Provided
You have probably seen this plant’s bright butter-yellow flowers; it is commonly abundant in old agricultural fields and along roadsides. The plants are sometimes present in the thousands, producing a brilliant floral display in the fall, sometimes even surviving 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, particularly on the coastal plain and piedmont, and is not often seen in the mountains. It can create quite an autumnal display at the edge of an old field, alongside sunflowers, morning glories, asters, and ragweed.
This plant belongs to the bean family, Fabaceae (or Leguminosae, if you prefer). The flowers are characteristic, with five unequal petals exhibiting bilateral symmetry. This floral structure is repeated in many 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-petal architecture. Some rather romantic botanists have likened the flowers’ shape to butterflies, describing them as papilionaceae. After blooming, the ovary of each flower expands into an inflated pod (or legume), which is initially green but eventually becomes nearly black. When the pods are ripe and dried out, the shiny black seeds rattle around inside, like miniature maracas. (In fact, the genus name of the plant is derived from the rattlesnake genus, Crotalus, in reference to the rattling seeds.)
The plant's foliage is somewhat unusual for a member of the bean family. Most herbaceous members of this family have leaf blades divided pinnately into a number of discrete leaflets, such as kudzu, clover, and lespedezas, which have three leaflets, or wisteria, which has seven to nine. However, our Mystery Plant is unique in that its leaves are simple—somewhat rounded at the tip and tapering down to the base—not divided into leaflets.
In addition to being beautiful, this plant (which is native to southern Asia) is useful in controlling erosion and building up the soil through the biochemical process of nitrogen fixation. The species was introduced to the Southeast for these reasons and, being rather weedy, has spread. However, there is a downside: all parts of the plant are somewhat poisonous to poultry and livestock, making it an agricultural liability. It is, however, great for teaching in the classroom, and I’ll soon need to go out and harvest a bucketful for my Botany Boot Camp class.
John Nelson is the retired curator of the Herbarium at the University of South Carolina. As a public service, the Herbarium offers free plant identifications. For more information, call 803-777-8175 or email johnbnelson@sc.rr.com.
[Answer: "Rattlebox”, Crotalaria spectabilis]
