If you plan on planting warm season grasses, you should apply Roundup in the fall, so your lawn is ready by the following spring. You should time it so that you plant no later than August or September.
According to the manufacturer, you should wait at least three days after planting to ensure healthy grass growth.
The best time to apply Roundup before seeding depends on two factors: grass type and the weather.
This herbicide blocks a specific enzyme pathway that prevents the production of essential plant proteins. Once absorbed by plant tissues, it quickly spreads throughout the plant’s system and completely stops its growth.
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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.
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
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 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.
How long you must wait to plant grass seed after treating your lawn with a Scotts Weed B Gon product depends on which product you use. When fighting broadleaf weeds such as dandelions with the basic Weed B Gon weed killer, wait at least three weeks to sow your grass seed. This product specifically targets the seeds of broadleaf weeds, but it can stop or slow down your grass seed’s germination as well.
When fighting grassy weeds, you must wait a little longer to plant your grass seed, whether you’re seeding your entire lawn, overseeding or treating bare patches. With the Weed B Gone Plus Crabgrass Control, wait at least four weeks to plant seed. Because this is specially formulated to stop grassy weed seeds from germinating, it also can stop your regular grass seed from growing.
Without Crabgrass Preventer
Keeping your lawn weed-free is a constant battle that uses cultural practices, such as deep waterings, to encourage grass to choke out weeds, as well as chemical products, such as using Weed B Gon, to kill weeds before they start. These chemicals often kill grass seed as well, keeping it from germinating along with the weed seeds, so wait before you sow your seed.
Although you can’t spread grass seed at the same time you spray your lawn with Weed B Gon, other options exist to treat your lawn for weeds at the same time you plant seed. These typically come in the form of weed and feed products that offer fertilizer along with the herbicide. Look for products specifically designed to be spread with grass seed, often noted on the label as a starter fertilizer plus weed control.
When you’ve already planted your grass seed and want to apply a pre-emergent herbicide to keep weeds from competing with your new grass for water and nutrients, have a little patience. Applying a weed-control product such as Weed B Gon too soon can kill young grass. Instead, wait until you’ve mowed the new grass at least three times before using herbicides on your lawn.