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Overseeding a Cool-Season Lawn to Fill in Bare Spots

Q. I've seen people 'overseeding' their lawns, and was surprised that I couldn't find any mention of it in your A to Z archives. I think my lawn might be a candidate for overseeding, but how can I tell for sure? Is there a problem with overseeding a different type of grass than is currently present? Is there a resource for identifying what type of grass I already have?
    ---Dane in Abington, PA
A. You're right—while overseeding is mentioned in a couple of my lawn care articles, there's nothing really in depth or easily findable. And this is the perfect time of year to repair cool-season lawns—so let's do it!

Now, the term 'overseeding' can have several different meanings, so let's choose the one I always think of: the patching and/or spot repairing of certain areas in lawns. For this kind of work, the qualifying 'rule', says our resident lawn care expert, Dr. Nick Christians of Iowa State University, is 30/70. If 70% of your lawn is in fairly decent shape and 30% or less is bare spots and ratty areas, you're a perfect candidate for spot repairs. But if more than half your lawn is bare, ratty or composed of large patches of unacceptable weeds, Nick says to seriously consider tearing it all up and completely reseeding.

Now surprisingly, there's often no problem with using a different type of grass than the old lawn is composed of. (The big exception, says Nick, is that you shouldn't patch areas in a bluegrass lawn with a turf-type tall fescue.) In fact, a lot of high-quality bagged grass seed is already a mixture of types. I'm looking at one right now that's a combination of three fescues, a bluegrass and a perennial rye. The reason this won't (or shouldn't) grow a patchwork quilt is that the varieties have been selected to have matching blade shapes and colors, and thus produce a consistent and visually pleasing lawn. (That's why you should always buy a named brand of seed when you sow a new lawn—so you can match it up when you have to do repairs.)

If you know what seed you started with, buy it again or ask an expert what would match the blade shape and color of your existing turf. Your local county extension office could be a great resource for this; if not, they should be able to point you in the right direction. If you don't know what you have, dig up about a square foot of turf (including the root system) and either show it to the folks at your local Extension office or take it around to a couple of nurseries/garden centers that specialize in premium grass seed.

OK? Let's move on to the actual work. Now that the summer heat stress period is over, it's safe to cut a cool season lawn down to around two inches once (and only this once) to better see what you're doing. Use a mulching mower; the pulverized clippings will help feed the new grass.

For areas that are truly bare, just rough up the soil with a cultivator. If there are clusters of weeds, soak the soil deeply and slowly pull them out. Cut 100% weedy sections out of the soil with something like a linoleum knife. If the lawn is full of thatch, dethatching and raking would be an excellent idea.

Once you're satisfied with your prep work, the ideal next step would be to spread an inch of compost over the entire lawn; this would provide a perfect bed for the new seed and take care of feeding the new and existing grass for the fall. Otherwise, spread an inch of high-quality yard waste compost over the prepared areas, sow the new seed, cover it lightly with more compost and then water the entire lawn as gently as possible for twenty to thirty minutes every morning until the new grass sprouts—which should be pretty fast, as the soil temperature is still nice and warm.

When the first sprouts appear, back off to morning watering every other day for about a week. Then go longer and less frequently—say an hour twice a week for another week or so. After that, fall into the pattern you should follow for the life of the turf—one inch of water delivered in one long soaking once a week until winter hits. (Obviously, don't water if you get rain; and try not to drop your seed right before a predicted deluge.)

Once all the new grass has been up and growing for about two weeks, give your entire lawn its fall feeding if you didn't spread compost over the whole thing before seeding. You can use corn gluten meal or a bagged organic fertilizer labeled for lawns. Do not use a so-called 'conventional' chemical fertilizer; it could burn up your tender new grass.

Cut the entire lawn at three inches from now on—no lower. And if leaves typically fall onto your lawn, be prepared to suck them off with a blower/vac this year. Let your new grass get really well established before you rake it.

And if you have a warm season grass like zoysia, Bermuda, or St. Augustine, don't do anything in the Fall. The time for you to renovate your lawn is when it greens up in the Spring.

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