Crotalaria pallida Seeds | Smooth Rattlebox
LATIN NAME: Crotalaria pallida
COMMON NAME: Smooth Rattlebox
Plant Profile
The specific name of its best-known synonym, mucronata, refers to the small sharp point at the tip of each leaflet, and that point is the quickest way to confirm the plant in the field. Leaflets are oval to obovate and carried in threes on a short stalk, with two tiny stipules at the base.
A seedling does not look like its parent. The first leaves are simple and rounded, and only as the plant gains size does the trifoliate adult form appear, so young plants in a stale seedbed are easily mistaken for something else. Shade and crowding delay the change, leaving a colony with plants at several stages.
The flower is a typical pea flower with a beaked keel, and the pod that follows swells into a thin-walled cylinder with a hard tip. Inside, the seeds loosen as they dry and knock against one another, which gives the genus its common name and gives a grower an easy way to tell when the crop is ripe.
Science & Biotechnology
Pyrrolizidine alkaloids are the reason this genus must be handled with care. They are esters built on a retronecine base, and the group includes monocrotaline and usaramine; the plant stores them as harmless N-oxides, but the liver converts them back into reactive esters that bind to cell proteins and DNA. Damage is cumulative, and the alkaloids pass into milk and honey, which is why feed and food programmes screen for them.
The pathway starts with homospermidine synthase, the enzyme that commits the plant to alkaloid production; the same enzyme evolved independently in several plant families, one of the neatest cases of convergent evolution in plant chemistry. Later steps assemble the retronecine core and esterify it into monocrotaline and related alkaloids.
Because the alkaloids survive drying, laboratories screen herbage, silage and honey by liquid chromatography coupled to mass spectrometry, usually against a panel of the common esters. For a grower the practical upshot is simpler than the chemistry: the seed is the most concentrated part of the plant, so a cover crop is cut long before the pods fill.
Uses & Benefits
The plant is useful in three ways, and all three depend on timing.
Sown thickly, it smothers weeds, fixes nitrogen in root nodules and produces plenty of soft biomass for the compost heap or the spade. Cut it at first flower and mow or dig the tops in; leaving it to set seed turns a useful cover crop into a weedy nuisance and puts the alkaloid load at its highest.
Stems harvested at flowering carry a soft bast fibre related to the sunn hemp of South Asia. Bundle the stalks, ret them in water until the fibre slips from the woody core, then wash, dry and comb the strands for cordage or rough cloth.
Growing from Seed
The seed coat is hard enough to delay water uptake, so break it before sowing: a brief soak in hot water or a nick with a file is enough. Any well-drained soil suits it, and the plant is not fussy about fertility.
Nodules form freely with the slow-growing rhizobia this genus partners with, so inoculation is seldom needed in tropical soils. The main problems are aphids, whitefly and pod-boring caterpillars, plus root rots in heavy ground. Follow it with a leafy crop that can use the nitrogen left behind.
Our Take
For a warm-climate rotation, put this beside Sesbania cannabina, another fast legume cut for biomass, and Tephrosia candida, which holds a slope while it feeds the soil. Crotalaria cunninghamii is the ornamental cousin, grown for its green bird-like flowers rather than for any crop.
Related Reading
Gardeners who grow Smooth Rattlebox often pair it with these related subjects: