Pre-emergent herbicides are a foundational tool in lawn care, designed to prevent weed seeds from sprouting by creating an invisible chemical barrier near the soil surface. Grass seed is the desired product, used to thicken an existing lawn or establish a new one. The direct answer to whether you can apply both at the same time is usually no; standard pre-emergent products will prevent the grass seed from growing, just as they stop weeds. Achieving both weed control and new turf growth requires understanding the science behind these products and proper timing.
The Conflict: How Standard Herbicides Affect Seeds
Standard pre-emergent herbicides, such as those containing prodiamine or dithiopyr, inhibit cell division in all germinating seeds. These chemicals create a barrier in the top layer of soil that targets mitosis, the process by which a plant grows its initial roots and shoots. When a seed sprouts, the emerging root or shoot tip contacts this herbicide barrier.
This contact disrupts the rapid cell multiplication needed for the seedling to establish itself. The young plant, whether a weed or a turfgrass seedling, cannot form the root structure required to absorb water and nutrients, causing it to die. The herbicide cannot distinguish between a weed seed and new lawn seed. Applying a standard pre-emergent simultaneously with grass seed will chemically prune the roots, preventing the new lawn from becoming established.
The longevity of this chemical barrier, known as its residual activity, makes simultaneous application impossible. Many conventional products remain active in the soil for 6 to 12 weeks to provide season-long weed protection. This extended activity means the herbicide remains potent enough to affect any grass seed sown into the treated soil, even weeks after application. This mechanism explains why timing the application is so important.
Seed-Safe Alternatives for Simultaneous Use
The conflict between weed prevention and new seeding can be overcome with specialized herbicides designed for use at the time of seeding. The most common active ingredient in these “seed-safe” products is mesotrione, often sold under the trade name Tenacity. This chemical offers a different mechanism of action than standard pre-emergents, allowing desirable grass to thrive while still controlling weeds.
Mesotrione works by inhibiting the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD) in susceptible plants. This enzyme is required for the plant to produce carotenoids, pigments that protect chlorophyll and the plant’s cellular structure from light. When HPPD is blocked, the weeds turn a distinctive white or bleached color before dying because their chlorophyll is destroyed.
Many cool-season turfgrass species, such as fescues, ryegrass, and Kentucky bluegrass, metabolize the mesotrione compound quickly and safely. This selective action allows the grass seed to germinate and establish successfully while controlling common grassy and broadleaf weeds pre-emergently. These specialized products are not safe for all grass types, such as many warm-season varieties, so compatibility with the specific seed must be confirmed.
Sequencing Applications for Optimal Results
For those using a standard pre-emergent, successful seeding requires careful timing to ensure the herbicide barrier has dissipated. If you apply the pre-emergent first, you must wait for its residual activity to break down before sowing grass seed. Depending on the active ingredient and application rate, this waiting period typically ranges from 6 to 12 weeks, though some labels may require up to 16 weeks. Always consult the product label for the precise re-seeding interval.
Conversely, if you seed first, you must allow the new turf to become established before applying a standard pre-emergent. Applying the herbicide too early, even post-germination, can damage the delicate, immature root systems of the new grass. A reliable guideline is to wait until the new grass seedlings have been mowed a minimum of three to four times. This ensures the grass has developed a strong enough root system to withstand the chemical application.