot (110 °F) water bath before letting the water cool to warm temperature for a 12-hour soak.
Never soak any seeds longer than 24 hours; this will "drown" them.
If you start them indoors, use biodegradable peat pots to accommodate their taproots and reduce the risk of transplant shock. Mist the top of the soil while watering from the bottom. Indoor and outdoor seed environments should remain consistently moist until the seedlings are well on their way.
- Soil preparation: Loosen compacted soil and help maintain consistent moisture by mixing in a good amount of aged compost.
- When to plant outdoors: As soon as soil temperatures are consistently 70°F or above.
- When to plant indoors: 6 to 8 weeks prior to last spring frost.
- Seed depth: Approximately 1/2" deep, or twice the width of the seed.
- Seed spacing: Sow, plant, or thin 18" to 24" apart.
- Days to germination: 14 to 28 days at 70°F; treated seeds germinate on the shorter end of this timetable
When growing senna from seed, indoor starts may benefit from some humidity, but be sure to remove any plastic covering for a couple of hours each day to prevent mold. We like to use a sterile seedling mix for slow-to-germinate species to help reduce the risk of fungus and rot.
Before transplanting your senna seedlings, which should be 3" to 5" tall, score and soak your biodegradable pots to help them break down.
Gardening with the Seed Needs family
We're excited to offer this North American native plant, especially since so many of our customers want to grow species that provide habitat and food to at-risk and endangered beneficial insects. Wild senna definitely meets that criteria while helping to protect other garden plants. Please contact us if you have questions about Senna hebecarpa or any other plant in our catalog, and keep an eye out for our new additions; we're always adding new ornamental, herb, and vegetable species, as long as we can source them from responsible suppliers of healthy, natural genetics. And don't forget to bookmark our gardening blog, where we share valuable gardening advice for growing our favorite plants!
On a final note: We're fans of Han Solo, but we're still wrestling with the reality that he shot Greedo in cold blood, and George Lucas tried to cover it up. Just sayin'.