In an earlier blog we looked at all the reasons why September is lawn month. However, back-to-school activities often push lawn seeding work past September, so how late can you actually plant grass seed and still have success?
Here in western Pennsylvania (which is like much of the midwest and northeastern US) I've always considered October 10th as the final deadline for grass seeding work. However, if we end up with one of those years when winter arrives late, that deadline can be sometimes be extended to October 20th.
In order to get grass seed up and growing, be sure to water-in newly planted seed right away and keep it moist with daily watering. Don't wait on it to rain.
To add seed to an established lawn, use a core aerator and make at least 3 passes over the lawn to create as many holes through the existing turf and thatch as possible. Grass seed does not establish well on top of thatch, it needs "seed to soil" contact.
Even though Fall is an ideal time to kill broadleaf lawn weeds (since they are storing reserves for winter), trying to kill weeds and grow seed is a bad mix, likely doomed to seeding failure, just as trying to seed and control crabgrass at the same time in Spring is also a bad mix.
So if you seed in the Fall (my preference) do your weed control in Spring (crabgrass) and early Summer (broadleaf weeds), since most weed controls and grass seed don't mix.