Plant Profile: Cassias
By Craig Huegel
Reprinted from The Understory
(Editor's note: Since this article was written, scientific names have changed per Guide to the Vascular Plants of Florida Richard Wunderlin, 1998. New names appear in parentheses.)
The growth and popularity of butterfly gardening has seen the introduction of many native plants into "the trade." Plants that were ignored by the nursery business suddenly have become commercially available because of their utility to the butterfly gardener. One such group is the cassias, or sennas.
Cassias are common throughout the world and examples can be found across all lines of growth forms, from weedy annual forbs to robust trees. They are members of the bean/pea family (legumes). As such, they produce the typical bean-type flower with a keeled lip and these flowers produce pods that contain hard seeds. In most species, the flowers are bright yellow and the leaves are compound—consisting of many small leaflets on each leaf stem, and, like most legumes, they tend to be rather weedy. Growth is rapid and seed production is great.
Butterfly gardeners discovered cassias because they are the larval food plant for several of the most attractive sulfur butterflies in Florida. The Cloudless, Orange-barred, and Sleepy Orange Sulfurs all use cassias and, therefore, this group of plants is an indispensable part of a complete Florida butterfly garden. From my personal observations, this group of butterflies will use a wide variety of cassias available in the nursery trade including the non-native species. The one major attribute that seems to hold as important is that egg laying occurs mostly during the flowering period. Sulfurs seem to be very particular about this and few will lay eggs on new growth if flowers are not also present.
As I've alluded above, there are many non-native cassias sold in Florida, including some ornamental trees. I feel that we do best to plant our native species wherever possible. Luckily, we have several good ones to choose from. Below is a brief description of some of those useful for our climate here in central Florida.
Bahama cassia (Cassia bahamensis, now Senna mexicana var. chapmanii). In my opinion, this is the premier cassia for the Florida butterfly garden. Bahama cassia is a robust shrubby plant that seems to be in bloom for most of the year. Because of its steady flower production, it also seems to have caterpillars on it for much of the year too. Native to south Florida, the Caribbean, and tropical South America, it occurs naturally at the edges of hammocks, often near the mangrove forest edge along the coast. Bahama cassia is very salt tolerant and will tolerate saltwater inundation for brief periods. It is not tolerant of freezing temperatures, however, but because of its great seed production, it normally reappears following a freeze as a carpet of seedlings. This cassia may ultimately grow to a height of seven feet, but more likely will not exceed four feet in Pinellas County. It is a brittle shrub with a rounded crown. Plant this cassia in partial sun in average to moist soil. Water it well as it becomes established.
Partridge pea (Cassia fasciculata, Chamaecrista fasciculata). Partridge pea is a very common annual or short-lived perennial "weed" found throughout Florida in a wide variety of sites. It is most common, however, in disturbed areas or in open fields. Partridge pea blooms in mass in late summer or early fall. From my observations, sulfur butterflies ignore it until then. This cassia can be an interesting addition to a mixed wildflower garden where it can "move around." Although it is an annual, it produces enough seeds to allow it to persist through the years. This plant generally is less than 12 inches tall at maturity. Its feathery foliage is attractive, but the real beauty lies in its great mass of blooms.
Privet cassia (Cassia ligustrina, now Senna ligustrina).This is another shrubby cassia native to south Florida. Privet cassia is a tall, narrow-crowned plant, unlike Bahama cassia which is much broader in aspect. I can find little information in the literature regarding the natural distribution of this plant. From my personal experience, I find it to be vary similar to Bahama cassia in regards to soil and sun. I have not tested it for salt tolerance. It will freeze in temperatures below freezing, but it has always come back in the form of seedlings. Privet cassia does not bloom as steadily as Bahama cassia. It has a tendency to produce a flush of blooms sporadically throughout the year.
Sensitive plant (Cassia nictitans, now Chamaecrista nictitans var. nictitans). In many respects, this plant is very similar to partridge pea. Major differences are in its foliage which is "soft" to the touch and will fold up when touched or brushed against. Although all cassias tend to fold their foliage during the evening hours, this plant has sensitive foliage that folds when something/someone contacts it. Use this plant like you would a partridge pea. The flowers tend to be a bit smaller and less showy.
Invasive Cassia: The Florida Exotic Pest Plant Council lists one non-native Senna as a Category 1 exotic invasive: Senna pendula var. glabrata, or climbing cassia. It may also be known as Christmas cassia, Christmas senna, or Senna bicapsularis. Native to South America, this plant has been grown in Florida as an ornamental since the 1940s. It displaces native vegetation in tropical hammocks, coastal strands, and canal banks. See the FLEPPC website for additional description and photos.