The University of Minnesota recommends fertilizing grass lawns in early September but has called it quits on doing a second application in November.
Whether you lavish care on your lawn or ignore it, there's no escaping this fact: To look good, grass needs to be fertilized.
Fertilizing not only makes grass green, it's also a green thing to do. Healthy, well-fed grass develops dense roots that hold the soil, preventing it from running off your yard and polluting the water.
But for fertilizer to benefit the grass and the environment, it needs to be applied correctly.
You've probably heard that it's important to keep fertilizer off sidewalks, driveways and streets, where it can run off and pollute streams, rivers and lakes. But you may not know that timing plays a key role in keeping fertilizer on our lawns and out of our lakes.
Fertilize in early fall
Without a doubt, early fall is the best time to fertilize. That's because most of the fertilizer applied on or around Labor Day goes straight into the grass. In the past, a second, later application -- in early November -- also was recommended. It was thought that this second application helped grass to green up earlier in the spring. But new research from the University of Minnesota casts doubt on that practice.
Brian Horgan, an associate professor of horticultural science, has been studying how much nitrogen is taken up by grass when fertilizer is applied at different times of the year. His research has shown that when fertilizer is applied in November, most of the nitrogen never gets to the grass. Instead, it runs off the surface of the soil or leaches into groundwater.
The research is behind new guidelines about how and when to fertilize. But there's not a one-size-fits-all approach. When and how you should fertilize depends on whether or not you irrigate your lawn and the quality of the soil.
(Good soil has at least 3.1 percent organic matter; poor soil has less than that. To check the amount of organic matter in your soil, get your soil tested through the University of Minnesota's soil testing lab at soiltest.cfans.umn.edu.)
Here are some fertilizing schedules to follow if:
You're lazy : If you don't irrigate your lawn and want to fertilize only once a year, do so around Labor Day. Apply fertilizer at a rate of 1 pound of nitrogen per 1,000 square feet. (Remember, the first number on the bag of fertilizer is the percentage of nitrogen it contains. A 10-0-5 fertilizer would contain 10 percent nitrogen, so you'd need to apply 10 pounds of fertilizer to deliver 1 pound of nitrogen.)
You have poor soil: If you have poor soil and don't irrigate your lawn, you'd be wise to fertilize twice a year. Apply 1 pound of nitrogen per 1,000 square feet around Labor Day and another lighter application around Memorial Day. The spring application should be at half the rate of the fall application, or a half-pound of nitrogen per 1,000 square feet.
You irrigate good soil: Irrigated lawns tend to need more fertilizer because the water moves nitrogen quickly through the soil and because the rapidly growing grass absorbs fertilizer more quickly. For irrigated lawns with good soils, three applications are best. Aim for 1 pound of nitrogen per 1,000 square feet on Labor Day, a light application (half a pound of nitrogen per 1,000 square feet) at the first mowing in spring, and another application around Memorial Day.
You irrigate poor soil: For irrigated lawns with poor soils, you should also fertilize four times. Do a full application (1 pound of nitrogen per 1,000 square feet) on Labor Day and light applications (half a pound) at the first mowing in spring, around Memorial Day and the first week of August.
Type of fertilizer
When fertilizing in late summer or early fall, choose a fertilizer that delivers about half of its nitrogen in slow-release form. (It might be called slow-release, water-insoluble or controlled-release, depending on the manufacturer.) Slow-release fertilizers can cost a little more, but they're particularly effective in fall.
For early spring applications, a fertilizer with 20 to 25 percent slow- release nitrogen will be a little cheaper and get the job done.
Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He's also the author of "How Trees Die," "The Truth About Garden Remedies" and "The Truth About Organic Gardening" (Timber Press, $12.95).