Renovating a lawn often requires two tasks: eliminating weeds and filling in bare patches with new turf. Many homeowners turn to “Weed & Feed” products, which combine fertilizer to nourish established grass with herbicides to kill undesirable plants. Planting grass seed, or overseeding, repairs thin areas and increases turf density. However, the chemicals intended to kill weeds actively interfere with the success of new grass growth.
Why Weed and Feed Products Hinder New Grass
The primary conflict stems from the herbicide component of the “Weed & Feed” formula. These products often contain pre-emergent herbicides designed to stop weed seeds from establishing themselves in the soil. They work by creating a microscopic chemical barrier near the soil surface. When any seed begins to germinate and sends out its first root, it absorbs the herbicide.
This chemical absorption inhibits the cellular division necessary for the young plant to develop a root system and shoots. This mechanism is non-selective; it cannot distinguish between weed seeds and desirable grass seeds. While the fertilizer component benefits existing grass, the weed-killing agent presents a significant obstacle to any new seeding effort.
Strategy 1: Prioritizing New Grass Seed
If the objective is to quickly establish new grass in large, thin areas, seeding must happen first. The optimal time is typically late summer to early fall when soil temperatures are ideal for germination. Preparation involves raking the soil to expose bare spots and ensure good seed-to-soil contact.
After spreading the seed, apply a starter fertilizer. This fertilizer is formulated specifically for new lawns and is high in phosphorus, which promotes strong root development in young seedlings. Crucially, starter fertilizer does not contain herbicides, ensuring the new grass receives nutrients without chemical damage. The newly seeded area must then be kept consistently moist with light, frequent watering to encourage sprouting.
Strategy 2: Prioritizing Weed Elimination
When a lawn is heavily infested with weeds, applying “Weed & Feed” is the preferred initial step. This strategy clears the competition so new grass can be planted into a less hostile environment later. Most formulas contain post-emergent herbicides, which are absorbed through the leaves of actively growing weeds, such as clover or dandelions, killing the existing plants.
Some “Weed & Feed” products also include pre-emergent chemicals for preventing new weed seeds from germinating. For post-emergent action to be effective, the product is typically applied to damp grass so the granular herbicide sticks to the weed leaves. Following application, the lawn should not be mowed for several days, giving the herbicide time to move throughout the weed’s system.
Essential Waiting Periods and Follow-Up Care
The decision to weed first or seed first sets the timeline for the subsequent step. If applying a “Weed & Feed” containing pre-emergent herbicides, wait typically four to twelve weeks before planting new grass seed. This delay allows the herbicide to break down in the soil. Always consult the product label, as specific formulations may require longer waiting times.
Conversely, if new grass seed is planted first, a waiting period is required before applying any broadleaf weed killer. New grass seedlings are sensitive and can be damaged by post-emergent herbicides until they are well-established. It is recommended to wait until the new grass has been mowed at least three times, or roughly four to eight weeks after germination, before applying weed control. Newly seeded areas require consistent, shallow watering multiple times per day, and the first mowing should occur when the blades reach three to four inches.