Perennial Broad-leaved Weeds
White clover is a procumbent perennial, native in grassy and rough ground. It is common throughout Britain and is recorded up to 2,400 ft. It is most frequent on soils of pH 5.0 to 8.0. It is common on clay soils but is rarely found on peat. White clover is sensitive to shade, drought and severe frosts. It is less tolerant of saline conditions than many of the pasture grasses it is associated with.
White clover is widespread in pastures and meadows, frequently cut grass verges and lawns. It is absent from tall grass. It often occurs as seedlings on arable land. In unsown set-aside land in Scotland, white clover was the most frequently recorded species and constituted the second highest ground cover. In a study of seedbanks in some arable soils in the English midlands sampled in 1972-3, white clover was recorded in 59% of the fields sampled in Oxfordshire and 53% of those in Warwickshire but only in low numbers. White clover seed was found in 9% of arable soils in a seedbank survey in Scotland in 1972-1978.
White clover is a highly variable species with many varieties and over 70 commercial cultivars. Much of the seed for cultivated white clover has been imported from Holland, Czechoslovakia, Poland and more recently Canada and New Zealand. White clover has been an important constituent of fodder since the 17th century and is the most important pasture legume in Britain. It fixes nitrogen through its root nodules and increases soil fertility. The foliage is generally palatable to grazing animals but some genotypes contain cyanogenic glucosides. When consumed in quantity this can disrupt the rumen of domestic stock. White clover is an important source of winter food for wildfowl.
White clover can carry the potato nematode (Ditylenchus destructor) as well as economically important viruses.
White clover flowers from June to September. The flowers are mainly out-breeding and insect pollinated. Seeds start to become viable 12 days after flowering but take a month to ripen fully. There are 3-6 seeds per pod. For clover grown as a seed crop the yield of seed varies with cultivar and growing conditions and ranges from 13 to 151 kg per ha.
Fresh white clover seed that has not suffered abrasion from mechanical harvesting or soil action has a high percentage of impermeable seeds and is largely dormant. Scarification increases the level of seed germination from 21 to 99%. Light has no effect on germination. In the field, seedlings emerge from February to November but mainly from March to May. In a sandy loam soil, field seedlings emerged from the top 0-30 mm of soil with most from the surface 20 mm.
Clover seedlings begin to develop stolons 7 weeks after emergence. The stolons produce adventitious roots at the nodes almost immediately. In an open situation the main stolon can exceed 95 cm in length while in crowded conditions the length is around 38 cm. Vegetative growth generally begins in April and reaches a maximum in June-August. Plants may flower in their first year but clones vary in their capacity to flower. Some do not flower regularly.
Persistence and Spread
White clover forms a relatively persistent seedbank. In one burial experiment, seed buried at 20, 55 and 105 cm gave sporadic germination of up to 5% over the first 30 years. Seed buried in mineral soil at 13, 26 or 39 cm depth and left undisturbed retained 6 to 23% viability after 1 year, 5 to 7% after 4 years and 1% after 20 years. In another burial experiment, seed buried in soil did not germinate after 5 years. Seed buried in a peat soil at 26 cm retained 7, 6 and 2% viability after 1, 4 and 20 years respectively. Commercial seed retained only 4% viability after 1 year of soil burial but some seeds were still viable after 5 years. Under dry storage the seeds gave 55% germination after 5 years. Seed stored under granary conditions retained 88% viability after 1year, but few seeds were viable after 4 years.
In New Zealand, pasture soils grazed by sheep contained fewer seeds in the seedbank than pasture grazed by cattle. Ungrazed pasture used as a seed crop had the highest seed numbers in the soil. In a 10-year old neutral grassland, white clover was common in the vegetation cover and in the soil seedbank.
Where seeding is prevented, white clover can survive almost entirely through vegetative reproduction. The creeping shoots root at the nodes and can form a large clonal patch. Plant longevity is difficult to estimate but some clones are known to have survived in situ for over 60 years.
Seed dispersal is important for the colonization of new habitats. Apparently-viable seed has been found in samples of cow manure. White clover was the most numerous seed to survive in dairy farm manure that had been composted. Seeds survived passage through sheep. Most seeds were destroyed after 3 months in a dung heap but 5% remained viable. Viable seeds have been recovered from wormcast soil. Passage through the earthworms increased the germination of both hard and non-hard coated seeds.
White clover is tolerant of heavy grazing, trampling and cutting. In undergrazed pasture it is suppressed by the taller growing grasses. The time of grazing can affect the growth of white clover relative to the grass. Heavy grazing of a white clover/perennial ryegrass sward in March, April and May leads to a substantial increase in white clover at the expense of the ryegrass. Grazing after April leads to a decline in the white clover but not of ryegrass. The frequency and intensity of grazing may also affect the relative balance of different clones or cultivars of white clover within a field. The highly selective nature of sheep grazing can have a differential effect on white clover clones. The leaves of different cultivars often have characteristic white marks and sheep have shown a distinct bias towards particular markings. White clover is trample resistant and high stocking rates on grassland can cause a shift in species composition in favour of white clover and perennial ryegrass.
In roadside verges, increased cutting frequency increased the incidence of white clover. In grass-clover swards, infrequent cutting (3 times per year) leads to domination by the grass. Frequent cutting (6 times per year) allows the white clover to dominate the sward.
In Suffolk it has been noted that during the winter wood pigeons feed almost exclusively on white and red clover leaves. At this time they may consume 50% of the clover in the pasture.
Clover seeds were not killed by field steaming treatments applied by a mobile field steamer.
A number of insects and their larvae feed on the roots, leaves and seeds of white clover. Slugs and snails also graze the plants. Fungal pathogens infect white clover particularly in the colder months of the year.
Updated November 2007.