How Long After I've Planted Grass Seed Can I Apply Weed Killer?

Herbicide Application Time

• Herbicides applied too early can seriously impede the germination of seed or damage any emerging seedlings. The best time to apply any weeds to a newly seeded grass area is after the new grass has been mowed two to three times, recommends Trey Rogers in "Lawn Geek." Avoid the use of any chemicals even if the product claims to be safe, as seedlings are highly sensitive to control ingredients as opposed to more mature grasses.

Preemergent Herbicides

• The use of preemergent herbicides is not recommended for areas that will be seeded. The preemergent herbicides retard germination of all seeds, grass and weeds included. The only preemergent herbicides that will not affect new grass seed are those containing siduron, recommends Trey Rogers in "Lawn Geek." Siduron will help to keep weeds in control until it is safe enough to start using postemergent herbicides.

Cultural Weed Management

• The emergence of weeds in newly seeded grass is common even in well-prepared sites. Keep weeds from spreading by removing them manually as soon as you notice them and before they get a chance to produce seeds. Irrigating, fertilizing and mowing the grass at proper times will also help manage weeds without the use of chemicals. Many weeds that start to grow with grass seedlings tend to go away with mowing alone.

Weed Management Suggestions

• Lawn care experts recommend bringing as many weeds under control as possible prior to starting new lawns either with seed or sod. Minimizing weed population is the best way to give grass seed a healthy start, suggests Warren Schultz in "The Chemical-Free Lawn" and "Ortho's All About Lawns," and turf specialists at the Colorado State University. Reducing weeds also helps to cut down the time required for lawn maintenance later.