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Desmodium tortuosum (Sw.) DC. 1962
(Fabaceae – the pea family)
subfamily: Faboideae – the bean subfamily
common name: Florida Beggar-weed
This plant is an import from tropical and subtropical America, and is now naturalized in tropical and subtropical Australia. It thrives among low trees, particularly in loamy soils, and will occupy disturbed or vacant cleared areas and in disturbed natural vegetation. There is a great deal of it on road verges and waste land on the island. It is one of the most troublesome weed pests in peanut crops, particularly in south-eastern USA.
It is an annual, biennial or perennial herb that grows to about 180 cm tall here (but in crops it is sometimes seen up to 3 m tall), its stems covered with sticky hairs. The leaves are alternate, spiral, compound with 3 leaflets, stipulate, with petioles up to 4.5 cm long. The blades of the leaflets are 3–11 cm long and up to about 4 cm wide, elliptic or ovate, the base tapering, the margins entire, and the blade hairy.
The flowers are in racemes 3–8 cm long, predominantly pink or purple, of a very irregular pea-shape. The seed pods are flattened, with 3–7 segments equally indented on both sides, and covered in hooked hairs. The pods break apart and stick to animals’ fur, and to human clothing.
Florida Beggar-weed infesting peanut crops is an example of a tall weed in a comparatively short-statured crop. The weed germinating within a few weeks of the crop often forms a dense canopy over the peanuts by harvest time. It is a late-season weed that escapes pre-emergence control measures and becomes apparent too late for effective post-emergence control. Not only does it compete for light, water and nutrients with the peanut plants (and can thus reduce the yield by up to 40%), but it makes harvesting tricky, interfering with the digging, turning and curing aspects of the harvest.
Although it is now seen as one of the most damaging weeds among leguminous crops, Florida Beggar-weed, up to about 1950, was a popular forage crop. Heavily seeded on land of moderate to high fertility, it matured 70 – 80 days later, yielding up to 13 tonnes per hectare of dry matter, and returning to the soil up to 5,000 seeds per plant for the next crop. As a result of all this planting in the past, it is now a plague in areas most suitable for peanuts and soybeans. As well as its suffocating effect on crops, the plant also harbours several insect and viral pests of field crops. Experiments are being conducted to find effective biological controls for the plant.
Photographs taken in Picnic Bay 2009, 2010
Page last updated 12th November 2016