Seasonal weed seeds begin sprouting in late February and early March when the rest of the lawn is still dormant. If you are a lawn purist the appearance of these seasonal weeds can be a little depressing right at the beginning of the season, but remember that with careful planning, and a schedule of treatment, you can get control of your lawn before the heat of the summer sets in.
The best time to lay pre-emergent is in March and April, although it depends on your climate and altitude. Pre-emergent by it’s definition is designed to be applied to the lawn as it is just waking up from winter dormancy. Once it is applied to the lawn it forms a 30 day plus chemical barrier on the soil line that prevents the germination of weed seeds, or lawn and flower seeds. It must be applied as the soil is warming up to be the most useful. Pre-emergent does not kill weeds that have already emerged, or those that have already put down roots and established themselves. Pre-emergent also will prevent the growth of grass seed at the soil line, so it is best used to control weeds several weeks before you intend to patch the lawn with grass seed.
When to Apply:
If your zone thaws and begins showing fresh grass growth in March and April, then you should apply Pre-emergent in March. If you see new growth in April and May, then you should apply Pre-emergent in April. Always try to stay one step ahead of the weeds in your yard, and and earlier application is usually best as opposed to one that is applied when the weeds seeds are already emerging. Don’t waste the chemical by applying before the ground is fully thawed, or when frost is still a common occurrence, the only effect you will achieve here is expensive rain run-off out of your yard, and into the sewage system.