The term “weed and feed” refers to a granular lawn product that combines a fertilizer to nourish the grass with a weed killer, or herbicide, to eliminate unwanted plants. The fertilizer component promotes a dense, green lawn, but the herbicide is the part that dictates the long waiting period before new grass seed can be successfully planted. The chemicals designed to kill weeds do not distinguish between weed seeds and your desirable grass seeds, making it impossible for the new seeds to germinate or for the seedlings to survive initial growth. The waiting time is necessary to allow the active herbicide ingredients to break down in the soil into harmless compounds.
The Mechanism That Prevents Germination
Selective herbicides commonly found in weed and feed products, such as 2,4-D, MCPP (Mecoprop-p), and Dicamba, function as synthetic auxins. Auxins are natural plant hormones that regulate growth and development, and these synthetic versions cause uncontrolled, abnormal growth in broadleaf weeds, leading to their eventual death. Grass plants, which are monocots, are generally tolerant of these chemicals because they possess specialized cells that prevent the herbicide from disrupting their internal vascular structure.
However, grass seedlings are extremely sensitive to these chemicals because they have not yet developed the necessary protective mechanisms to withstand the herbicide’s effects. The delicate process of germination and early root growth is easily disrupted by the lingering presence of the synthetic auxins in the soil. Even if the seeds sprout, the young shoots may show symptoms like twisting or stunting before dying. The breakdown of these chemicals in the soil is primarily facilitated by soil microbes and environmental factors like moisture and temperature.
Standard Waiting Periods for Common Herbicides
The most common weed and feed products contain post-emergent herbicides, which are designed to kill visible, actively growing broadleaf weeds. Active ingredients like 2,4-D, MCPP, and Dicamba are included in these formulations and require a specific time to dissipate before seeding. The general recommendation for planting grass seed after applying a product with these post-emergent chemicals is to wait a minimum of three to four weeks.
This waiting period allows the chemical compounds to degrade sufficiently in the soil. Always consult the specific product label, as it may specify a different timeline based on the exact chemical concentration and formulation. While some formulations may allow seeding sooner, four weeks is a reliable minimum. Seeding too soon risks poor germination and patchy growth.
The Extended Wait Time for Pre-Emergent Products
When the weed and feed product contains a pre-emergent herbicide, used to prevent annual weeds like crabgrass, a different scenario arises. Pre-emergent chemicals like Prodiamine or Dithiopyr (Dimension) work by establishing a chemical barrier near the soil surface. Instead of killing the seed, they interrupt cell division in the tiny seedling as it begins to grow, effectively killing it before it can emerge from the soil.
Because these herbicides are specifically designed to remain active in the top layer of the soil for an extended duration, the necessary waiting time before seeding is significantly longer. The residual effect of these chemicals can last for several months, often requiring a wait of 60 to 120 days before new grass seed can be safely planted. Applying seed prematurely will cause the pre-emergent chemical to kill your desirable grass seedlings just as it would kill crabgrass.
Steps to Prepare the Soil for Seeding
Once the waiting period has passed, you can take steps to maximize the success of your new grass seed. Encourage the breakdown of remaining chemical residue through consistent watering. Frequent, deep watering during the waiting period helps to leach the herbicide deeper into the soil profile and promotes microbial activity.
Before spreading seed, lightly dethatching or aerating the soil can improve seed-to-soil contact and encourage better root penetration. Consider the soil temperature, which is a more accurate indicator for germination than the air temperature. Cool-season grasses, for instance, germinate best when the soil temperature is in the 60 to 75 degree Fahrenheit range, typically corresponding to late summer or early fall seeding. Applying a starter fertilizer that is free of herbicides at the time of seeding will provide the young plants with the necessary nutrients for strong initial growth.