September through December is the best time to fertilize a cool-season lawn, research has shown.
Not sure if you have cool-season grass? The answer is simple. If it stays green all winter, then it is cool-season. If it turns brown in the fall, assuming it is healthy, it is a warm-season lawn.
This only applies to grass. Most perennial plants, like trees and shrubs, should not be fertilized from August until the end of October. Fertilize an azalea now and it may start growing instead of entering dormancy like it is supposed to in the fall. Summer annuals can be fed as long as they are growing.
Lawns can be fed in the spring but only with light applications. Too much nitrogen can lead to summer disease problems. Most cases of brown-patch disease happen when too much nitrogen was applied in March or April.
What should the lawn be fertilized with? For decades most people used the good old stand-by: 10-10-10. They figured if it was good enough for the farmers crops, it was good enough for their lawn. There are two problems with that. The fertilizers used by farmers are formulated to be applied to agricultural crops. They do not work as well in landscapes.
The other problem occurred in recent years. About five or so years ago most, if no all, states passed laws that said phosphorous cannot be used in lawn maintenance fertilizers unless there is a recent soil test that states it is needed. The middle 10 in 10-10-10 says that the fertilizer is 10% phosphorous by weight. Therefore, 10-10-10 will be illegal to use on lawns in most cases. You can still use it to establish a new lawn, but there are better products.
