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.
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.
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.
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.
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.
You can sow seeds in as little as a week or even sooner after spraying glyphosate, a systemic, nonselective weed killer. Glyphosate moves from the leaves to the roots of plants, destroying the entire plant, but leaving no residue in the soil. The chemical affects many types of plants, including weeds, grasses and desirable plants, but after the liquid is absorbed into the plant, it doesn’t pose any further threat. You can safely sow ornamental flower seeds a day after spraying with glyphosate and grass and vegetable seeds, three days after, even though the herbicide takes up to seven days to destroy weeds. If you remove the dying weeds too soon, live roots could remain in the soil, ready to regrow. Another systemic weed killer that doesn’t affect seeds is pelargonic acid.
It makes sense to be cautious about sowing seed after using weed killer. Certain herbicides can harm sprouting seeds and young plants. However, while you must wait several months to sow seed after applying some weed killers, you only need to wait a few days after applying others. The reason for this difference lies in the effect of the active chemicals in the individual products. Read the label carefully and follow all the directions when applying a weed killer.
Sowing Seed After Applying Glyphosate
Sowing seed after applying a pre-emergence weed killer disturbs the chemical barrier on the soil surface, which means that weed seeds may germinate too.
Pre-emergence weed killers prevent seeds from sprouting. They create a chemical barrier on the soil surface that suppresses seed development. What this means is, if you sow your own seed after applying a pre-emergence weed killer, the seed isn’t likely to grow. However, some pre-emergence products only affect grassy weeds, so you can safely sow most vegetable and flower seeds after applying these herbicides. The same doesn’t apply to reseeding or overseeding your lawn. Grass seed won’t sprout until a pre-emergence weed killer has decayed and become ineffective. For example, it isn’t safe to sow lawn seed until four months after applying a crabgrass preventer.
Many selective weed killers leave little or no trace in the soil, and they target certain plants while leaving others unharmed. Generally, these types of herbicides destroy either grassy weeds or broadleaf weeds. You can safely sow most seeds in your vegetable or flower patch a day after applying selective herbicides, such as sethoxydim, clethodim and bentazon, for grassy weeds. These herbicides only affect your desired plants if the plants belong to the grass family. For lawns, herbicides that destroy broadleaf weeds are effective, but it isn’t safe to reseed until a month after applying these products, unless the label states differently.
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.