“Weed and Feed” is a convenient, dual-purpose lawn care product that combines fertilizer to nourish existing turf with a herbicide designed to eliminate unwanted plants. The answer to whether this product kills grass seed is yes. The chemicals included in the “weed” component are non-selective regarding their target and will severely inhibit or completely prevent the germination of new grass seed. Applying this combined product simultaneously with seeding, or too close to the seeding date, will lead to failure of the new lawn establishment.
How Weed and Feed Herbicides Affect Grass Seedlings
The detrimental effect of Weed and Feed on new grass establishment stems primarily from the action of its herbicides, which are generally categorized as pre-emergent or post-emergent. Pre-emergent herbicides are the most problematic for new seed, as their function is to create a chemical barrier in the soil that prevents seeds from successfully sprouting. These chemicals work by inhibiting cell division in the root or shoot of the emerging plant.
This mechanism cannot distinguish between the seed of a weed, such as crabgrass, and the seed of desirable turfgrass. The chemical barrier prevents the delicate root and shoot structures of the new grass seed from developing properly after germination, effectively killing the seedling before it can establish.
The residual activity of these pre-emergent chemicals can remain in the soil for an extended period, continuing to inhibit any new seeding attempts.
Post-emergent herbicides, which are designed to kill actively growing broadleaf weeds, present a serious risk to young seedlings. Ingredients like 2,4-D or dicamba are systemic, meaning they are absorbed by the plant and translocated throughout its tissues, disrupting growth hormones. Immature grass seedlings, with their thin cell walls and limited root systems, are highly susceptible to damage from these chemicals, even if they manage to germinate.
The young grass blades lack the maturity and protective waxy cuticle of established turf, making them vulnerable to chemical burn or growth distortion from the post-emergent components. Even if the herbicide does not cause immediate death, it can severely stunt the seedling’s growth, making it less competitive against existing weeds and environmental stresses.
When to Apply Weed and Feed Before or After Seeding
The timing of application is crucial and depends entirely on the specific herbicide ingredients in the Weed and Feed product. If the product contains a pre-emergent herbicide, applying it before seeding requires a significant waiting period to allow the chemical to break down in the soil. For many common pre-emergent ingredients, this required waiting period before new grass seed can be safely planted ranges from two to four months.
Certain long-lasting pre-emergent formulas, particularly those designed for season-long control of weeds like crabgrass, may require a waiting period extending up to nine months. The herbicide’s active ingredient and its half-life determine the precise breakdown time. It is mandatory to read the product label for the manufacturer’s specific seeding interval, as ignoring this interval will result in the total failure of the grass seed to germinate.
Applying Weed and Feed after seeding also requires a waiting period until the grass is fully established and mature enough to tolerate the herbicide. The general rule is to wait until the new turf has been mowed at least three to four times, which typically takes six to eight weeks after germination. By this time, the grass has developed a robust root system and thicker leaf tissue, giving it the necessary resilience to withstand the post-emergent herbicide component.
Attempting to apply a Weed and Feed product to a lawn that has only just begun to sprout will likely result in the death of the young grass plants due to chemical sensitivity. For the fertilizer component, a specialized starter fertilizer without any herbicides should be used immediately after seeding to provide beneficial nutrients.
Safer Weed Control Options During Lawn Establishment
To effectively manage weeds while establishing a new lawn, alternative strategies that isolate the fertilization process from the herbicide application should be employed. The most straightforward method is to use a starter fertilizer that contains no herbicides. This ensures the new seedlings receive the necessary phosphorus and other nutrients without chemical interference. This approach feeds the new grass without introducing any growth-inhibiting compounds into the vulnerable soil.
For immediate weed control during the seeding period, manual removal, such as hand-pulling, is the safest non-chemical option. While labor-intensive, this method eliminates competing weeds without distributing any herbicides that could harm the delicate grass roots. This is particularly effective for removing large, established weeds before they can go to seed and further contaminate the soil.
If chemical control is necessary, there are specialized herbicides that are specifically labeled as safe for use during the seeding process. Products containing the active ingredient mesotrione are often used. This herbicide can control certain weeds both before and after they emerge while allowing certain new grass seeds to germinate and grow. This targeted approach offers the benefit of weed control with significantly reduced risk to the new turf.