Plant 85

Stipa gigantea Link (Poaceae)


Giant needle grass

Giant needle grass is an immigrant to our gardens from the southwestern Mediterranean. Its large tussock form, grey-green leaves, golden inflorescences and ability to survive drought make it a stately addition to ornamental landscapes and a promising species as climates get warmer. Stipa is a large genus of northern temperate grasses, with many species scattered across the steppes of Central Asia, whilst others become serious weeds in strongly seasonal climates outside Eurasia.

Most plants cannot move, so must rely on broken body parts or pollen and seed movements to disperse their genes across space. Grass genes get around on the wind as pollen grains are blown from the anthers that hang from the diminutive flowers and are trapped on feathery stigmas. The flowers of grasses are aggregated into spikelets surrounded by dry scales. As fertilised flowers mature into fruits, the scales that make up the spikelet, and surround the flowers, take on protective functions. Some grasses have spines, hooks, elaborate arms (awns) and bristles on their spikelets. These features may harden and become sophisticated structures to help seeds escape their parents and bury the fruits in the soil.

Needle grass fruits are torpedo-like, and well named; they have hard, pointed tips, surrounded by harpoon-like hairs at one end and long, twisted awns at the other end. The hairs point away from the tips of the fruits, whilst the awns are hygroscopic. As the awns twist and turn with changes in humidity levels, the fruits are buried; they are drilled into sandy or broken soils. The mechanism is called trypanocarpy, an apt description derived from the ancient practise of boring holes in skulls to release evil spirits. In this case, weeds may be the 'evil spirits' released by these grains. In addition to being an effective burial mechanism, fruits can be trapped and driven into the fur of mammals to aid dispersal or into the trousers or flesh of unwary travellers; the hairs around the tip prevent the fruit being pulled out.

Such grasses are merely nuisances for some, but to farmers in areas dominated by needle grasses, land may become useless for fodder or hay when the grasses are fruiting; the armed fruits embedding themselves in the mouths and skins of grazing animals. Many farmers would perhaps concur with an early twentieth-century commentator writing about another troublesome species: 'the eradication of this grass ... would be an act of mercy'.

Further reading

Silberbauer-Gottsberger I 1984. Fruit dispersal and trypanocarpy in Brazilian cerrado grasses. Plant Systematics and Evolution 147: 1-27.

Stephen Harris