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Clover, most commonly Trifolium repens, is a perennial weed with shamrock-like leaves and fragrant white flowers. It colonises gardens and lawns using runners that fix into the ground, and competes with other plants for space to grow. Once it's established in your garden it will quickly spread, causing particular problems when it appears in the lawn, as it's difficult to remove.
Solution
Organic
In the lawn, your best bet is to dig out affected areas and replace patches of lawn with weed-free turf. In other parts of the garden, just dig it out. Mind you, it's worth bearing in mind clover has nitrogen-fixing nodules on its roots, so it acts as an excellent green manure when dug back into the soil.
Chemical
In lawns, use a weedkiller such as a 2, 4-D-based herbicide in spring or early summer, when growth is at its most vigorous. Repeat throughout summer where necessary. Apply in cool, moist, calm conditions when there's least risk of accidentally damaging nearby garden plants.
