Weeds and other undesirable intruders fall into three categories: annuals, perennials, and coarse weed grasses.
Annual weeds often pop up in newly sown lawns, and usually disappear with mowing. Perennial weeds are a bigger problem. They're not usually eradicated by mowing, which means you have to treat them with a weedkiller. Never apply weedkiller to a new lawn. Give your lawn nine months of growth before you apply the weedkiller.
These are the toughest weeds to eradicate. Selective weedkillers are ineffective because the weed grasses have similar characteristics to your lawn grasses. The only effective way to get rid of them is to cut out the roots with a sharp-pointed knife.
No matter how well you prepare your site, some weeds will always appear. Don't rush to sow your lawn. Leave the seedbed fallow for a few weeks. This gives many of the weed seeds a chance to germinate. You can then remove or destroy them before you sow your lawn.
In the early stages of a new lawn, you can pull out most weeds by hand. But take care not to disturb the tender grass seedlings. You can also cut out weeds with a sharp-pointed knife.
You can treat an established lawn with weedkiller anytime from May to September, but not when there's a drought. The weedkiller will be ineffective on the weeds and may scorch your lawn.
A modern grass-seed mixture, such as Turfline® GrassFix, gives you a dense, durable lawn with the ability to compete strongly against weeds. Adding Microclover® increases anti-weed competition even further and strengthens the grasses in your lawn.