The general answer is yes, you can and should apply fertilizer to new grass seed, but only a special type formulated for new growth. Using the wrong fertilizer can seriously damage your efforts to establish a healthy lawn.
Understanding Starter Fertilizers
Starter fertilizers are distinct from general lawn products used on established turf. They focus on nurturing the delicate early stages of grass development by providing nutrients directly to the seeds and new roots while they are most vulnerable. The different ratio of nutrients is the primary distinguishing factor between starter and regular fertilizers.
The three numbers on a fertilizer bag represent the percentage of Nitrogen (N), Phosphorus (P), and Potassium (K)—known as the NPK ratio. Starter fertilizers feature a significantly higher middle number, indicating a greater percentage of phosphorus. Common ratios often appear as 10-20-10 or 18-24-12, as this high phosphorus content is the most important element for new grass.
Phosphorus is directly involved in the energy transfer within plant cells and is the nutrient necessary for root development. For a new seedling, establishing a robust, deep root system is more important than rapid leaf growth. Strong roots allow the young grass to absorb water and nutrients more effectively, helping it survive environmental stresses like drought and heat.
While nitrogen promotes green, leafy growth, applying too much to a seedling encourages fast top growth at the expense of root development, creating a weak plant. The balanced starter formula provides just enough nitrogen for initial growth while supporting the deep-root foundation with phosphorus. Potassium, the third nutrient, helps promote disease resistance and overall hardiness.
Applying Fertilizer for Optimal Germination
The timing of starter fertilizer application should occur very close to the moment of seeding. Since nutrient needs begin immediately upon germination, the fertilizer must be present in the soil when the roots emerge. You can apply the granular fertilizer just before broadcasting the seed and work it lightly into the topsoil, or you can spread it immediately after the seed is down.
If applying fertilizer before seeding, lightly rake the area to mix the granules into the top 1 to 2 inches of soil, positioning the phosphorus closer to the emerging roots. When applying after seeding, avoid working the product in, as this can disturb the seeds. The nutrients must be close to the seed to be effective, since phosphorus does not move easily through the soil profile.
To ensure even coverage and prevent over-application, use a broadcast or drop spreader. Consult the product label for the recommended application rate, often measured in pounds per 1,000 square feet. Set the spreader to a lower setting and apply half the fertilizer walking in one direction, and the second half walking perpendicular to the first path. This cross-hatch pattern helps prevent missed spots and reduces the risk of fertilizer burn caused by overlap.
Immediately following the application of both seed and fertilizer, a light but thorough watering is required. This dissolves the fertilizer granules, activating the nutrients so they can move into the soil. The watering must be gentle, using a fine spray, to prevent the grass seed and fertilizer from washing away or accumulating in low spots.
Common Mistakes That Damage Seedlings
The biggest mistake when establishing a new lawn is using a standard, high-nitrogen fertilizer instead of a starter product. These general-purpose lawn foods are formulated for established grass and contain high concentrations of nitrogen salts. When applied to tender new seedlings, these salts draw water away from the fragile roots and the seed itself, resulting in “fertilizer burn.”
Fertilizer burn can prevent seeds from germinating entirely, or it can scorch the young sprouts, causing them to turn yellow or brown and die. Because a new lawn is not yet established, it is more susceptible to the high salt index of standard fertilizers than mature turf. Over-application, even of a starter fertilizer, can also lead to this damaging salt buildup.
Another pitfall is the use of “Weed & Feed” products on a newly seeded area. These combination products contain a fertilizer mixed with an herbicide, often a pre-emergent weed killer. The herbicide component is designed to prevent weed seeds from sprouting, but it cannot distinguish between weed seeds and grass seeds.
Applying a Weed & Feed product prevents new grass seed from germinating, leaving a bare patch of soil. If the product contains a post-emergent herbicide designed to kill existing weeds, it will be too harsh for the delicate grass seedlings and will likely kill them. Wait until the new grass has been mowed several times and is fully established—typically six to eight weeks—before applying any product that contains an herbicide.