Ranunculus bulbosus L.
Perennial Broad-leaved Weeds
Bulbous buttercup is a native perennial with an annually renewed swollen corm-like stem base from which arise one or more aerial shoots. It is found in dry grassland and on grassy slopes. It is primarily a lowland buttercup and is found on the ridges in ridge and furrow pastures. It is common throughout England and Wales but is less frequent in Scotland. Seedlings fail to establish in very wet conditions and this may restrict distribution in Scotland. It is often abundant on well-drained or even thin soils due to the ability to survive summer drought. In loose soils, such as on dunes, bulbous buttercup can adjust its depth by up to 10 cm if covered with blown sand.
Buttercups are serious weeds of old pastures and hay meadows. Bulbous buttercup is characteristic of lightly grazed pasture.
A number of varieties have been described. The petals are usually yellow but white flowered forms occur.
Bulbous buttercup is the most toxic of the buttercups due to a high protoanemonim content. The foliage is harmful if eaten fresh but is not poisonous when dried in hay. Cattle and horses usually avoid the plant but poisoning can occur with new stock that has not come across it before. The seeds and bulbs of bulbous buttercup form a major part of the diet of partridge.
The first flowers are seen in March, but peak flowering is normally from mid-May to June. Flowering is not linked to daylength. Each flower head contains 20-30 seeds. The mean number of seeds per plant is around 70. Often seed will have ripened and been shed by early July. Fresh seed is normally dormant and requires a period of after-ripening to achieve maximum germination. Seedlings emerge from July to October but most do so in the autumn. The optimum depth of emergence in sandy soil is 16 mm. Adequate but not excessive soil moisture is important for germination and early seedling growth.
After seed has set in July bulbous buttercup dies down leaving a bare patch in the grass. It passes the summer as a corm that may remain dormant from July to September. The corms are buried 10-30 mm down in the soil and are 10-30 mm in diameter at the time of flowering. The corm is densely packed with starch plus around 1% of the glycoside ranunculin.
Corms do not need a period of cold to break dormancy only adequate moisture and favourable temperatures. Growth may begin again as early as August depending on rainfall levels. Sometimes this may lead to a brief second flush of flowers from some new shoots. By September the lateral bud of a corm that has flowered or the terminal bud of a corm that has not flowered becomes active and grows out from the old corm. If the old corm has become buried by loose soil the new shoot can extend itself up to the surface. Leaves develop and a small rosette is formed. The vertical corm-like stock overwinters and the rosette produces new leaves in March. The main foliage leaves form a basal rosette that is susceptible to shading. The leaves curl down to make a circular depression in the grass that excludes the development of other plants. A new corm develops at the base of the shoot. The old corm transfers food reserves to the new corm in February/March then dies. It is impossible to determine the age of a plant because the whole plant is renewed annually.
Persistence and Spread
Bulbous buttercup survives the dry period of summer as a corm. Plants often occur in clumps or larger patches but this is not due to vegetative spread as the corm is replaced annually by a single daughter corm. If the new shoot is damaged though, several axillary shoots may develop. The ability of new shoots to emerge from deeply buried corms allows the plant to re-establish itself in newly sown leys after a permanent pasture has been ploughed up.
Seed is the main method of multiplication. Bulbous buttercup produces 10 times more seed than creeping buttercup (R. repens). In general, the older the pasture the greater the seed population in the soil. There is no obvious dispersal mechanism and seeds normally fall around the parent plant. Cattle may eat the seed heads during grazing and the seeds can pass unharmed through the digestive system. The seeds also survive digestion by birds.
The abundance of the bulbous buttercup in pasture depends on the management system. Bulbous buttercup becomes established where overgrazing has occurred or bare patches develop due to urine burn or dung heaps, and where fresh soil is exposed due to mole activity. The plant prefers sunny areas and cannot tolerate early competition from taller plants. Bulbous buttercup seldom persists in grass that is allowed to grow long enough to cut for hay or silage. It may be eliminated from pasture if 2 or 3 hay crops are taken unless the plant has died down naturally before competition becomes severe. Bulbous buttercup fails to survive when impeded drainage or other factors increase soil water levels while good drainage encourages the plant. Bulbous buttercup may disappear if irrigation is applied regularly. It is intolerant of trampling and is usually absent from footpaths that traverse grassland.
Pigs eat the corms with relish and do not appear to be harmed. They will graze on the corms after ploughing. Geese pull up and eat the plants. Other birds eat bulbous buttercup plants and seeds, and there is predation too by voles and mice.
Updated November 2007.