A perennial, it is a member of the Legume family - Fabaceae. It is a major problem in the lawn due to its low growth habit with trailing stems or stolons, which root at nodes along their length forming a mat of bright green foliage that is very noticeable. When the lawn is dry or poorly fed the clover thrives as it is able to fix it's own nitrogen in nodules on the roots. In wet weather it can be very slippery. It is unaffected by mowing due to its low growth habit and recovers rapidly from trampling.
The leaves are compound, with 3 broad leaflets (sometimes 4, if you're lucky!) 1.3 - 2.5cm long, with tiny teeth on the edges, a pale triangular mark appears on each leaflet.
The 6 - 13mm long pea-like flowers appear from May to September, are white or very pale pink on separate stalks from the leaves and with 40 to 100 flowers in a 2cm diameter, ball-shaped cluster. They turn brown with age.
Spreading is mainly by seed, which remain viable for a long time, so it will recur after growing plants have been removed. The seeds can survive passage through the gut of grazing animals and earthworms, so this is a means of dispersal.

In drier regions it is included in lawn seed mixes as it keeps the lawn green if the grasses die back. Also in agriculture Clover is regarded as a nutritious herb which improves the quality of fodder and adds nitrogen to the soil. Wood Pigeons feed on the leaves during the winter when most other vegetation is absent or low in nutrition.

Rake the lawn before mowing, this will raise the runners to be cut. Water during drought and keep the grass well fed. If it is in bloom remove the mowings from the lawn and do not add them to the compost heap or use them for mulching. It does not grow well in taller, dense grass, so allowing the sward to grow longer for a time should reduce the clover.
Lawn Sand applied in the spring works well as do selective weedkillers, though the latter will probably be more successful. The Lawn Sand contains Ferrous Sulphate and Ammonium Sulphate, which are both slightly acid, so as well as scorching the leaves it discourages the clover, which likes lime (alkaline conditions).
Due to the amount of seed which are present the application of weedkillers will have to be repeated to remove the new plants which germinate later. Spiking, aerating and feeding the lawn are important to keep the grass growing well, thus removing the conditions more favourable to such weeds.

Red Clover (Trifolium pratense) is a close relative, but much rarer.

See related Lesser Trefoil and Bird's-foot Trefoil. Also Wood Sorrel, Least Yellow Sorrel and Creeping Wood Sorrel, which have similar leaves.