Several legume Crotilaria species are native to Florida. Others are naturalized exotic pest plants that have or may become invasive, disrupt natural plant communities and are listed as noxious weeds or prohibited plants. There are over 600 Crotilaria species globally, mostly of African and Asian origin. Roots of legumes in the pea/bean Fabaceae family support nitrogen-fixing bacteria that improve soil quality.
All Crotilaria develop seeds in inflated peapod-like cases that dry and harden at maturity. Mature seeds inside (about 20 or more per pod) become loose and rattle when the pod is shaken — hence the common names rattlebox, rattlepod and shake-shake. The seeds can be toxic to domestic livestock if ingested in large quantities. The alien introduced species have prolific seed dispersal that can result in large colonies of nonnative crotalaria seedlings the following year. One management practice is repeated mowing in summer to prevent flowering and the resulting seed development.
How to tell the difference between the few (8) Crotilaria species that are grown in Florida is fairly simple. First: desirable native Crotilaria are low-growing plants found in scrub, flatwoods and moist woods while exotic Crotilaria are found in drier, disturbed and developed sites like roadsides and vacant lots. Second key is height: native species are low-growing or mat forming, under a foot tall, while exotics are upright and taller than a foot.
Leaf shape, edges and arrangement along the stem are key to plant identification. Simple leaves have one leaf blade with a midrib while compound leaves have several, usually small, leaflets on a central rachis. The margins (edges) of a leaf blade can be whole or smooth, called entire, without teeth or notches or may have teeth. Some leaves have pointed tips while others are indented like a stylized heart-shape, called retusa. Other leaves may have shallow or deep lobes, either pointed or rounded. Compound leaves may or may not have a terminal leaflet at the tip and the leaflets may be opposite each other on the rachis or arranged alternately. On perennials, next year’s leaf bud may be seen where the leaf joins the plant stem or branch.
All Crotilaria in Florida have yellow flowers, followed by an inflated peapod-like fruit. Native perennial Rabbitbells (C. rotundifolia) and annual exotic Showy Rattlebox (C. spectabilis) and Rattleweed (C. retusa) have simple leaves with smooth margins. Rabbitbells is low-growing and mat-forming with hairy upper leaf surfaces and may flower any month of the year. The exotics are tall. Showy Rattlebox (C. spectabilis) has simple leaves with pointed tips which alternate along the stems while Asian Rattleweed (C. retusa) has distinctive heart-shaped leaf tips.
If the Crotilaria leaf is compound with three leaflets, the species may be Low Rattlebox, Lanceleaf Rattlebox, Shake Shake (Velvety Rattlebox) or Smooth Rattlebox. If the plant is low growing or forms a mat, it is native Low Rattlebox (C. pumila). If the compound leaves are long, narrow and lance-shaped, it is exotic African Lanceleaf Rattlebox (C. lanceolata) that grows 4 feet tall. African Shake Shake (C. rotalaria incana) grows 6 feet tall and has long, shaggy hairs on its seedpods which contain about 40 viable seeds. Smooth Rattlebox (C. pallida) has no hairs on the seedpods.
The rare Avon Park Rattlebox, also called Harebells or rabbitbells (C. avonensis), is endemic to three small isolated sites in Central Florida. It grows naturally nowhere else on Earth and is legally protected. While alien but naturalized crotalaria are pretty wildflowers, gardeners should avoid spreading exotic pest plants.
The Florida Exotic Pest Plant Council Invasive Plant Lists is readily available at https://www.fleppc.org/list/list.htm. The U.S. Federal Noxious Weed List is at https://www.aphis.usda.gov and Florida’s prohibited plant list is at https://plants.usda.gov/java/noxious.
Jane Weber is a professional gardener and consultant. Semi-retired, she grows thousands of native plants. Contact her at firstname.lastname@example.org or phone 352-249-6899.