BACKGROUND: Overseeding is the action of adding grass seed to an existing lawn. This may be considered by people for spots where bare areas show, or even better over the whole yard! Overseeding is a great way to improve the density of grass that has become thin while introducing new innovative grass that is likely more resistant to disease, drought, and bugs.
THE RULE (KIND OF): if 50% or more of you lawn is in good condition, overseeding can be a positive effort worth trying. If more than 50 percent of the area is in poor condition (weeds, dog spots, etc), you will want to consider a new lawn from sod or seed (bigger project).
NOTE: Pick seed that is appropriate for your lawn type and area. You probably want to make sure you are buying perennial not annual grass so that it comes back the next year. It will say right on the label of the bag.
MOW LOW: We don’t typically advise this…. but because you are trying to grow grass from seed take into consideration that the seed will be competing with that existing (taller) grass and its nutrients (sunlight, water, fertilizer, etc). To give the seed a better chance MOW LOW in gradual steps. 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.
RAKE TO LOOSEN SOIL: You will need good seed-to-soil contact for the new seed to germinate. Prepare areas by raking. Use a light touch, so you break up the soil surface without raking out the existing grass.
RAKE TO HELP SEED SOIL CONTACT: We recommend applying seed with a spreader to get more even coverage. Use the recommended rate for your selected seed when overseeding. Rake the area again lightly with an upside down rake after you overseed to improve the seed-to-soil contact.
APPLY STARTER FERTILIZER: apply a starter fertilizer for improved and faster results. We think going half application rate 2 weeks apart is a real pro move if you have the time.
WATER, WATER: The grass seed must be wet in order to germinate. The soil should be kept evenly moist, which may mean several water applications per day for a few weeks. Try not to over do the watering leading to flooding the area and making the seed move.
After the grass blades sprout, you’ll still need to water a couple of times per day and try to take it easy on the lawn for the first couple mows. Definitely don’t go heavy traffic, it will die. Keep up with your regular fertilizer applications and enjoy!
The best time for overseeding lawns that have cool-season grasses is in September in northern climates. You are looking for that not too hot, not going to frost too soon time frame.