Ask a Master Gardener: Fall is the right time to fertilize the lawn
Unless you have had a soil test that tells you otherwise, feed 1 pound of actual nitrogen per 1,000 square feet of lawn.
Q: Is it too late to fertilize my lawn? What about aerating it? I meant to get around to that last month and never did.
A: Now is a very good time to fertilize the lawn. If you leave the clippings on your lawn when you mow, you may only need to do one fertilizer application a year, and late September to mid-October is the right time to do it.
Other seasons aren’t the best choice. Fertilizing in early spring can cause a flush of new growth that uses up energy the grass should be putting into its root system. Fertilizing during the hot months of summer can damage your lawn.
Unless you have had a soil test that tells you otherwise, feed 1 pound of actual nitrogen per 1,000 square feet of lawn. A soil test would tell you if you need any phosphorus or potash (potassium). Most lawn fertilizers have no phosphorus and a tiny amount of potash. Some have micronutrients like iron, but they are not needed unless a test shows you they are deficient. Too much of any nutrient is at best not helpful and can be harmful. So, look at the first number of the fertilizer bag, make sure the second is zero, and don't worry about the third.
Fertilizer labels show the percentage of each element (nitrogen, phosphorous and potash respectively) by weight in the bottle or bag. So, a 32 pound bag of of 26-0-3 would give you 8.32 pounds of nitrogen (32 x .26), or enough for 8,320 square feet of lawn. Look at the N-P-K numbers on the label and the weight of the bag, not the "feeds 10,000 square feet" claim.
Slow-release types are preferred. Most "fall" fertilizers are slow-release, but many regular ones are, also.
Make sure to water it in after application, either with a hose or sprinkler or timing your application to be just before it rains (but not when a big storm is expected as that can wash the fertilizer to low areas).
Liquid lawn fertilizer is a pet peeve of mine, probably because I used it for several years before actually reading the entire label. The front of one bottle I just pulled up online says "feeds up to 7,200 square feet". It is a 5-pound container of 36-0-6. Doing the math, it has 1.8 pounds of nitrogen (5 x .36), which is enough to feed 1,800 square feet. Another bottle, buried in the middle of the instructions, said that it provides only 6-10% of the amount normally recommended for a fertilizer application and that it would "supplement your regular feeding." So, skip it and use the granular.
You can still aerate your lawn now, too, but you’ll want to get on that quickly. Early October is as late as the University of Minnesota lawn-care experts recommend. Happily, they say you can also aerate in late spring.
The point of aerating the lawn is to break up compacted soil and help prevent thatch. This will help your grass develop a stronger root system. It’s best to do plug aeration, which pulls little cores of earth up. You can leave the little plugs on the lawn, where they’ll break down and provide nutrients to the grass. You may be able to rent a plug aerator; lawn services will also do this task for you.
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to firstname.lastname@example.org.