The Genus Crotalaria
This is a large genus of herbaceous and woody plants from warm climates around the world, mostly in Africa and Madagascar. A few species are native to the Coastal Southeast and several weedy species have become established here, mostly in disturbed sites. Some, at least, are food plants for bella moth larvae.
The bean family, Fabaceae, is one of the largest of the plant families. It is characterized by seed pods that are known to botanists as legumes. This is a large family that includes many important economic crop genera, including peanut (Arachis,) soybean (Glycine,) alfalfa (Medicago) and beans (Phaseolus.) Ornamental garden plants include orchid tree (Bauhinia,) redbud (Cercis,) coral tree (Erythrina,) lupine (Lupinus) and black locust (Robinia.)
This is an evergreen to deciduous shrub to small tree to about 10 feet tall. It bears bright yellow flowers through the growing season with strongest flushes in spring and fall. It is reported to grow in moist soils in South Africa but I consider it to be somewhat drought tolerant in northeast Florida. It grows best in full sun, or a bright partly shaded spot. It is reported to be cold hardy from zone 8 to 10. From my experience, it is reliably cold hardy in middle zone 9a.
Because so many of its relatives are weedy, plant any exotic Crotalaria with some caution. This species has grown slowly and has not set fruits in my unirrigated, zone 9a garden but it might behave differently in a different site or in a warmer zone.
This plant is rare in cultivation. It might be found in specialty catalogs. Propagation is by seeds and summer softwood cuttings.
In my garden, new growth starts in mid-March. Scattered flowers appear through the year. Heaviest flowering starts in late March, usually with another flush starting around late September. My plants are deciduous when winter temperatures drop into the mid-20’s F. Small branches may be killed by temperatures in the low 20’s F. The plant in my northernmost zone 9a garden died after two successive winters with temperatures in the upper teens F.
This is a native annual with stems that spread over the ground and reach only a couple of inches tall. Small but bright yellow flowers stand on short stalks above the foliage. Small, inflated seed pods up to an inch in length follow the flowers. In the wild, it grows in sunny, reasonably well-drained sites, often in poor soils. It will not compete with a healthy turf but can invade sparse lawns. Reportedly, it is cultivated in gardens but I find little cultivation information. Range maps show it growing on the eastern coast from approximately zone 6b south into tropical Central America.
I have not seen this plant in local nurseries. Plants can be propagated by seeds. It should be encouraged and enjoyed where it grows naturally.
My plants are naturally occurring natives. The earliest flowers appear in late March. They die back with the first hard freeze. It is a host for the brightly-colored bella moth. This is a white to tan-colored moth with orange and black markings on its forewings and bright pink to orange hind wings. I see this moth throughout the summer and fall when I walk across the lawn and cause them to fly up. I find bella moth larvae on two other legume genera, Baptisia and Sophora.
other Crotalaria species
A few native and several exotic (non-native) species of Crotalaria may be found in the Coastal Southeast. Some, like the showy rattlebox or Crotalaria spectabilis, are regarded as noxious weeds throughout their range in the United States. This annual weed prefers open, sunny sites and is found in disturbed areas, pastures, roadsides and open pine forests. I do not know how long these seeds can survive in the soil. Like some other members of this family, the seeds can remain viable in the soil for several years. I have been pulling seedlings from some sites for five years, so far. Each year, fewer seedlings appear.
Another common weedy species in much of the Coastal Southeast is Crotalaria lanceolata. It is a much smaller plant than Crotalaria spectabilis but the plant is more difficult to spot in a pasture and I have seen much larger numbers of this species within a given area. Persistent removal of this plant yielded good results. The number of seedlings has dropped rapidly each year over five years.