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.
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.
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.
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.
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.
In short, here’s what you should do if you encounter weeds in your newly seeded lawn:
If you find that the weeds are recurring past the 6-8-week mark, you may wish to consider using a selective herbicide to spot spray your weeds. Some weed killers such as glyphosate (Roundup) kill more than just weeds, so it is important to not apply these as if they are not done precisely, they can kill your grass. Shop bought selective weed killers will recommend when to apply their product and how often and you should read the instructions thoroughly and adhere to them.
However, you can get rid of these weeds just as fast as they have appeared.
The important thing to remember when new weeds appear in your newly sown lawn is not to act hastily – do not apply a Feed, Weed and Moss Killer type product of any kind on a newly sown lawn.
Although this can be frustrating and we can appreciate that a quick solution will be desired, the good news about these types of weeds is that they are largely shallow rooting and should come out with the first mow at the 6-8 week mark after sowing. If they don’t, they should be easy to pull out of the turf.
If you find that when the lawn is at least six months old and has been taken over by weeds or moss, you can use a Feed, Weed and Moss Killer product.
Post-Emergent Secrets to Success
Specialized Herbicide: To control some especially challenging weeds, like Nutsedge, Clover, Creeping Charlie or Bermudagrass, you’ll want to choose a specific herbicide that’s been proven to be effective. Ask a local garden center or your local Cooperative Extension System office to learn which herbicides will beat your toughest weeds.
Broadleaf Weed: Leaves are broad and flat (not grassy or needle-like).
Non-Selective Herbicide: Kills any green and growing plant, whether or not it’s a weed.
Examples: Chickweed, Crabgrass, Lamb’s-Quarters, Annual bluegrass
If you have a yard, you have weeds. They may lurk in the lawn, thrive under a shrub or flourish in flowerbeds, making weed control a constant battle. It requires patience, persistence and knowledge – of both types of weeds and the weapons you have to eradicate them.
With pre-emergent herbicides, you’ll want to apply the chemical prior to the time weed seeds start germinating, which can be spring or fall depending on the type of weed. For example: cool-season weeds, such as Annual Bluegrass, are usually best controlled with a late summer to early fall application. If you apply too early, these herbicides will have degraded and are useless when seeds start to germinate. Most pre-emergent Crabgrass killers remain active in soil for 6-8 weeks. If you apply too late, the herbicide will not affect weeds that have already germinated.