Overseeding is the process of spreading new grass seed over an existing lawn to improve its density, fill in thin areas, and enhance overall health. Simply tossing seeds onto the turf rarely yields successful results because the seed needs to make firm contact with the soil to germinate effectively. Proper preparation of the established lawn is the most impactful factor in determining the success of new grass growth. The initial step in this preparation is adjusting your mower to a much lower setting than normal, which directly addresses the competition new seedlings face.
The Rationale for Low Mowing
Lowering the cutting height is a deliberate action taken to reduce the existing turf’s competitive advantage over the new seeds. When the established grass is cut short, it allows significantly more sunlight to penetrate the canopy and reach the soil surface below. This light stimulates the germination and early growth of the newly spread seedlings. If the existing grass is too tall, seeds can become trapped in the thick layer of organic material near the soil line, known as thatch, where they dry out quickly and fail to sprout.
Specific Mowing Height and Technique
The specific height for the pre-seeding mow should be significantly lower than your lawn’s regular maintenance height, typically aiming for a finished height between 1.5 and 2 inches for most common cool-season grasses. This range is short enough to expose the soil without severely damaging the established turf’s crown, where the new shoots emerge. Cutting the grass down to this height should be done immediately before spreading the seed, ensuring the lawn remains short throughout the germination period.
To achieve this maximum shortness efficiently, you should use a sharp mower blade, as a dull blade will tear the grass blades and stress the existing lawn unnecessarily. It is recommended to bag the clippings during this ultra-low cut, rather than mulching them back into the lawn. Bagging removes the cut material, preventing it from forming a dense mat that could shade the soil and smother the new seeds.
For particularly dense lawns, a technique known as double-cutting can be used to ensure the lawn is uniformly short. This involves mowing the entire area once at the target height, then immediately mowing a second time perpendicular to the first direction. This two-pass method helps stand up any blades of grass that were pushed over on the first pass, maximizing the exposure of the soil surface.
Essential Steps After Mowing
Achieving the low cut is only the first step; the soil must be prepared to receive the seed. The action following the mow should focus on scarifying the lawn surface, which means lightly scratching or disturbing the top layer of soil. This step creates small grooves and pockets where the seed can settle and be covered by a tiny amount of soil, increasing germination rates.
A thorough raking with a stiff spring-tine rake helps remove any remaining loose debris or thatch that the mower missed. For lawns with a substantial thatch layer, mechanical dethatching or light core aeration can be performed to break up compacted material and improve soil structure. These actions ensure that the new seeds are nestled slightly into the surface, allowing for true seed-to-soil contact. The seed, once placed in the prepared pockets, gains access to consistent moisture and is protected from being washed away or eaten by birds.
Mowing Schedule After Overseeding
After the seed has been spread, the focus shifts from preparation to protection, and the mower should be stored away for a period. The first cut after overseeding should not occur until the new seedlings are well-established, typically meaning a wait of approximately two to four weeks. The timing depends more on the height of the new grass than the calendar. The new blades should reach a height of at least 3 to 4 inches before they are strong enough to withstand mowing.
The first cut must be done with extreme care to avoid uprooting the fragile new growth. For this initial pass, the mower deck must be raised to its highest setting, ensuring only the very tips of the established and new grass blades are removed. Following the one-third rule—never removing more than one-third of the grass blade height in a single mow—is especially important during this period to prevent shocking the young plants.
The first few mows should be performed only after the soil surface is dry to prevent the mower wheels from damaging the seedlings or causing ruts. This careful approach protects the seedlings as they develop their permanent root systems and transition into a regular maintenance routine.