Overseeding involves spreading new grass seed over an existing lawn to improve density and fill in bare areas without completely tearing up the existing turf. Homeowners often ask if fertilization can be combined with seeding to save time. The answer is yes, provided you use a specific type of product designed for new growth. Applying the correct nutrients during seeding is crucial for successful establishment. The proper selection and application of fertilizer will determine if the seeds germinate into a thick, healthy lawn or fail entirely.
The Necessity of Starter Fertilizer
New grass seedlings require nutrients distinct from those needed by mature lawns. This specialized product is called a starter fertilizer, formulated to support the fragile beginning stages of plant life. Starter fertilizers are characterized by a high N-P-K ratio in the middle number, representing phosphorus (P). Common formulas, such as 10-20-10 or 18-24-12, indicate a higher percentage of phosphorus relative to nitrogen (N) and potassium (K).
Phosphorus is an essential macronutrient that directs energy toward developing a robust root system beneath the soil surface. A strong initial root structure is crucial for anchoring the plant and enabling it to efficiently absorb water and other nutrients as it matures. Since phosphorus is relatively immobile in the soil, applying it directly at the time of seeding ensures the young roots have immediate access to this necessary element.
Nitrogen, the first number in the ratio, primarily promotes foliage growth above the soil. Too much nitrogen too early causes seedlings to prioritize grass blades before roots are established. This creates a weak plant highly susceptible to stress from drought, heat, or disease. Starter fertilizer provides the necessary nitrogen for initial greening while prioritizing the phosphorus needed for subterranean development.
Common Fertilizers That Harm New Seed
Using the wrong product is the most significant mistake when combining seeding and fertilization, as it can destroy seeds or young sprouts. Fertilizers containing pre-emergent herbicides, often sold as “Weed and Feed,” must be avoided entirely during overseeding. Pre-emergent herbicides create a chemical barrier that prevents all seeds, including grass seeds, from successfully germinating and forming roots.
Standard lawn fertilizers designed for established turf also cause damage due to their high nitrogen content and quick-release formulation. These products contain mineral salts, such as ammonium sulfate, which create a salt desiccation effect in the soil. High concentrations of these salts draw moisture away from the newly sown grass seed via osmosis. This dehydration prevents germination or causes lethal “fertilizer burn” to delicate seedlings.
Quick-release, high-nitrogen fertilizer can cause the grass blades to turn a scorched, straw-like color. Furthermore, the aggressive growth stimulated by high nitrogen leads to shallow roots that cannot support the rapidly expanding leaf mass. This results in a fragile lawn ill-equipped to handle environmental stresses.
Step-by-Step Combined Application
Successful overseeding requires preparing the existing lawn to ensure maximum seed-to-soil contact.
Preparing the Lawn
Mow the lawn lower than normal, typically 1.5 to 2 inches, so sunlight can reach the new seedlings and prevent shading from existing turf. After mowing, it is beneficial to dethatch the area or perform core aeration. Aeration removes plugs of soil and organic matter, relieving compaction and opening pathways for water and nutrients.
Application Process
Once the soil is prepared, apply the grass seed and starter fertilizer sequentially using a calibrated spreader for even coverage. Spreading the seed first, followed immediately by the starter fertilizer, is highly effective. The fertilizer granules help settle the seed into the small openings created by aeration or light raking, improving seed-to-soil contact.
Follow the recommended application rate on the starter fertilizer bag, aiming for 0.5 to 1 pound of nitrogen per 1,000 square feet. A slow-release starter formula is preferred, as it provides a consistent nutrient supply over a longer period and reduces the risk of a damaging salt surge. After spreading both materials, lightly rake the area to settle the seed and fertilizer into the soil surface.
Initial Care for Germinating Seedlings
After application, focus shifts entirely to maintaining consistent moisture in the upper soil layer. Successful germination requires frequent, light watering to keep the top inch of soil consistently moist, but never saturated or waterlogged. Depending on conditions, this may require watering two to three times daily for short cycles of five to ten minutes each. Allowing the seed to dry out during the germination period will likely kill the emerging sprouts.
Once the new grass reaches three to four inches, it is ready for its first mow. Temporarily reduce watering before mowing to allow the soil to firm up, preventing the mower wheels from sinking or disturbing the young plants. The mower blade must be sharp, and the height should be set to remove no more than one-third of the blade height.
As the grass matures, adjust the watering routine to encourage deep root growth. Gradually reduce the frequency and increase the duration of each watering to soak the soil six to eight inches deep. The second fertilizer application should occur after the new grass is well-established, typically four to eight weeks after seeding, using a standard lawn food product.