Crabgrass, a summer annual weed, aggressively competes with new grass seedlings for water, light, and nutrients. A single plant can produce over 150,000 seeds that remain viable in the soil for years. Successfully introducing new turf into a crabgrass-infested area requires a strategic approach that coordinates seeding with specific weed management practices. The goal is to establish a dense, healthy stand of turfgrass that can eventually outcompete the dormant crabgrass seeds.
Timing Your Lawn Project
The time of year selected for overseeding is the most important factor in the battle against crabgrass. Crabgrass seeds begin to germinate when the soil temperature reaches a sustained 55° Fahrenheit for several consecutive days, which typically occurs in the late spring or early summer. Seeding a lawn during this period subjects the fragile new grass to immediate and intense competition from the emerging crabgrass population.
The ideal window for introducing new grass seed is in the late summer or early fall, generally four to six weeks before the first expected frost. This timing allows the new turfgrass seedlings to establish a strong root system without the pressure of summer heat or the immediate threat of crabgrass germination. The cooler soil temperatures and increased moisture levels of the fall provide optimal conditions for the new grass to thrive.
Attempting to seed in the early spring is considered a secondary option and carries a much higher risk of failure. While the moderate temperatures are conducive to grass growth, they also coincide with the imminent germination of crabgrass. New seedlings will not be mature enough to withstand the summer heat or the application of the selective herbicides needed to control the crabgrass that will inevitably emerge.
Suppressing Existing Crabgrass Before Seeding
Existing, mature crabgrass plants must be eliminated before any new seed is applied, as a dense canopy will prevent new seedlings from reaching the soil and sunlight. Standard pre-emergent herbicides are ineffective against fully grown weeds. These mature crabgrass plants are summer annuals that will die naturally with the first hard frost, but removal is necessary for fall seeding.
For small, isolated patches, manual removal by hand or with a weeding tool is an effective solution, ensuring the entire plant is pulled before it drops new seeds. For larger areas, a post-emergent herbicide designed to target crabgrass is necessary, with products containing the active ingredient quinclorac being a common choice. Quinclorac is selective and works by interfering with the weed’s growth processes.
A waiting period is required between the application of a post-emergent herbicide and the distribution of new seed to prevent damage to the new turfgrass. A waiting time of seven days or more is often recommended to allow the chemical to dissipate or break down. The dead plant material left behind by the herbicide should be mowed and bagged or lightly raked to ensure the new grass seed can make direct contact with the soil.
Reseeding with Crabgrass Prevention in Mind
The physical process of reseeding must be integrated with specialized chemical control to ensure success. Before applying seed, the lawn must be prepared by mowing the existing turf to a low height and then performing light dethatching or core aeration to open up the soil surface. This preparation increases seed-to-soil contact, which is a significant factor in successful germination.
A unique challenge in this process is the need for a pre-emergent herbicide that will not harm the new grass seed. This is accomplished by using a seed-safe pre-emergent, most commonly one containing mesotrione. Mesotrione works by inhibiting photosynthesis in many weed seedlings, including crabgrass, but is formulated to be safe for most newly sown turfgrasses.
This seed-safe product is typically applied simultaneously with the grass seed or immediately afterward, often combined with a starter fertilizer to provide necessary nutrients. The seed must be distributed uniformly, and the area should be lightly raked to barely cover the seed, followed by a consistent watering regimen. Keeping the top layer of soil moist is crucial for germination and will also activate the pre-emergent barrier against any crabgrass seeds that try to sprout.
Post-Establishment Weed Management
After the initial seeding and germination, the focus shifts to nurturing the new grass until it is fully established, which is usually defined as having been mowed three to four times. This period is when the new turf is most vulnerable, and maintaining a dense stand of grass through proper cultural practices is the best defense against weed encroachment. Regular, deep, but infrequent watering promotes deep root growth, while mowing at the highest recommended height for the grass type helps shade the soil and suppresses weed growth.
Even with the use of a seed-safe product, some crabgrass or other weeds may emerge. It is important to resist the urge to apply traditional post-emergent or non-seed-safe pre-emergent controls too early, as these chemicals can severely damage the developing turf. The three-to-four-mow rule or a specific time frame, often 60 days or more, must be strictly followed before the new grass is robust enough to handle the more aggressive herbicides.
Once the lawn is established, the long-term strategy involves applying a traditional, non-seed-safe pre-emergent herbicide early the following spring, before soil temperatures reach the 55°F threshold. This application targets the crabgrass seeds that survived the previous season and sets up a protective barrier for the mature turf. A dense and well-maintained lawn remains the most effective long-term deterrent against all weeds.