How to Overseed Your Lawn
If you live in a hot-summer, mild-winter area where warm-season grasses predominate, you can overseed the lawn with cool-season grasses, which keep the lawn green all winter. Because annual ryegrass fills in so quickly, it’s usually used for overseeding, but you can also use perennial ryegrass or one of the fescues.
The best way to overseed is to dethatch and aerate the lawn, then reseed, and top-dress (add a thin mulch). Then, when the weather starts to warm the following spring, you aerate and dethatch again, but you don’t have to reseed because the warm-season grass comes back by itself.
The preceding method is quite a bit of work, so here’s an easier way to overseed:
In fall, use a heavy-duty reel-type mower. Set the cutting height very low so that it is just above the soil line, and mow to scalp the lawn.
Rent or borrow the reel mower if you don’t own one.
Scalping is one of those perfectly descriptive jargon words: It means exactly what you’d think. Lawn experts use it in two ways. One is as when you intentionally remove most or all the grass. Scalping is also an accident and occurs when a mower is set too low or otherwise digs too deeply into a patch of grass.
Rake up the debris.
Compost it by itself, since some warm-season grasses (even pieces of them) can be weedy.
Sow cool-season seed.
Put it down heavier than normal (up to 10 pounds per 1,000 square feet for annual ryegrass).
Topdress, water, and fertilize.
Apply a thin layer of organic matter and then fertilize and water like you would for a newly seeded lawn.
When the cool-season grass becomes established, mow it at the proper height over the winter. As the weather warms up in spring, fertilize and start cutting the lawn at the lower height for the warm-season grass. Soon, the warm-season grass again predominates and the winter grass disappears.