Home & Garden

Why plant senna? It’s low maintenance, drought tolerant and attracts butterflies

Striking flowers of candlestick senna.
Striking flowers of candlestick senna. Fairchild Tropical Botanic Garden

Desert cassia (Senna polyphylla) is a popular little landscape tree, small, hardy, and flowering often in direct sun, but it seems less trendy lately. I’ve seen a few topple from high wind, which may be a reason for their fall from popularity, but maybe not.

Any plant with “desert” in the name makes me fear it’ll succumb to rot from too much water and high humidity, but this is a misnomer, as the Senna polyphylla has no problem with heavy rain, provided the soil drains well. As its common name suggests, it was classified within the Cassia genus for a time, but now it is back to Senna, despite its common name.

It stays nicely compact to about 10 feet tall and in full sun provides nice pops of yellow flowers nearly year round. However, I have seen desert cassia toppled easily in wind, many of them remaining staked to supports for months. My own tree developed lots of bare twigs that seemed dead, snapping off easily. There is a fungal affliction called rust disease that has infected certain Senna species and caused leaf drop, though I find no mention of it attacking Senna polyphylla in particular.

Image 1 - 2019-09-03T151159.854.jpg
Desert cassia produces small flowers and foliage, but lots of both. KENNETH SETZER Fairchild Tropical Botanic Garden

In any case, late winter and early spring are good times to prune desert cassia and other sennas, watching particularly for dead, diseased, or crossed, rubbing branches.

Sennas tend to look alike: bushy, small leaflets, and smallish yellow flowers. One exception, a personal favorite, is Senna alata. Known as the candelabra or candlestick senna, its inflorescences are as thick as a candle and six or more inches long. They appear in warmer months, and even the flower bracts are a matching intense, bright golden yellow. The fern-like foliage is similar to other members of the pea family, Fabaceae, but on a large scale, absolutely gigantic compared to desert cassia’s tiny little leaflets.

Sennas are larval host plants for sulphur butterflies. Those are the butterflies that rarely alight long enough for a sharp photo. However their larvae, aka caterpillars, are also stunningly patterned and easier to observe. Senna flowers attract many other butterflies and skippers as a nectar source; ants also seem to love them, at least on the candlestick senna.

Image 3 (84).jpg
Sulphur caterpillar on Senna species. Fairchild Tropical Botanic Garden

You may choose to go native with the seldom-cultivated Bahama senna (S. mexicana var. chapmanii). Confusing names, yes, but this one’s native to Cuba, the Bahamas, and South Florida’s coastal scrub/pinelands. On the small side, this bushy plant grows only about four feet tall and is a great addition to small, sunny locations in a native garden and as a possible groundcover in rocky, sunny areas.

Wild privet senna, S. ligustrina, is another native and similar to Bahama senna but taller, and more tree-like. The two native sennas are sometimes available at native plant nurseries. Bahama senna will be offered at the Members’ Day Plant Sale at Fairchild on Oct. 5.

There are many other Senna species, but the only troublemaker for us in the genus that I’m aware of is Christmas senna, also called climbing cassia (S. pendula var. glabra), a scrambling shrub causing trouble in Australia and a Florida Exotic Pest Plant Council Category 1 Invasive for Central and South Florida.

Otherwise, low maintenance, drought tolerance, flowers for us and for wildlife all make sennas a go.