Overseeding involves introducing new grass seed to an existing lawn to improve its density and appearance. A “weed and feed” product combines fertilizer with a weed-controlling herbicide in a single application. Combining these steps requires careful planning, as the chemicals designed to eliminate weeds can severely harm the new, fragile grass seedlings. Proper timing is paramount to ensure the successful establishment of the new turf without causing damage.
The Critical Waiting Period After Overseeding
The most practical guideline for applying weed and feed after overseeding is to wait until the new turf has reached maturity. A general timeframe is four to eight weeks following germination, but the true indicator is physical development. A more reliable measure is the “three-mow rule,” requiring seedlings to be mowed a minimum of three times before applying any broadleaf herbicide.
The need for this delay is directly related to the development of the grass’s root system and leaf structure. Young grass plants must establish a robust root network to withstand the stress of chemical application. If the herbicide is applied too soon, the delicate seedlings lack the physiological defenses and root anchorage necessary for survival, leading to widespread die-off.
This recommended waiting period allows the seedlings to transition from their initial fragile state to a more resilient, mature turf. Waiting for the third or fourth mowing ensures the grass blades are actively growing and the root system is sufficiently developed to absorb nutrients without collapsing under the chemical stress. Ignoring this period risks negating the effort and cost of the entire overseeding process.
Understanding the Conflict Between Herbicides and New Grass
The “weed” component is typically a post-emergent broadleaf herbicide designed to disrupt growth processes in target plants. These herbicides (e.g., 2,4-D, dicamba, MCPP) are often synthetic auxins that overload the weed’s system, causing erratic growth and death. Immature grass seedlings, though generally tolerant when mature, are highly sensitive to these chemicals during early development.
The young grass blades readily absorb the herbicide, which can interfere with the formation of the plumule and disrupt the establishment of a strong root structure. Applying these chemicals too early can inhibit root growth, preventing the plant from properly anchoring itself and absorbing water and nutrients.
The high nitrogen content in the “feed” portion can also contribute to plant damage if applied excessively. While fertilizer helps growth, the concentration in a weed and feed formulation can chemically burn the tender leaf tissue of a young seedling recovering from germination stress.
The combination of growth-disrupting chemicals and high nutrient concentration creates an environment the immature turf cannot handle. The waiting period allows the grass to reach a stage where its physical structure can safely process the fertilizer and resist the damaging effects of the herbicide.
Timing Weed Control Before Overseeding
Controlling weeds before overseeding is a practical strategy, but it also demands a specific timeline. Many common weed control products, particularly pre-emergent herbicides, remain active in the soil for an extended period, and their residue will prevent the new grass seeds from germinating. Pre-emergent herbicides work by forming a chemical barrier near the soil surface that kills seeds as they sprout.
The required waiting period for seeding after a pre-emergent application can range widely, often from 6 to 12 weeks, and sometimes up to four months, depending on the specific chemical used. For example, a common crabgrass preventer containing ingredients like prodiamine or pendimethalin requires a substantial delay because the chemical must fully degrade before the new seeds can safely germinate.
However, certain specialized herbicides, such as those containing mesotrione, are exceptions. These products can sometimes be applied on the same day as seeding because they selectively target weeds without harming new grass seedlings. For typical broadleaf weed and feed products applied before seeding, a waiting period of at least 30 days is recommended to allow the active chemicals to dissipate sufficiently and prevent suppression of the desired grass seed.