The question of whether to apply fertilizer and grass seed together is common for those seeking an efficient way to start a new lawn or thicken an existing one. The answer is yes, you can combine these two tasks, but success hinges entirely on selecting the proper type of fertilizer. Using the wrong product, particularly one designed for weed control, can easily negate all your efforts by preventing the grass seed from ever sprouting. A strategic approach involves understanding the specific nutritional needs of a newly germinating seed versus an established lawn.
Choosing the Right Fertilizer for Seeding
The most significant distinction in fertilizer choice is between a starter fertilizer and a standard product that contains pre-emergent herbicides. A starter fertilizer is specifically formulated to provide immediate, accessible nutrients to new seedlings as they begin to grow. This type of product is designed to be gentle on the vulnerable grass seed and is foundational for successful establishment.
Starter fertilizers are characterized by a high concentration of phosphorus, represented by the middle number in the N-P-K ratio. Phosphorus is the primary nutrient that fuels root development, which is extremely active in a new grass seedling. Suitable N-P-K ratios often include a high middle number, such as 10-10-10, 20-10-10, or blends like 24-25-4. These products ensure developing roots have the necessary energy for strong anchorage before the plant focuses on producing green blades above ground.
In contrast, you must avoid using standard “weed and feed” fertilizers when seeding. These products contain pre-emergent herbicides designed to stop seeds from sprouting. The herbicides create a chemical barrier in the soil that targets the initial stages of germination. This action inhibits cell division in the developing root system, effectively killing the seedling before it can establish itself above the soil surface. This chemical action is indiscriminate and will kill your new grass seed just as effectively as it kills target weed seeds.
Proper Steps for Simultaneous Application
Before spreading any material, you should prepare the soil to ensure the best possible seed-to-soil contact. If the area has a thick layer of dead grass or compacted soil, light dethatching or aeration can significantly improve the environment for the new roots. Loosening the top layer of soil with a rake is helpful to create small grooves where the seeds can settle.
The process of applying seed and the appropriate starter fertilizer can occur on the same day, and they can be applied one after the other. While it is possible to mix the seed and the fertilizer granules in a spreader, it is generally better to spread them separately to ensure a more even distribution of both materials. Applying the fertilizer first or the seed first does not significantly impact the final result.
Once both the seed and the starter fertilizer are on the ground, gently rake the area again. This light action helps work the materials slightly below the soil surface, maximizing seed-to-soil contact without burying the seed too deeply. Following this, a light pass with a lawn roller can further press the materials into the soil, which helps prevent the seeds from washing away during initial watering.
Essential Care Immediately Following Seeding
The most important factor immediately following the application is the watering regimen, which must be precisely controlled for successful germination. Unlike an established lawn that needs deep, infrequent watering, newly sown seeds require moisture in the top layer of soil constantly. You should water the area lightly and frequently, typically two to four times per day for short durations of five to ten minutes each.
The goal of this frequent, shallow watering is to keep the top one to two inches of soil continuously damp, but never saturated to the point of pooling water. Allowing the seeds to dry out completely, even for a short period, can cause the germination process to stop. Once the seeds have sprouted and the new grass is approximately one inch tall, gradually reduce the frequency of watering while increasing the duration to encourage deeper root growth.
Minimize all foot traffic on the newly seeded area during the germination and initial growth phase. Walking on the area can displace the seeds or damage the fragile new seedlings, disrupting establishment. The first mowing should not occur until the new grass blades reach a height of three to four inches, which takes four to eight weeks depending on the grass type. When mowing for the first time, set the mower to its highest setting and only cut off the top one-third of the blade height to avoid stressing the young plants.