Small squareweed

Mitracarpus hirtus, small squareweed or button weed, is an erect annual herb in the Rubiaceae family native to Mexico, the Caribbean, and tropical South America, and now widely naturalised throughout tropical Africa, Asia, and Australasia as a weed of cultivation and disturbed ground. Plants grow 10 to 60 centimetres tall on slender, four-angled, sparsely hairy stems with opposite, lance-shaped to narrowly elliptic leaves 2 to 6 centimetres long, the bases connected by a fringed interpetiolar sheath of bristles characteristic of the genus. The inflorescence consists of dense, sessile, axillary glomerules at the upper nodes, packed with very small white funnel-shaped flowers each only two to three millimetres across. Fruits are tiny dehiscent capsules splitting transversely to release minute pitted seeds. The species thrives in seasonally dry tropical climates on light, sandy or loamy soils.