Botanical name: Senna marilandica, syn. Cassia marilandica
Family: Fabaceae (legumes)
Height: 3 to 6 feet (1 to 2 m)
Width: 2 to 3 feet (60 to 100 cm)
Exposure: sun to light shade
Soil: well drained
Flowering: late summer, early autumn
Hardiness zones: 4b to 9
If you want to give your landscaping a touch of the tropics, but dragging a palm tree outside in June then bringing it inside in September doesn’t appeal to you, you could always plant a Maryland senna. This perennial belongs to genus renowned for its tropical trees and shrubs and I’ve seen many exotic senna trees in my travels, notably the stunning emperor’s candle stick (Senna alata) with its upright stalks of brilliant yellow flowers. With Maryland senna, I feel I have much the same thing in my own backyard, but on a much smaller scale. With its pinnate leaves like miniature palm fronds topped by bright yellow flowers, it makes my garden feel a lot more exotic. It doesn’t even seem logical that such a tropical-looking plant could survive temperatures of -30 ̊F (-35 ̊C) and yet it does.
Maryland senna is a perennial that thinks it’s a shrub! And evolutionarily, that is indeed what it is. Its ancestors were originally tropical shrubs that, over the millennia, learned to tolerate increasingly colder climates. It’s strategy? When temperatures drop, it retreats underground where it’s warmer, its crown just under the surface of the soil, but then in early summer, it produces tall, upright stems. Not just your typical soft perennial stems prone to flopping, but woody ones, like those of a shrub, as if it thought its branches would be permanent. They won’t be, of course: they freeze to the ground in winter and the plant then starts all over again.
Despite its long-ago tropical origins, the Maryland senna is itself quite widely distributed in northern climates, found not just in Maryland, as its botanical name suggests, but through much of the Eastern half of the United States, almost to the Canadian border.
The plant is slow to awaken in the spring, but quickly makes up for lost time, producing woody stems that reach up to 6 feet (2 m) by late summer. They start directly from the stump, without branching, giving a “shrub” with a flared, symmetrical habit. The dull green leaves are pinnate and composed of four to nine pairs of oval leaflets reminiscent of the leaves of the black locust (Robinia pseudoacacia).
In mid-summer or early fall, depending on the local climate, masses of yellow flowers appear on the upper part of the branches. The senna is a legume, but doesn’t have typical pea-shaped flowers. Instead, the buds open wide into a five-petaled bloom with dark brown stamens. The flowering lasts about a month and attracts a lot of bees and butterflies.
After flowering, long sword-shaped seed pods appear whose seeds can be harvested for propagation. Otherwise, leave them on the plant in the fall, as they attract seed-eating birds in the winter. There is no noticeable fall coloring.
The plant forms a single dense clump at first, but it has lateral rhizomes which can produce suckers, forming a colony over time. However, this multiplication takes place over many years and it could scarcely be called invasive.
Maryland senna prefers full sun or only very light shade, becoming droopy and failing to bloom when it doesn’t get enough sun. In nature, it’s found in rich and rather humid soils. However, like many plants, it’s more adaptable in cultivation and grows well in just about any soil, moist or dry, acid or alkaline, fertile or rocky. As a legume, it lives in symbiosis with bacteria that fix nitrogen from the air, which means that the plant provides its own nitrogen. In other words, there is no need to fertilize it too diligently.
Usually, sennas are propagated by seeds which germinate after 2 months of moist cold stratification. Their germination is irregular, taking from 1 to 12 weeks, but thereafter, growth is rapid. You can also remove and replant elsewhere offsets if and when they appear. Mature clumps can also be divided, but a saw or ax will likely be needed.
Since sennas are large shrublike perennials, use them as if they were shrubs, that is, as a screen, hedge, backdrop for a flower bed, etc. They can also be naturalized in a meadow or along a stream.
Try this plant with silvery leafy plants, such as Elaeagnus × ‘Quicksilver’ or various wormwoods (Artemisia spp.). They also make a wonderful background for Shasta daisies (Leucanthemum × superbum) and daylilies (Hemerocallis spp.).
Infrequent. In their natural range, the larvae of various species of sulfur butterflies (very attractive, with bright yellow wings) feed on them, but they’re mostly present only in the plant’s natural range and even then, usually don’t do much noticeable damage.
The ants that frequent sennas are looking for the sweet nectar produced by special nectar glands located on the stems. The ants in no way harm the plant and it’s believed that the plant produces the nectar especially to attract them, as ants are ferocious predators that will kill most other insects that try to invade their territory.
The Other Hardy Senna
There are over 260 species of Senna, but the majority are of tropical origin. At least one other species is quite hardy, perhaps even hardier than S. marilandica: so-called wild senna or American senna (S. hebecarpa, syn. Cassia hebecarpa).
This species shares much the same distribution as Maryland senna, but extends further north: as far as southern Ontario. The few nurserymen who offer it give it a hardiness rating of zones 4a to 9, while Maryland senna gets a rating zone of 4b to 9. That’s not much of a difference, but if you live in zone 4a, it’s important!
Wild senna is of about the same dimensions as Maryland senna: about 3 to 6 feet (1 to 2 m) tall and 2 to 3 feet (60 to 100 cm) wide.
So … what’s the difference between S. marilandica and S. hebecarpa? During the flowering season, you can’t, in fact, tell the two apart! But the seed pods give them away. Those of S. hebecarpa are hairier and open in the fall, so the seeds drop before winter. Those of S. marilandica are less downy and remain closed through much of the winter, only dropping their seeds at the end of the season. There are also some minor differences in the flowers, but from a gardener’s point of view, there is no discernable difference in blooming plants. Buy one or the other: you don’t have to have both.
You may know of “senna” as a medicinal plant, as dried senna leaves are widely available. They’re derived from the leaves of a different species, one originally from Africa: Senna alexandrina, syn. Cassia angustifolia. It was already well known to Europeans as a medicinal plant when they arrived in America. They discovered that indigenous peoples used the leaves of native sennas in much the same way, especially for its laxative effects.
Where to Find Plants?
Neither Maryland senna nor wild senna are found in just any nursery. Try a local nursery that specializes in perennials. In the United States, a native plant nursery is likely to carry one or both.
Or do as I did and order seed by mail. The nice thing about buying seeds is most companies ship worldwide, so you can order them from any country. Here are a few sources: Prairie Moon Nursery, Sheffield’s Seed Company and Prairie Nursery.
Hardy sennas: grow them for their beauty and ease of growth … and so you have something your neighbor doesn’t!