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When to Overseed Grass Seed

Overseeding means distributing grass seed over an existing lawn in an effort to bring it back to a lush, healthy state. Generally, gardeners overseed after experiencing lawn problems or when they have an older lawn that looks thin and worn out. Overseeding can be done with the same kind of grass already growing, or it can be used to introduce a new type of grass. The timing of overseeding is crucial to its success.
  1. Fungal Diseases

    • A host of lawn fungal diseases can leave your lawn with bare patches. Diseases such as leaf spot, melting out and necrotic ring spot can cause grass to die out or thin. Thin areas or bare spots allow weed seeds to germinate and pop up across the lawn. If your lawn has experienced a fungal disease, treat the lawn with a fungicide before overseeding. You might need to dig up the diseased grass and remove it from the yard. The quicker you overseed after a fungal disease treatment, the fewer weeds you will find in bare areas.

    Thatch Layer

    • Dethatching a lawn reduces the thatch layer between the grass and soil, making it easier for water to reach the roots. A layer of thatch thicker than 1/2 inch can contribute to bare areas and fungal diseases. Dethatching itself also can cause bare areas. After dethatching aerate the lawn to allow moisture and oxygen to penetrate it, then overseed to prevent weeds from taking over. The best time to dethatch is when the grass is vigorously growing, because dethatching often stresses out grass.

    Dormant Lawns

    • Gardeners growing only warm-season grass types find that their lawns turn brown and go dormant in the late fall. To keep a green lawn throughout the year, overseed with cool-season grass, according to the University of California. Cool-season grass types include ryegrasses, fescues and bluegrasses. Each grass variety has a specific distribution rate, so check the grass seed bag. A cool-season yard will last from the late fall to spring months and go dormant right when your warm-season grass greens up.


    • Fall is an optimal time to overseed grass. The soil is still warm from the summer, but air temperatures have cooled off. In addition, you do not have to worry about damage to your newly seeded lawn from the summer heat. Weed populations typically decrease in the late fall, so your new grass has little competition. Moreover, grass seed needs a lot of sunlight to germinate. Trees during this time have lost some of their leaves and your yard is receiving more sun than during the growing season.