To seed or not to seed? That is the (spring time) question!
The truth is, you can throw seed down on your lawn any time that you want. But will this provide the seed the best chance for germination? Probably not. There are better times of the year to seed. The optimum time to sow grass seed in northern Ohio is between late August and the whole month of September. If seeding cannot be done before mid October, it would probably be best to postpone until spring because there may not be enough growth to survive the winter. For spring seeding, the earlier the better once you can get out to create a seed bed. For dormant or winter seeding, that is seed that is sowed around November 15th or later, the seed will not germinate until the following spring.
We have two good grass growing seasons in northern Ohio. The spring and fall. These are considered growing season because the weather is cooler for our cool season grass types and there is typically more rainfall in the spring and fall. These things help our turf plants grow well and focus on healthy root growth. When the heat of summer comes and the rainfall decreases, our lawns usually slow down considerably and root development almost completely ceases. The reason that fall is the best time to seed is because the new grass will have time to mature and develop its root system before winter and then get another growing season in the spring to develop more-so before the heat of summer.
When seed is sown in the spring, or with dormant seeding, the new grass has one growing season before the harsher summer conditions. Lawns that are sown in the spring, have a greater risk of being infested with undesirable annual weeds, including crabgrass because they do not have as much time to mature and thicken. Infestations of summer weeds are reduced in successful early fall seedings, because good lawn density develops before the weed seeds can germinate the following spring. If you do decide to seed in the spring, be certain that a crabgrass pre-emergent (crabgrass preventer) is not applied on or near your seeded area because these products will also prevent the germination of grass seed too!
The same theories apply when over-seeding an established lawn as well. If you are looking to thicken your already existing lawn, one procedure that will help is aerating and over-seeding. The holes and plugs left behind from the aerating process provide bare soil surface that seed requires for germination. Fall aeration and over-seeding is preferred over spring time, but if you missed the fall opportunity, spring would be your next best choice. Again, just remember if you do this, make sure that you or your hired lawn care professional does not apply a crabgrass pre-emergent. A better choice would be to apply a starter fertilizer over your lawn for that fertilizer application as a substitute.