Yes, you should kill existing weeds before seeding your lawn. Successful lawn establishment depends on minimizing competition for resources, and the presence of weeds will severely limit the growth of new grass. This preparatory step is necessary because most effective weed control products cannot be applied to young, vulnerable grass seedlings without causing significant damage. The goal is to provide the new grass seeds with an environment where they can germinate and develop a robust root system.
Why Pre-Seeding Weed Control Is Necessary
Weeds aggressively compete with new grass seedlings for water, light, and nutrients, which are limited resources in a newly seeded area. Grass seedlings are initially weak and cannot outcompete established weeds that have extensive root systems and canopy coverage. Weeds often absorb essential nutrients, such as nitrogen, phosphorus, and potassium, more rapidly than young turfgrass, effectively starving the new growth.
Some weeds also employ a biological strategy called allelopathy, releasing chemical compounds into the soil that actively inhibit the germination and growth of surrounding plants. This chemical interference can significantly reduce the viability of your new grass seed, leading to thin, patchy results. Skipping pre-seeding weed removal means consistent watering will also encourage dormant weed seeds already present in the soil to sprout alongside the grass.
Once grass is seeded, the homeowner is temporarily unable to use the most effective weed killers without harming the new turf. Selective herbicides, which target broadleaf weeds but spare grass, are too potent for tender grass seedlings and must be avoided for several weeks after germination. Pre-emergent herbicides, designed to prevent weed seeds from sprouting, will also stop the desirable grass seeds from germinating.
Choosing the Right Weed Removal Method
The most effective method for total lawn renovation is chemical control using a non-selective herbicide, such as glyphosate. This product works systemically, absorbing through the leaves and translocating to the roots to kill the entire plant. Because glyphosate breaks down rapidly upon contact with soil, it poses little residual threat to new seeds once the waiting period is observed.
For maximum effectiveness, especially against tough perennial weeds with extensive root systems, a seven-day waiting period is recommended between application and any soil disturbance. This delay allows the herbicide enough time to fully move throughout the plant and kill the roots, preventing regrowth. Homeowners should always check the product label, as some formulations contain ingredients that can persist in the soil and harm new seeds.
Non-chemical options are also effective, particularly for smaller areas or organic approaches. Soil solarization involves covering moistened soil with clear plastic sheeting for four to six weeks during the hottest part of the year. The plastic traps heat, raising the soil temperature high enough to kill existing weeds, weed seeds, and pathogens.
Manual and mechanical removal, such as tilling, must be approached with caution. Tilling deeply brings dormant weed seeds from lower soil layers up to the surface, where they readily germinate. If tilling is used, it should be followed by a second kill step, such as a light re-application of herbicide, to eliminate the new flush of weed sprouts before seeding.
Essential Soil Preparation Before Seeding
Once the existing weeds are dead, the next stage involves preparing the soil to create an optimal seedbed environment. Physical preparation begins by removing all the dead plant matter, or thatch, to ensure the new seed can make direct contact with the soil. This step is vital for successful germination, as seeds left on top of debris will dry out quickly.
A soil test should be conducted to determine the soil’s current pH and nutrient levels, guiding any necessary amendments. Grass thrives in a relatively neutral pH, and adjustments may require adding lime for acidic soil or sulfur for alkaline soil. Incorporating organic matter, such as compost, into the top few inches of soil can improve water retention and provide a slow release of nutrients.
Aeration is beneficial, especially in compacted areas, as it creates small holes that allow air, water, and nutrients to penetrate the root zone. Leveling the soil surface with a rake ensures a uniform grade and prevents water from pooling in low spots, which could wash away the new seeds. After any deep soil work, a light rolling or watering should firm the surface slightly.
Understanding the Waiting Period and Pre-Emergent Constraints
Following the application of a non-selective herbicide like glyphosate, a waiting period of a few days up to two weeks is generally required before seeding. This interval allows the herbicide to fully break down in the soil, preventing any lingering residue from harming the new seeds. The specific waiting time depends on the product concentration, the weather, and the sensitivity of the grass seed variety being used.
A more significant constraint involves pre-emergent herbicides, which are designed to form a chemical barrier in the soil to stop weed seeds from sprouting. These products cannot be applied concurrently with grass seed because they cannot distinguish between weed seeds and desirable turfgrass seeds. Applying a pre-emergent too soon will prevent the new lawn from growing entirely.
Traditional pre-emergent products can remain active in the soil for three to four months, meaning no seeding can occur during that time. If seeding is done first, the lawn must reach a level of establishment—typically after six to eight weeks, or after the grass has been mowed several times—before a pre-emergent can be safely applied. This delay is necessary to ensure the young grass is mature enough to withstand the treatment without its root development being inhibited.