Lawn grasses can become seriously weakened by long periods of drought and lack of plant nutrients. Under these conditions yarrow (Achillea millefolium) thrives and, once established, is a difficult weed to control.
Encourage vigorous grass growth by applying a proprietary top-dressing in early spring at 1-4 kg per sq m (2-7.5lb per sq yd) working in well with a rake or broom. Apply a further dressing in September at a heavier rate.
Feed the grass when it is growing actively with proprietary spring and summer lawn fertilisers, taking care to follow the manufacturer's instructions.
Mow regularly when in growth and lightly rake. After two or three days cut again removing the long growth. Repetition of this treatment weakens the weed and strengthens the grass.
Yarrow is fairly resistant to many selective weedkillers although repeated applications of weedkillers containing 2,4-D, plus mecoprop-P or dicamba can be effective. While this may eventually eliminate the weed, feeding and top-dressing will encourage better grass growth, and make conditions less favourable for the weed.