Overseeding Lawns

Instructions for Northern Yards Using Cool-Season Grasses

Grass and trees, ground perspective
Jason Todd/Photographer's Choice RF/Getty Images

Is your lawn composed of cool-season grasses? If so, it may well take a beating from the heat to which it is subjected in summer. After all, by definition, these grasses crave the cooler temperatures of spring and autumn. But there's something you can do to undo summer's assault. It's called "overseeding lawns."

"Overseeding lawns" is just what it sounds like. Namely, you are sowing seed over existing grass, in order to make your lawn thicker and lusher. Therefore, such a sowing makes sense only if the existing grass is healthy enough and abundant enough to be worth keeping. If you currently have only 50% coverage, then you need to tear up the old lawn and start a new lawn from seed.

Note that overseeding lawns with warm-season grasses is also possible, although the reasoning behind the operation is different in that case (and annual ryegrass is commonly used). The present article is specifically about working on Northern lawns that are made up of cool-season grasses.

Preparations for the Job

Since you will be sowing seed not over an open stretch of soil, but rather over existing grass, take into consideration that the seed will be competing with that existing grass. If nothing else, the latter will rob the seed of some sunlight. To give the seed a better chance, mow the existing grass, cutting it shorter than you normally would. Normally, you should cut grass to a height of 3 to 3.5 inches. In this case, reduce that height to 1.5 to 2 inches. Also, bag or rake up the clippings in preparation for overseeding lawns, even if normally you do not. You want to give seeds the best chance of making good contact with the soil, and clippings would just get in the way.

Overseeding is intended to be a minor task, not a major operation, so if large areas of the lawn have little or no grass in them, you will want to have filled in those bare patches at an earlier time. Plan accordingly. If you know you will be overseeding in the fall, take care of the bare patches in spring.

You should also have the soil tested by your local county extension office well in advance of overseeding. For all you know, the soil may be lacking in nutrients or have an incorrect soil pH. As part of the testing, the extension office will tell you what (if anything) you need to do to improve the soil.

Another step that it may be necessary to take to promote contact between seeds and soil is core aeration, or "lawn aeration." This step will help reduce lawn thatch, which stands in the way between grass seeds and the soil that they would like to call home. Core aerators (or "lawn aerators") can be rented from local rental centers.

You may need to add a layer of topsoil before overseeding the lawn in specific problem areas. For instance, due to shallow tree roots popping up on the lawn, your topsoil layer may be too thin. Spread 1/4 inch of screened topsoil over such an area (up to 2 inches, in severe cases), and rake it in.

Buying the Seed

The gardening section of a home improvement store will carry bags of grass seed suited to your area. The bag of grass seed that you buy should have information on the back concerning recommended seeding rates (for starting new lawns) and overseeding rates. Note the difference: You do not need to spread as much seed when overseeding lawns as when starting new lawns.

Grass seed is often sold in different "blends." Thus, even if you buy a bag of grass seed that says, "Kentucky blue grass" (one of the cool-season grasses), what you are buying may be a mixture of different grass seeds. This is why one is unable to state, point-blank, that the recommended rate for overseeding lawns for Kentucky blue grass (Poa pratensis) is such and such: It would depend on the particular blend of Kentucky blue grass that you bought. For instance, for one blend of Kentucky blue grass, the recommended rate for overseeding lawns might be 1 to 2 pounds for every 1000 square feet that you need to cover. But for another, the rate might be 2 to 3 pounds. This is why you must read and follow the instructions on the back of the bag.

Tips and Techniques for Overseeding Lawns

For the actual operation of overseeding the lawn, use a fertilizer spreader. Despite the name of this gadget, it is also meant for spreading grass seed. Set the spreader to the overseeding rate as recommended on the bag of grass seed, then load some seed into the hopper. You are now ready to go:

  1. Simply push the fertilizer spreader, releasing seed as you go, across the whole lawn, refilling the hopper as necessary.
  2. Right after you are done overseeding, apply a starter fertilizer for better results.

Those two tasks are the easiest in the whole process (you can accomplish them in one day on all but the most massive of lawns). It is the prep work and the care for the seedlings afterwards that requires the most labor, by far.

Caring for the Seedlings Afterwards

The grass seed must be watered properly (starting right after the above tasks have been completed), in order to germinate. Use just a fine spray, as you don't want to create a flood. The soil should be kept evenly moist, which may mean several waterings per day (depending on the weather), for several weeks.

After the grass blades sprout, you'll still need to water a couple of times per day. If you know your schedule will not permit this, the time to look into automatic irrigation systems is before overseeding lawns, not after. While it is all right to mow your lawn as you normally would after overseeding, avoid walking around a lot on the lawn until the new grass seedlings are 3 inches tall.

Five weeks after the grass has sprouted, apply a quick-release nitrogen fertilizer at the recommended rate; repeat in another six weeks.

The best time for overseeding lawns that have cool-season grasses is in September; the second-best time is in March or April.