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Macroptilium atropurpureum (DC.) Urb. 1928
pronounced: mak-rop-TILL-ee-um at-roh-purr-PURR-ee-um
(Fabaceae — the pea family)
subfamily: Faboideae – the bean subfamily
common names: Siratro, Purple Bush-bean
Macroptilium is from the Greek μακρος (macros), large and πτιλον (ptilon), a downy feather; atropurpureum is botanical Latin for dark purple. I have not been able to discover the derivation of the name Siratro.
This is a climber and a creeper in the pea family, native from Texas south to Peru and Brazil, and in the Caribbean, but is widely naturalized in much of the tropics. It is widely used in coastal Queensland and NSW as a pasture plant.
The plant has a deep, fleshy taproot, the young stems are finely pubescent, and the bright green leaves, 2–7 cm long, are trifoliate with ovate leaflets. They are grey-green on the lower surface. The two lower leaflets will often have an extra rounded lobe, and the leaves have silky hairs on their underside. The stems are trailing, climbing and twining.
The inflorescence is an erect raceme, with 6–12 often paired flowers on a short rachis, the flowers being deep purple with a velvety sheen, reddish at the base, with a sweet-pea flower shape. They are borne on long spikes most of the year. After flowering, narrow pods up to about 10 cm long appear. Each contains usually 12–15 seeds, speckled, light brown to black, flattened ovoid. The long hirsute pods shatter violently when ripe, and can throw seed for several metres.
This species is useful as a tropical forage crop in full sun, and can also be used to prevent soil erosion. It tolerates a wide range of soils and pH, and tolerates more salinity than other leguminous forage crops. It is well-adapted to drought by virtue of its taproot and the pubescent leaves that reduce evaporation. It will die off on heavy frost, and then resprout from the crown in warmer conditions. It is commonly seen growing on roadsides, in disturbed sites, and in areas not grazed by livestock. As an environmental weed, it can form dense infestations along forest edges, and will grow over native shrubs, grasses or young trees, effectively smothering them. In recent years it has become common in vegetation around waterways, and in coastal sand dune vegetation. It can also be a problem in revegetation sites, where it smothers young trees and shrubs before they become established. It can spread by runners, but most spread is by seed, as stolon development is usually fairly weak. The seeds, as well as rooting in the surrounding soil, can spread great distances through water movement, and following ingestion by cattle. Siratro is rampant on Magnetic Island, on roadsides and in vacant or waste land.
As with most legumes, the roots nodulate to fix nitrogen in the soil, making it a good green manure; the down-side is that seed remaining in the soil can regenerate large populations of the plant in the succeeding crops.
This is a food plant for the caterpillars of several butterflies, including:
Photographs taken at Picnic Bay 2009, 2012
Page last updated 24th January 2018