Mystery Plant: Butterfly-shaped flower an odd member of the bean family

You have probably seen this one’s bright butter-yellow flowers, the plants commonly very abundant in old agricultural fields and roadsides. The plants are sometimes present in the thousands and they can produce a brilliant floral display in the fall, sometimes even through a light frost. This species occurs nearly throughout the southern states and well into the Mississippi River valley. It is most commonly seen at lower elevations and is especially common on the coastal plain and Piedmont, not too often seen in the mountains. It can make quite an autumnal display at the edge of an old field along with sunflowers, morning glories, asters and ragweed.

This plant is a member of the bean family, the family name being “Fabaceae” (or “Leguminosae,” if you prefer). The flowers are characteristic with five unequal petals exhibiting a bilateral symmetry. This architecture is repeated again and again in different members of the bean family: the largest petal, at the back, is called the “banner” or “standard.” Two narrower petals on each side are “wing” petals, and two even narrower “keel” petals at the bottom embrace and protect the stamens and pistil. If you find this plant in bloom, you can easily take a flower apart with your fingers and see this five-petaled architecture. Some rather romantic botanists likened the flowers in shape to butterflies (and the flowers can thus be described as “papilionaceus”).

After blooming, the ovary of each flower expands into an inflated pod (or legume), at first green but eventually becoming nearly black. When the pods are ripe and dried out, the shiny black seeds become detached within and readily rattle around inside like miniature maracas. (In fact, the genus name of our plant is derived from the rattlesnake genus, Crotalus, in allusion to the rattling seeds.)

The foliage of the plant is somewhat unusual for a member of the bean family. Most herbaceous members of the family have their leaf blades divided pinnately (along either side of the midrib) into a number of discrete leaflets: consider kudzu, clover and lespedezas, which have three leaflets, but also wisteria, which has seven to nine or so. Our Mystery Plant is an oddball in that its leaves are simple, somewhat rounded at the tip, and tapering down to the base, not at all divided into leaflets.

In addition to being beautiful, these plants (which are native to southern Asia) are useful in controlling erosion and building up the soil by the complicated biochemical process of nitrogen fixation. The species was introduced into the Southeast for these reasons and, being rather weedy, has spread. There is a dark side, though: All parts of the plant turn out to be somewhat poisonous to poultry and livestock, so it is now something of an agricultural liability. It’s great, though, for teaching in the classroom, and I’ll soon need to go out and harvest a bucketful for my Botany Boot Camp class.

(Answer: “Rattlebox,” Crotalaria spectabilis)