During May and into June a yellow-flowered annual with opposite, spear-shaped, coarsely-toothed leaves can be found in grasslands throughout Britain and Europe, and large parts of North America. By the end of June the flowers have been pollinated and the green calyx inflates to surround a capsular fruit. Inside the fruit there are a small number of kidney-shaped, flattened, winged seeds. As the fruit ripens during the summer months the seeds loosen and the plant will rattle in the wind; hence the plant's common name. The generic name, Rhinanthus, literally means 'nose flower' and is a reference to the flower's resemblance to a nose in profile.
The leaves of yellow rattle are yellowish green which gives a clue about its life style. It is a root-hemiparasite and does not need to rely on its own photosynthesis for carbohydrates. Carbohydrates, together with water and mineral nutrients, come from its hosts, which are most frequently grasses and legumes. The connection between the yellow rattle and its host is called a haustorium; a physical and physiological link between the xylem systems of two different plants.
Seeds of some hemi-parasitic plants germinate in response to chemicals released from the roots of host plants. In the case of yellow rattle the seeds germinate in the early spring, in response to seasonal cues, following a cold shock to break dormancy; soils have almost no yellow rattle seed bank. When yellow rattle seeds germinate they show little selectivity as to the roots they initially attack but potential hosts respond to yellow rattle attacks in different ways. Legumes and grasses show few responses, whilst other herbs will modify the chemistry of their cells wall when attacked, making them difficult to infect and unlikely to be successful hosts.
Yellow rattle is most commonly found in open grassland, roadsides, and even dunes, in areas of low soil fertility; it does not tolerate shading. By diverting nutrients from the plants that it parasitizes, yellow rattle reduces the growth of its hosts. This ability of yellow rattle to modify plant community structures has been used with considerable success to encourage the growth of species other than grasses and legumes, thereby increasing overall species diversity in hay meadows. Yellow rattle has become an important management tool for encouraging species diversity in traditionally-managed meadows. Traditional hay-making practises are effective at maintaining yellow rattle populations, with annual fluctuations reflecting the interplay between host and hemi-parasite.
Pywell, RF et al. 2004. Facilitating grassland diversification using the hemiparasitic plant Rhinanthus minor. Journal of Applied Ecology 41: 880-887.
Westbury DB 2004. Rhinanthus minor L. Journal of Ecology 92: 906-927.