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
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.
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.
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.
When properly preparing your lawn for fall overseeding, you created a pathway for grass seed to reach the soil. If grass seed didn’t reach the soil, it would not grow. Raking, verticutting, dethatching, or core aerating are all methods to ensure seed to soil contact.
Disturbing the soil (verticutting, aerating, dethatching or raking) exposes soil and any pre-existing weed seeds that lay dormant in the soil. These weed seeds could be from last year or several years ago, suspended under the soil, too deep to germinate waiting for their opportunity to spring into life.
“Yes, You Did it Right”
Each year during the fall over seeding season, we get a lot of folks that come in to the Grass Pad and say, “I followed your fall lawn renovation program; I used your weed-free grass seed. Why do I have weeds in my yard?” Our response is “Well, it means you really did it right!”
Dandelion, clover, spurge, and numerous other broadleaf weeds are stimulated as well. If you should have just a few weeds here and there, pull them by hand. However, if you are overrun, fall is an excellent time to control those broadleaf weeds. Perennial broadleaf weeds are busy sucking up much-needed nutrients to store for overwintering. An application of Gordon’s Trimec or Speedzone can be applied 28 days after grass seed germination. A fall application of Gordon’s Trimec with Uncle’s Stikit, spreader sticker or a granular application of Loveland Weed and Feed will be quickly absorbed by the broadleaf weeds and is sure to kill to the roots.
Grassy weed seeds appreciate the same fertilizer and additional watering from your fall renovation program and flourish. Don’t panic – annual grassy weeds will be slow to germinate as soil temperatures cool in fall. Mother Nature will take care of these annual weeds at first frost. Using Prevent, crabgrass preventer in mid-April will eliminate their return.