Known via a range of common names which include devil bean, large yellow rattlebox and wedge-leaved crotalaria, Crotalaria retusa is a highly ornamental plant that you can grow in your garden. A member of the bean family (Fabaceae), it produces bright yellow flowers that are borne on an upright spike which is presented promindantly above a mass of jade green leaves. For ornamental purposes, this plant is best grown in mass in either a flowerbed or as a border plant.
Sometimes found growing as a weed in wastelands, Crotalaria retusa grows as an annual herbaceous shrub that can attain a height of about 1 m. This plant can only be from seeds and the process is an easy one but trying to get some seeds can prove to be difficult. It loves a sunny spot that is also well draining. Being a legume, Crotalaria retusa can fix its own nitrogen from the atmosphere and hence can be grown in nitrogen-poor soils.
Besides being useful as an ornamental plant, Crotalaria retusa has an interesting characteristic that will probably interest kids. The pod-like fruit this plant produces, when mature and dry, becomes a rattlebox that can be shaken and heard. The common name of this plant ‘rattlebox’ is derived from this interesting feature. The genus name Crotalaria is derived from the Greek word, κροταλον, which refers to the musical percussion instrument castanet. If you are looking for a candidate to grow in your sensory garden, plant Crotalaria retusa in it to confer the sound/audible component.
Like other legumes, Crotalaria retusa can be grown as a green manure, where mature plants can be worked back into the soil to add nitrogen. It is also grown as a source of plant-derived fibre and dye. Crotalaria retusa is also grown as a medicinal plant. According to Plant Resources of South East Asia (PROSEA), its roots are used against coughing up blood while its leaves are mixed with those of another relative, Crotalaria quinquefolia, and consumed or applied externally against fever, scabies, lung diseases and impetigo. Flowers and leaves are both edible as vegetables due to their low alkaloid content and are purportedly sweet. Seeds are roasted and eaten in Vietnam. Note that this plant is also nematode-resistant and studies have found dried plant parts can be worked into the soil as a soil amendments to deter and reduce root galling by the root-knot nematode, Meloidogyne incognita.
Lastly, note that Crotalaria retusa is a butterfly host plant. According to the Butterfly Circle website, Crotalaria retusa is the food plant for the pea blue butterfly (Lampides boeticus). Unlike other butterfly caterpillars that chew up leaves, the first two instars of the pea blue bore into flower buds of this plant and consume the flower parts contained within. The larger 3rd and final instar caterpillars will move on to eat the developing seeds within seed pods. For more information on the pea blue butterfly, please refer to the webpage below:
Do your part for the wildlife and environment around us, try to grow this plant without the use of pesticides. In rapidly urbanising Singapore, we are losing loads of our native species of flora and fauna without many of us knowing. Butterflies have an important role in our environment and one of the most obvious role they play is that of a pollinator where they help to transfer pollen that help plants to set seed and ensure the survival of subsequent generations.