A crabgrass preventer is a pre-emergent herbicide designed to stop certain weed seeds from establishing in the soil. Grass seed requires successful germination, which involves absorbing water and developing a root system before the shoot emerges. Most traditional crabgrass preventers will stop the successful germination of desirable grass seed. This interference occurs because the herbicide’s action is indiscriminate, affecting any seed’s ability to sprout and grow roots once the chemical barrier is present.
How Crabgrass Preventers Work
Pre-emergent herbicides do not kill existing plants but instead create a thin, chemical barrier just beneath the soil surface. This layer acts as a chemical shield, targeting seeds as they begin to germinate. The active ingredients work by inhibiting the process of cell division, specifically in the root tips of the emerging seedlings.
When a seed absorbs water and germination begins, the initial root, or radicle, pushes out. As this root encounters the herbicide barrier, the chemical prevents its cells from dividing and elongating. The seedling is unable to establish a functional root system, causing it to die shortly after germination, often before it ever breaks the soil surface.
Managing Standard Preventers and Waiting Periods
Common pre-emergent herbicides, such as those containing prodiamine, pendimethalin, or dithiopyr, are designed for residual control, meaning they remain active in the soil for an extended period. This long-lasting effect is beneficial for season-long crabgrass prevention but presents a conflict when attempting to sow new grass seed. The chemical must degrade sufficiently before new seed can safely be planted.
The residual life of these chemicals varies significantly based on the specific active ingredient, the application rate, and environmental factors like temperature and moisture. Consequently, the waiting period after applying a standard preventer before new seeding can occur typically ranges from approximately two to four months. For example, some product labels recommend waiting 60 days, while others advise a full 12 to 16 weeks before attempting to overseed.
This extended delay is necessary because the herbicide must be broken down by soil microbes and moisture to a concentration that no longer inhibits root growth. If seeding is attempted too early, the seed will germinate, but the root will encounter the chemical and fail to establish. If a homeowner must seed sooner than the recommended period, they may need to physically disrupt the chemical barrier by heavily raking or aerating the soil to bring untreated soil to the surface before planting.
Pre-Emergent Options Safe for New Grass
Fortunately, not all pre-emergent herbicides pose a risk to newly sown turfgrass, offering a solution for those who need to seed and prevent crabgrass simultaneously. Specialized products containing the active ingredients mesotrione or siduron are formulated to allow the germination and establishment of certain turfgrass species. These chemicals operate on a different mode of action than standard preventers, providing selective control.
Mesotrione, commonly found in products labeled for use at the time of seeding, is one such exception. This herbicide works by inhibiting a specific enzyme pathway in susceptible plants, causing them to turn white and eventually die, yet it is tolerated by most common turfgrass seedlings. Its application window is flexible, often allowing for use immediately before or at the time of seeding, but its residual control is shorter, typically lasting only 21 to 30 days.
Siduron is another specialized chemical that provides crabgrass prevention without harming most cool-season grass seedlings, like Kentucky bluegrass or perennial ryegrass. Products containing this ingredient are designed to be applied directly when seeding, allowing the homeowner to address both weed control and turf establishment in a single step.