There will always be debate about when is the best time to perform work in your yard and this includes the right time for planting grass.
The answer is really not that difficult if you take a few seconds to think about it. But there are many factors that you should consider and choosing the exact best time may not be best for your yard.
Fall is a time when all of your plants go dormant. For most of us this means by the end of September or definitely by mid October you will begin to see leaves starting to change their colors and flowers blooming less often. By November many of your trees may now be bare and your annual flowers have wilted from cooler night air.
In the Spring you have the birth of new plants and leaves begin to sprout on your trees as they come out of dormancy and begin capturing the suns rays to turn it into food.
So really the best time to plant is in the Spring. It is at this time when the plants can begin their growth and progress into full sized plants before the summers heat hits them.
If you were to plant in the fall you would be asking immature plants to weather dormancy before they have reached their full potential. The smaller grass plants would have a weak root system and most of them would die in the first few frosts.
In addition raking of leaves can destroy young root systems and leaving the grass covered in debris from trees will have a bad effect on their growth.
The exceptions
If you have a large section of dead grass or exposed soil with no root system there is a strong chance that erosion will remove many of the nutrients needed for a healthy yard.
You can also end up with a really messy muddy yard in late fall or winter if you get hit by a seasonal storm or temporary patch of warm days that melts snow.
If you see you have a problem then you should try to cure it as soon as you can in the late summer early fall by over planting the area and hoping for the best.
Some seed will grow and what is left will hopefully remain in the soil ready for the spring but you should understand there is a real probability that you will need to rework the area in the spring.
What is the best thing to do for your yard in the fall?
Adding a lot of nutrients in the fall is also not a great idea because many of the products will get washed off your yard from rain and snow melt.
Thatch that builds up in your grass should not be removed later in the fall because it will help stay off erosion.
Probably the best thing you can do is give your grass some extra water early in late summer and early fall and increase its blade height by raising your lawn mower setting.
More grass will mean better insulation of roots in winter months and protection from runoff.
If you continue to use your yard later in the season foot traffic will not wear away the longer dormant grass as easy as it will shorter cut grass.
Final Note
When is the best time to start new grass? It really comes down to what you expect and what problem you are trying to fix.
Grass can grow very quickly even in late fall but if you are trying to improve the general condition of good grass or fix patchy lawns of mixed grass types then the spring is always best. Planting in the spring gives the plant a full season of growth
So, for best results only plant in the Summer or Fall if you are trying to prevent erosion, keeping in mind next spring you might need to do some touchup work.