“Weed and Feed” is a popular lawn care product combining fertilizer to nourish existing grass and herbicide to control weeds. Overseeding is the process of spreading new grass seed over an existing lawn to fill in bare spots and improve density. The desire to perform both tasks simultaneously is common, aiming for a quick solution to thin grass and weed problems. This convenience introduces a direct conflict because the chemical designed to eliminate unwanted plants cannot distinguish between a weed seed and a desirable grass seed.
The Immediate Conflict
The straightforward answer to combining these two lawn care tasks is that it should generally be avoided. The weed-killing component of the “Weed and Feed” product is the primary reason why simultaneous application fails. This herbicide is formulated to prevent growth or actively kill existing plants that are not established turfgrass. Applying it alongside new grass seed will almost certainly destroy the seeds before they have a chance to germinate and establish.
The chemicals are designed to remain active in the soil for a period of time to provide ongoing weed control. If new grass seed is introduced during this active window, the herbicide treats the seed as an invasive plant. This prevents the new grass from ever sprouting, resulting in wasted seed and an unsuccessful attempt at thickening the lawn.
How Herbicides Prevent Germination
The mechanism behind this conflict depends on the type of herbicide included in the “Weed and Feed” formula. Products often contain a pre-emergent herbicide, which creates a chemical barrier in the top layer of the soil. This barrier stops seeds from successfully germinating and developing roots, effectively preventing weeds like crabgrass from sprouting. Unfortunately, the pre-emergent cannot differentiate between weed seeds and newly scattered grass seeds, so it halts the growth of both indiscriminately.
Another component can be a post-emergent herbicide, which is designed to kill weeds that have already emerged, such as dandelions or clover. These chemicals are typically absorbed through the leaves of the target plant. While post-emergent herbicides do not create the same long-lasting soil barrier as pre-emergents, they can still harm delicate, newly sprouted grass seedlings. Young grass is particularly vulnerable to chemical treatments that established turf can tolerate.
Safe Alternatives for Combined Care
For homeowners who want to fertilize and seed close to the same time, there are safe alternatives to traditional “Weed and Feed” products. The safest approach is to use a starter fertilizer alongside the grass seed, as these products contain the necessary nutrients but no herbicides. Starter fertilizers are formulated with a higher phosphorus content, which promotes strong root development in new seedlings. This nutrient profile supports the young grass without introducing any growth-inhibiting chemicals.
There are also specialized, seeding-safe weed control products available that can be used concurrently with new grass seed. One notable example is the herbicide mesotrione, often sold under the brand name Tenacity, which can be applied at the time of seeding. This chemical works by inhibiting photosynthesis in certain weeds, causing them to turn white and die, while newly planted grass seeds remain unaffected. By selecting a weed control product specifically labeled as safe for new seed, you can address existing weeds without jeopardizing germination, but always verify safety for your specific grass type.
Establishing the Correct Lawn Care Sequence
If you choose to use a standard “Weed and Feed” product, separating the application from seeding with a specific waiting period is the best practice. When the herbicide is applied first, you must allow the chemical to break down in the soil before introducing new seed. The required waiting period is typically between four to eight weeks, depending on the specific herbicide ingredients and the manufacturer’s instructions. This waiting time ensures the herbicide residue will not interfere with the seed germination process.
Conversely, if you decide to seed first, you must wait until the new grass is well-established before applying a broadleaf weed killer. A general guideline is to wait until the new seedlings have grown enough to have been mowed at least three to four times. This usually takes approximately six to eight weeks after germination. Waiting allows the young grass plants to develop a robust root system and mature leaves, making them strong enough to tolerate the stress of the herbicide application without being severely damaged.