Can You Put Weed Preventer Down With Grass Seeds?

Pre-Emergent Herbicides

• Weed preventer products contain compounds that disrupt the natural processes of germination in seeds -- whether of weedy plant or flower, veggie or grass seeds. Scattering pre-emergent herbicides, as weed preventer is more generically called, will prevent the grass seeds from germinating. Pre-emergent granules or sprays create a barrier film atop the soil surface that kills any plant sprouts that touch the chemical film as they grow upward. If you rake or walk across the treated soil surface, you break the film barrier and potentially allow any seeds to germinate and sprout upward without touching any herbicide.

Product Insight

• If you are establishing cool-season grasses in your lawn, such as a bluegrass, ryegrass or fescue, one chemical herbicide is usable to prevent weed seed germination. The ingredient siduron is often blended into weed preventer or crabgrass-preventer mixes with cool-season grass seeds as it will not prevent these lawn grass' seeds from sprouting. Siduron will stop the weedy grasses such as foxtail and crabgrass from germinating.

Is Herbicide Needed?

• Using weed preventer products is most important if you sow seeds at a time of the year when you expect lots of weeds to sprout -- such as in spring and summer when days are long and it's warm. Many weed seeds do not germinate well or as in high of numbers in fall as the days shorter. Cool-season lawns are best started from seed in late summer to early fall, such as from late August to mid-September across the temperate United States. This correlates well to a time frame when most weed seeds are not naturally sprouting and the grass seed can grow and establish well before any weeds sprout the following spring. Scattering a light straw mulch atop a seeded lawn, at a rate of one bale per 1,000 square feet, can help prevent weeds while the grass sprouts. Only use straw mulch that's free of weed seeds, though.

Warm-Season Grasses

• Most warm-season grasses, which are mostly used in the southern half of the United States where winters are mild, are started by sod or plant sprigs or plugs. Weed preventer with those forms are OK. Carpetgrass, bahiagrass, common Bermudagrass and centipedegrass can also be started from seed. Warm-season grasses are sown and sprout in the spring and summer, when many weeds -- especially crabgrass -- also naturally sprout quickly. In these situations, it's vital to prepare a planting area that is already void of weeds and not use any pre-emergent herbicides. Irrigate to promote rapid germination of the warm-season grass seed so it chokes out and prevents any weeds from getting started. Crabgrass seeds sprout more if the soil surface alternates between moist and dry. Irrigate evenly to have a moist soil to benefit the warm-season grass seed and not the crabgrass.