Weed management should be completed before seeding the lawn with a non-selective herbicide seven to 14 days before you till the soil. A second application of the herbicide may be required to kill any weeds you missed during the first treatment. Wait another seven days until tilling the soil if a second application is used.
Remember that all herbicides are different and the exact time you must wait to apply weed killers to newly planted grass will vary from one product to another. Also, some herbicides cannot be applied to certain species of turfgrass. For best results, always refer to the herbicide bottle’s label.
You can control weeds in newly planted grass seed and seedlings without the use of herbicides. Manually pulling the weeds by hand when they first appear keeps them from producing seeds and prevents the problematic plants from spreading, according to the University of California Statewide Integrated Pest Management Program website. They suggest keeping the newly planted grass weed free with proper mowing, irrigation and fertilization. Since newly planted turfgrass has short roots, keep the root zone moist by watering the soil lightly. However, avoid over saturating the soil. After the turfgrass has become established, promote deep and healthy root growth by watering infrequently but deeply.
Weed Control after Seeding
Before you plant grass seed, you should always prepare the area by removing any weeds that may be growing in the location. Even with careful preparation of the planting site, weeds can still develop among the newly planted grass seed. Weed killers, however, can harm grass seeds and seedlings if applied too early or improperly.
Some pre-emergent herbicides can safely be used during seeding and usually come mixed with a seed starter. These products have the active ingredient Siduron – also known as Tupersan – that works by suppressing weed seeds while improving root development of the new grass. The fertilizer and pre-emergent herbicide mix is applied with a drop or rotary spreader using a rate of 2 1/2 pounds per 1,000 square feet. The spreader setting and actual application depends on the brand of starter fertilizer plus weed control you use, and you should always follow the instructions found on the label.
A general rule of thumb is to wait at least until you have mowed the new grass four times before using any standard postemergent broadleaf herbicide. A standard pre-emergent herbicide should not be applied until at least three to four months after seeding the area.
Marylee Gowans has written about gardening for both online and print publications. She attended the University of Akron, graduating with a Master of Fine Arts in creative writing. In 2009, she received master gardener certification from the Master Gardeners of Summit County, Ohio.
Can I spray weed killer on recently seeded lawn? I don’t want the weeds to take over before my new lawn gets established.
Lawns seeded in spring tend to get weedy, which is one reason why autumn is the recommended time for planting cool-season grasses (such as Kentucky bluegrass and tall fescue). Before applying an herbicide, wait until new turf grows enough that it requires mowing three times. Young grass plants are sensitive to weed killers and may be damaged if the herbicide is sprayed too soon.
Q: I reseeded my fescue lawn and the new grass is coming up really good. There are a lot of weeds also coming up with the new grass. How long should I wait to spray weed killer after seeding the lawn?
A: If you disturbed the soil before you reseeded, I can understand why you have weeds now. Weed seeds lie dormant in the soil for several years and only sprout if brought to the surface by aerating or tilling. The label on broadleaf weed killers typically says not to use the product on a newly seeded lawn until it has been mowed three times. Be patient; you don’t want your weed control attempts to harm the grass you spent so much time planting.