Overseeding is a fundamental practice that rejuvenates thinning turf by planting new grass seed directly into the existing lawn. The success of this process relies heavily on proper preparation, and mowing is a necessary first step. Without reducing the height of the existing grass, the new seeds will struggle to reach the soil and receive the light and moisture they need to germinate and establish.
Why Mowing Low is Essential
Mowing the lawn to a low height immediately before overseeding maximizes seed-to-soil contact. For grass seed to germinate, it must physically touch the soil surface, a process that is prevented by a dense canopy of existing grass blades. By dropping the cutting height, you expose the soil surface and create a clear path for the seeds to fall into the turf layer.
The recommended height reduction is significant, often involving setting the mower one or two notches lower than the normal cutting height. For most cool-season grasses, this means aiming for a height of about 1.5 to 2 inches. This practice temporarily stresses the mature grass, slowing its growth and reducing the competition it poses to the emerging seedlings.
This temporary “scalping” also serves to increase the amount of sunlight reaching the soil level. New grass seedlings require a high level of light to photosynthesize effectively. Taller grass blades would shade the young seedlings, causing them to grow weak and spindly as they stretch for light. It is crucial to bag or rake up all the resulting clippings, as leaving them on the lawn creates a smothering layer that blocks both light and seed-to-soil contact.
Soil Preparation Steps
After the low mow, the next phase focuses on physically opening the soil to ensure the seed has an optimal environment for germination. One of the most common obstacles is thatch, a layer of organic matter that accumulates between the soil and the green grass blades. If this layer exceeds half an inch in thickness, it acts as a barrier, preventing seeds from reaching the mineral soil.
Dethatching is necessary, which involves using a specialized machine to mechanically remove the excess thatch layer. Regardless of thatch levels, aeration is often employed to relieve soil compaction and create small pockets for the seeds. Core aeration, which pulls out small cylindrical plugs of soil, is preferred over spike aeration because it allows better penetration of air, water, and nutrients.
The small holes left by the core aerator act as perfect miniature seedbeds by ensuring the seed is nestled within the soil. Following aeration, applying a starter fertilizer is highly beneficial for the new seedlings. This fertilizer should be high in phosphorus, as phosphorus is the nutrient that supports strong, rapid root development in young plants.
Seeding Application and Initial Care
With the lawn properly prepared, seed application requires even coverage. The recommended overseeding rate for cool-season grasses typically ranges from 3 to 5 pounds per 1,000 square feet, depending on the specific grass type. Applying too much seed can be detrimental, causing the emerging seedlings to compete intensely and result in weak, spindly turf.
To achieve a uniform distribution, divide the total amount of seed in half and apply it using a broadcast or drop spreader. The first half should be spread walking in one direction, and the second half should be applied by walking perpendicular to the first path, a method known as cross-hatching. This ensures the entire area receives the intended seed density.
The first two weeks following application are defined by an intensive and specific watering regimen to keep the seeds viable. The soil surface must be kept consistently moist to facilitate germination, requiring frequent, light watering two to four times daily. Avoiding deep saturation prevents washing away the seeds or promoting fungal diseases. Once the new grass reaches approximately three to four inches in height, the watering schedule can gradually transition to deeper, less frequent soakings to encourage the development of robust, deep root systems.