Pre-emergent herbicides are chemical products designed to create a protective barrier in the soil, preventing weed seeds from successfully sprouting. When considering lawn maintenance, the most direct answer to whether a standard pre-emergent will kill grass seed is yes. These herbicides are non-selective in their impact on germinating seedlings, meaning they cannot distinguish between undesirable crabgrass seeds and desirable turfgrass seeds. Applying a standard pre-emergent product near the time of seeding will prevent new grass from establishing itself.
How Standard Pre-Emergents Work on Grass Seed
Standard pre-emergent herbicides do not prevent a seed from germinating; instead, they stop the seedling immediately after germination occurs. The herbicide is applied to the soil surface before the target weeds sprout, where it forms a chemical zone. Once a seed germinates, the tiny root structure, known as the radicle, emerges and begins to absorb water and nutrients.
When this root or shoot absorbs the herbicide, the chemical interferes with the process of cell division, or mitosis. By halting the rapid cell division necessary for a seedling’s initial growth, the herbicide stops the plant’s development, causing the newly sprouted plant to die before it can emerge from the soil. This mechanism explains why applying a pre-emergent after a weed has fully emerged is ineffective, as the product is designed to affect the delicate, developing root system.
Because the chemical’s mode of action targets the cell division process common to all germinating seeds, it poses a significant threat to new turfgrass. The barrier cannot differentiate between the seeds you want to grow and the weed seeds you want to eliminate. Therefore, applying chemicals like Prodiamine or Dithiopyr will result in the loss of both weed and grass seedlings. Established, mature turfgrass plants are unaffected because their root systems are already fully developed and not dependent on rapid cell division for initial growth.
Understanding Residual Soil Activity
Residual soil activity is the length of time the pre-emergent chemical remains active in the soil, continuing to inhibit cell division. This duration is a factor when planning to seed a lawn after an herbicide application. The chemical’s persistence is necessary to provide season-long weed control, but it also dictates the required waiting period before new grass seed can be safely planted.
The duration of residual activity depends heavily on the specific active ingredient, the application rate, and environmental factors like soil type, moisture, and temperature. Common ingredients like Pendimethalin and Prodiamine can remain active for six to twelve weeks, and sometimes longer. Warm soil temperatures and high microbial activity lead to faster breakdown, while cooler temperatures prolong the chemical’s life.
It is necessary to consult the product label for the exact required waiting period before seeding. The label provides specific guidance for the active ingredient and application rate, which is the reliable way to prevent the chemical from damaging new grass seed. Seeding before the residual activity has dissipated will result in poor germination and wasted seed.
Options for Seeding and Weed Control Simultaneously
For situations requiring both weed control and seeding in the same season, chemical and cultural strategies can bypass the limitations of standard pre-emergents. The primary chemical exception is the active ingredient Mesotrione. This herbicide works differently than standard pre-emergents, providing a window of safety for newly sown turfgrass.
Mesotrione works by inhibiting an enzyme called HPPD, which is necessary for pigment production in certain plants. This unique mode of action allows desirable cool-season grasses to germinate and grow, while susceptible weeds turn white, or “bleach,” and die. This chemical can be applied at the time of seeding, providing pre-emergent control against many broadleaf and grassy weeds without harming the new turf.
Cultural Practices for Weed Suppression
Beyond specific herbicides, cultural practices aid in suppressing weeds during the establishment phase. Applying a starter fertilizer provides the new grass with the immediate nutrients needed for rapid establishment, helping it to outcompete any weeds that sprout. Maintaining consistent, light irrigation encourages quick germination, allowing the new turf to fill in bare spots before weeds take hold.
Mowing the new lawn at a higher height of cut (typically three inches or more) creates a dense canopy that shades the soil surface. This shading limits the sunlight available to weed seeds, many of which require light to germinate. By combining a seed-safe herbicide like Mesotrione with these cultural practices, you can establish a thick lawn while minimizing weed pressure.