Crabgrass preventer is a pre-emergent herbicide designed to create a chemical barrier in the top layer of soil, stopping weed seeds from successfully sprouting. The core problem is that most of these herbicides cannot differentiate between an unwelcome crabgrass seed and a desirable turfgrass seed, creating a conflict when seeding a lawn. The length of time you must wait before seeding depends entirely on the specific chemical ingredient used in the product.
Why Crabgrass Preventers Stop Grass Seed
Pre-emergent herbicides work by targeting the initial stages of a seed’s life cycle, specifically inhibiting cell division and root development in germinating seeds. When a seed encounters the treated soil layer and begins to sprout its first root, the chemical is absorbed, causing the seedling to die before it can establish itself. This chemical mechanism is not selective.
The protective barrier is designed to remain active in the soil for an extended period to provide season-long weed control. If turfgrass seed is planted while this barrier is still effective, the desirable seeds will absorb the herbicide. This results in the failure of the new grass to germinate, leading to wasted seed and a sparse lawn.
Determining Your Required Waiting Period
The waiting period is determined by the active ingredient and its residual activity in the soil. Common active ingredients are Prodiamine, Dithiopyr, and Pendimethalin, which provide extended weed prevention. These chemicals must break down to a concentration level that no longer interferes with turfgrass seed germination.
For products containing Prodiamine or Pendimethalin, the standard recommendation is to wait 8 to 16 weeks (two to four months) after application before seeding. Dithiopyr-based products often have a similar waiting period. The exact waiting time is influenced by product concentration, soil type, and environmental factors like temperature and rainfall, which affect chemical degradation. Always check the specific product label, as it provides the most accurate information on re-seeding intervals.
Preventers That Allow Immediate Seeding
There are specific exceptions to the long waiting period, primarily involving the active ingredient Mesotrione. This herbicide works differently from traditional pre-emergents because it targets a specific enzyme in weeds involved in photosynthesis, causing them to turn white and die. Unlike Prodiamine or Dithiopyr, Mesotrione allows many common cool-season turfgrasses, such as Kentucky bluegrass, tall fescue, and perennial ryegrass, to germinate and establish successfully.
Products containing Mesotrione, often sold under brand names like Tenacity, can be applied immediately before or at the time of seeding. This provides a solution for homeowners who need to plant new grass in the spring but also require crabgrass control. Another, less common alternative is Siduron, which offers a shorter period of crabgrass control but is also generally safe for use when seeding. Using these specialized products allows for simultaneous weed prevention and lawn renovation.
Maximizing Success After Waiting
Once the required waiting period for the traditional crabgrass preventer has passed, taking specific steps will maximize the success of the new seeding effort. Preparing the soil is an important first step, often involving core aeration or dethatching to loosen compacted soil and improve seed-to-soil contact. If the herbicide barrier was particularly strong, lightly cultivating the top half-inch of soil can help break up any residual chemical layer.
Applying a starter fertilizer that is high in phosphorus will provide the necessary nutrients to encourage strong root development in the new seedlings. The most important factor, however, is the watering schedule, which must be light and frequent. New seeds should be misted several times a day to keep the top layer of soil consistently moist until the grass sprouts, usually for the first two to three weeks.
For cool-season grasses, seeding in the late summer or early fall is ideal. Soil temperatures are naturally conducive to germination and establishment before the next season’s crabgrass pressure returns.