Plant 135

Anthoxanthum odoratum L. (Poaceae)


Sweet vernal grass

Sweet vernal grass is one of the earliest-flowering, perennial grasses in the British flora. The grass is naturally distributed through temperate Eurasia, but has been introduced to the Americas and Australasia. It is very variable and occurs in a wide range of grasslands, in addition to ruderal habitats. The grass's scientific and common names summarise its characteristics: a fragrant scent and an early production of yellowish flower heads.

Sweet vernal grass flowers lack lodicules, structures equivalent to petals that force the flowers to open. The grass is strongly protogynous, its female parts are produced earlier than its male parts. Local spread of the grass appears to be due to wind dispersal of mature spikelets, with occasional long-distance dispersal by spikelets attaching to the outsides of animals.

Sweet vernal grass gives hay its characteristic scent and is common in meadows. In 1816, George Sinclair published Hortus Gramineus Woburnensis, one of the first books to apply a scientific approach to investigating the value of different grasses as fodder. Sinclair made the point that sweet vernal grass was not particularly productive since it had a high ratio of stem to leaf. Furthermore, cattle would only eat it when no better grasses were available. However, it was included in seed mixes for permanent pasture because hay was expected to have the odour of sweet vernal grass. The chemical coumarin is responsible for the grass's odour.

Coumarins are not confined to sweet vernal grass; they are found across the plant kingdom, in plants including tonka beans, melilotus, sweet woodruff and meadowsweet. In the early 1920s, some North American cattle suffered excessive bleeding, and frequently died, after eating mouldy silage containing melilotus. Fungi were converting melilotus coumarins into dicoumarol, a toxic anticoagulant. Although sweet vernal grass contains coumarins, the concentrations appear to be too low to cause animal husbandry problems. Warfarin, an anticoagulant, is a synthetic derivative of dicoumarol and was initially introduced in the late 1940s as a rodenticide. By the early 1950s it was found to be effective for thrombosis prevention in humans.

Sweet vernal grass probably reached the Americas and Australia in the eighteenth and nineteenth centuries in the wake of European immigration. Immigrants would take plants that were familiar to them, including the seeds of pasture grasses. Furthermore, hay, probably contaminated with sweet vernal grass, was needed by the people to feed their animals on the passage, fill beddings and pack belongings.

Further reading

Cope T and Gray A 2009. Grasses of the British Isles. Botanical Society of the British Isles.

Davies EG and Ashton WM 1964. Coumarin and related compounds of Anthoxanthum puelii and Melilotus alba and dicoumarol formation in spoilt sweet vernal and sweet clover hay. Journal of the Science of Food and Agriculture 15: 733-738.

Grant MC and Antonovics J 1978. Biology of ecologically marginal populations of Anthoxanthum odoratum. 1. Phenetics and dynamics. Evolution 32: 822-838.

Stephen Harris