Overseeding is the process of spreading new grass seed over an existing lawn to improve its density and color. This practice is particularly beneficial for cool-season grasses that naturally thin out over time. While seed quality and subsequent watering are important, proper lawn preparation is crucial for success. Focusing on the correct pre-seeding cut is a foundational step.
The Rationale for Aggressive Mowing
The instruction to mow the lawn at a significantly lower height than normal serves two main purposes. First, a short canopy is necessary to maximize seed-to-soil contact, which is essential for germination. If the existing grass is too tall, seeds get trapped in the thick layer of blades, preventing them from settling onto the soil surface to absorb moisture and nutrients.
Second, reducing the height of the existing turf dramatically increases the amount of sunlight reaching the soil. New grass seedlings require ample light energy to germinate and establish root systems. A tall, dense canopy creates shade, smothering the young seedlings. Cutting the grass low temporarily suppresses the existing turf, giving the new seed a competitive advantage for light and resources.
Recommended Pre-Seeding Cutting Heights
The general recommendation is to lower the mower to its lowest or next-to-lowest setting, aiming for a final grass height between 1 and 2 inches. Some specialists suggest an even more aggressive cut, down to 1/2 inch, to fully open the canopy and expose the soil. The goal is to expose the soil surface without completely scalping the ground down to bare dirt, which can stress the existing grass roots.
Gradual Reduction and Mower Care
If the lawn is currently very tall, attempting to reach the final low height in a single pass can severely shock the existing grass, a practice known as scalping. To avoid this stress, the height should be reduced gradually over two to three days. Adhere to the rule of never removing more than one-third of the blade length in a single mowing session until the target height is reached. Use a sharp mower blade, as a dull blade tears the grass, increasing the risk of disease.
Clipping Removal
The optimal cutting height varies slightly depending on the grass species, though most overseeding is done on cool-season varieties like Fescue or Kentucky Bluegrass. A common target for these grasses is 1.5 to 2 inches, which is shorter than their normal maintenance height of 3 to 4 inches. After the final low cut, all clippings must be bagged and removed from the lawn. Leaving a thick layer of clippings creates a physical barrier that prevents the seed from reaching the soil and can promote disease.
Essential Steps After Mowing and Before Seeding
Once the lawn has been aggressively mowed and debris removed, the next steps focus on preparing the seedbed. Even after a low cut, a layer of thatch—a buildup of dead organic matter between the grass blades and the soil—can block the seed. Dethatching or power raking may be necessary to mechanically remove this spongy layer, especially if it is thicker than half an inch.
Following thatch removal, breaking up compacted soil is important, as it allows for better movement of water, nutrients, and air to the seed. Core aeration is the most effective method, involving mechanically pulling small plugs of soil from the ground. The holes created by aeration provide ideal pockets for the grass seed to settle into, ensuring excellent seed-to-soil contact and protecting the seeds from wind and runoff.
Before applying the seed, ensure the soil surface is ready. This may include lightly raking the area to break up remaining clumps and smooth out uneven spots. Focusing on these preparatory actions—mowing, dethatching, and aeration—maximizes the conditions for the new grass seed, increasing the chances of developing a thicker lawn.