Lack of regular watering and feeding are among the commonest reasons. Soil compaction and cutting the grass too short and too often may also contribute to the appearance of weeds in a lawn. Use a long knife or hand fork to remove the weeds, then firm back the displaced sod. Weedkillers intended specifically for lawns work by translocation, moving within a plant from the leaves to the roots. The weeds begin to distort and shrivel within a few days of application.
Chemicals are selective and do no harm to lawn grasses at normal dilution rates.
Weedkillers should not be applied to newly laid sod, sprigs or plugs or seedling grasses within 6 months of germination. Weedkillers are usually bought in concentrated form and need to be diluted properly before use. At the start of the growing season fertilize the lawn and apply the weedkiller only when the grass and weeds are growing vigorously and the daytime temperatures are around 22 C. Hose end sprayers are commonly used but should be used only on calm days otherwise the weed killer can be blown onto flower beds and borders or even a neighbour's garden. I prefer to apply a weedkiller using a watering can with a fine rose. I divide the lawn into strips to ensure that the grass receives a complete and even coverage.