Weeds in a newly established lawn are a common problem for homeowners. Whether you have chosen to plant grass from seed or lay down instant sod, the presence of unwanted growth creates a desire to quickly apply weed killer. However, the delicate state of new turf is highly vulnerable to chemical stress, meaning that precise timing is paramount. Applying any herbicide too soon can severely damage or destroy your young lawn, making the correct waiting period the most important factor in successful weed control.
Understanding Herbicide Types
The timing rules for new grass are dictated by the mechanism of the two main types of lawn herbicides.
Pre-emergent herbicides prevent weed seeds from germinating by creating a chemical barrier in the top layer of the soil. If a weed seed attempts to sprout, it absorbs the chemical and dies before it emerges. This barrier is non-selective and will also prevent desirable grass seed from growing.
Post-emergent herbicides kill existing, visible weeds that have already sprouted. These products are absorbed through the leaves and translocated down to the root system. While many are selective and target broadleaf weeds without harming established grass, young plants lack the maturity to tolerate the chemical stress.
Timing Requirements for Newly Seeded Lawns
A lawn grown from seed is the most susceptible to herbicide damage and requires the longest waiting period. The primary concern is allowing the fragile grass seedlings to develop a sufficient root system and leaf surface area to survive the chemical application.
For applying a post-emergent herbicide, wait until the new grass has been mowed at least two or three times. This mowing threshold indicates that the seedlings are mature enough to have a developed root structure and enough leaf tissue to process the herbicide. Depending on growth rate, this waiting period typically lasts 30 to 60 days after germination.
Applying a standard pre-emergent herbicide is discouraged in newly seeded areas because it prevents future grass seed from growing. The only exception is the active ingredient mesotrione, which is safe to apply at the time of seeding. For all other pre-emergents, wait until the lawn is fully established, often a full growing season, before application.
Timing Requirements for Newly Installed Sod
Sod presents different establishment challenges than seed, as the grass blades are already mature, but the root system is not. The primary vulnerability of new sod is its ability to successfully root into the soil underneath the turf.
For post-emergent herbicides, the waiting period is generally shorter than with seed. Selective weed killers can typically be used once the sod has successfully rooted into the underlying soil, which often occurs within a two to four-week window. To check for root establishment, gently lift a corner of a sod piece; if it resists lifting, the roots are anchoring, and application can be considered.
Avoid applying standard pre-emergent herbicides until the sod’s root system is robustly established, which can take six weeks or longer. Pre-emergents inhibit root growth, and applying them too early prevents the sod from forming the deep, strong roots necessary for long-term health.
Safe Application Practices and Product Selection
Once the appropriate waiting period has passed, safe application practices are necessary to prevent damage to the young turf. Always begin by thoroughly reading the product label, specifically looking for language that confirms the herbicide is safe for newly seeded or sodded lawns. Many product formulations are designed for young turf and detail the exact waiting period required for a given grass type.
Always spot-treat individual weeds initially instead of applying a broadcast application over the entire lawn. This targeted approach minimizes chemical stress placed on the young grass plants.
Post-emergent herbicides are most effective when applied to actively growing weeds, ideally when daytime temperatures are between 65 and 85 degrees Fahrenheit. Avoid applying herbicides during drought stress or when temperatures exceed 90 degrees, as this increases the likelihood of turf injury. Application should also be avoided on windy days to prevent chemical drift. Finally, allow the herbicide a few days to absorb into the weeds before mowing or watering, as specified on the product label.