Fall Overseeding Guide for Cool Season Grasses

For cool season lawn owners (those in the northern 2/3 of the country) we get one chance each year to turn it all around. To un-do the damage from harsh conditions and blistering heat of summer. That time is the early fall; the best time to grow new grass from seed. This guide to overseeding is packed with tips and tricks to help make your lawn look great this fall and beyond. 

As your grass ages, there’s a tendency for it to thin out, especially after a summer of being on the lawn a lot. It is subject to wear and tear just like anything else. But there’s a secret to reviving a lawn. The pros know it but often homeowners aren’t always up to speed: overseeding.

Overseeding is the practice of adding new seed to your existing lawn. For cool season grasses, fall is the best time to add new seed. Spring is second best, but it should only be attempted in the spring if you are OK with weeds actively germinating alongside your new grass seed. Fall is the best time because broadleaf weeds are starting to die off, and the only thing growing in your lawn will be new grass. If done properly, overseeding can transform a lawn and develop new healthy grass to fill in bare spots and fix other problem areas. 

Whether this is your first lawn or you’re a seasoned pro, learning the basics of overseeding can dramatically improve the look of your lawn and give it a serious color boost heading into winter and next spring. 

  1. Make a plan. You’ll need to consider which seed to purchase, whether you need a hand spreader or a push spreader, your irrigation needs, and if you need to aerate, etc. Core aeration is great but not always necessary. To test whether you’ll need to aerate, grab a screwdriver and stick it about 6 inches into your lawn. If it goes in easily, then you’re all set. If you run into a lot of resistance, your soil is compacted and you’ll need to either rent an aerator or call a lawn service to come and aerate your lawn.  
  2. Mow low and bag the clippings. This is one of the only times you should mow your grass at a very low height. Set your mower to around 2-2.5 inches (the numbers on your mower deck correspond to inches). Be careful, as you should never cut off more than ⅓ of the total height of the grass blade at any one time, so you’ll need to start mowing one notch lower with each mow starting a couple weeks before you add seed. It’s important to mow lower because you’ll want to make sure that you give your new grass plenty of time to mature without disturbing it. Mowing involves walking over it and using heavy equipment that can damage your new lawn. And bag your clippings when mowing to prevent grass clippings from blocking the seed to soil contact. 
  3. Dethatch - On the topic of seed to soil contact, you'll want to remove as much debris as possible from your lawn before adding seed. This will ensure the best seed to soil contact. This article will explain more about how to dethatch your lawn.  It should look similar to this after you're done. Don't worry, you're not harming it! You're removing debris stuck in your lawn and creating the perfect seed bed by allowing the seed to get into the soil. lawn debris after dethatching
  4. Timing is everything. While labor day weekend is the unofficial start of overseeding, it’s  not as simple in practice. Cool season grasses will grow best when the soil temperatures are dropping to below 70 degrees, and the timing depends largely on your region. Overseed too early (when the summer is still in full swing) and your new grass will probably not have a high germinate rate. And the grass that does emerge will likely get scorched by the hot summer sun. Overseed too late and it may germinate, but it probably won’t establish itself before the first frost sets in. Grass typically needs to be mowed twice before it's considered hardened off for the winter. So there’s a narrow window that you need to aim for when planting new seed, and it’s only a couple weeks long. The turfgrass alliance recommends seeding at least 45 days before the average first frost. All of this is great to know, but it can be a little overwhelming. If you’re a Lawnbright customer, don’t sweat this step. We’ll shoot you a text when your soil temps in your area are optimal for seeding. 
  5. Timing REALLY is everything. And timing is not just about soil temps. You’ll need to consider the weather forecast over the course of the next seven to ten days. If there’s a tropical downpour on the way, you should probably wait until after they pass, as the seed will likely get washed away by heavy rain. It’s not a matter of how much total rain is in the forecast, but rather the intensity of the rain. This can be especially problematic in September, which is peak hurricane season on the gulf coast and in the south east. So keep an eye on the forecast.
  6. Spread the Seed - Use a hand spreader for small areas and a broadcast spreader for larger areas, following the bag rates for overseeding. For very small spots and bare patches, you can just spread it using your hand and lightly rake it in so it's covered. You can also go a little heavier in bare spots than areas where there’s existing grass. In northern climates we typically recommend Jonathan Green Black Beauty - Either the ultra or original version, depending on where you live. 
  7. Cover bare spots. If you’re overseeding bare patches, a cover like peat moss will be useful to keep the seed in place and prevent washout from heavy rains. Peat moss is also great for indicating whether the soil is dry or wet. If it dries out, it appears lighter than when wet, and it will indicate that you need to add water. If you don't have any peat moss, you can just add a light layer of dirt or gently rake in your seed. 
  8. Water. A LOT. This is often the most difficult step. Most overseeding failures occur because the new grass seed does not receive enough water. You need to change your watering strategy from deep and infrequent to shallow and frequent. Water 4 times a day, and only 10 minutes at a time. The point is to keep the seed constantly damp, without drenching it or letting it dry too long. And use common sense. On hot sunny days the seed will dry out quickly, so you may want to add an additional watering session or two, and water less on cooler overcast days. After about 10-14 days you should start seeing seedlings emerge, at which point you can back off your watering to twice a day, and then about a week after that you can return to a normal 1-2 time per week schedule. 
  9. Apply starter fertilizer. A few days after seeding you can apply a starter fertilizer or a product containing sea kelp product to push deep root growth. Lawnbright plans in cool season lawn regions assume fall overseeding, and fall boxes ship with either a starter fertilizer or root growth product based on soil needs. If you'd like add-on our starter fertilizer to your plan, you can purchase it separately

This may seem like a lot, but all of these steps will have you set up perfectly for lush, healthy turf come spring!