It's prime lawn care time

Early fall is a time of active growth for grass, both above and below ground. That means regardless of the current condition of your grass, it's prime time for lawn care. Any effort you put into your yard now will pay dividends not only this fall, but also next spring and summer.

Here's how to keep your lawn looking great:

Because turf grasses are growing so actively now, they're able to take up and make use of fertilizer most effectively.

Apply a high-nitrogen fertilizer in early to mid-September, then water the lawn lightly afterward to make sure the granules reach the soil and don't wash away. (Nitrogen is always the first of three numbers that give the nutrient analysis of any fertilizer.)

The University of Minnesota no longer recommends a second application later in autumn, because grass is less able to absorb nitrogen then. One fall application will do.

Though we often receive plenty of rainfall in autumn, it's not unusual to run into a dry spell.

If we haven't had rain for a week or so, you should water the lawn, especially if it's been warm or windy. It best to water deeply, rather than lightly, but how often you need to water depends on the weather. As the temperatures drop, you can water less frequently, but as long as grass continues to grow it will need water -- whether from the sky or your sprinkler.

Keeping the grass taller during summer (2 1/2 to 3 inches) results in deeper root growth. But once the weather cools off, you can gradually reduce the height of the grass. By the final mowing, your lawnmower blades should be set so the grass is only about 2 inches tall. If the grass blades are left too tall going into winter, they can pack down, which makes the grass more prone to disease.

Early September is the best time to overseed thin patches of grass. Soils are still warm, there's usually more rainfall, and nights are longer and cooler -- all favorable conditions for grass seed to germinate and grow rapidly. Plus, few weed seeds are programmed to sprout now, so there's less competition.

Scruff the soil so seeds make good contact rather than sit on a hard-packed surface. Aerifying the lawn before overseeding loosens the soil and creates an excellent surface for planting. Fertilize with standard lawn fertilizer or one formulated specifically for use when planting grass seed. Then water lightly as often as needed to keep the soil moist. Water more heavily and less frequently as the young grasses grow. Mow the areas that are overseeded when existing grasses grow too tall. Most important, do not use any form of herbicide in these areas until next year, including fertilizer/herbicide combinations.

An abundance of crabgrass has been one of this year's most common complaints. The repeated heavy spring rains interfered with pre-emergence herbicides. So even lawns that were treated for crabgrass may have lots of it.

Because crabgrass is an annual weed that dies over the winter, there's no point in using weed killers on it now. Instead, plan to apply a pre-emergence herbicide to infested areas next spring.

Natural products containing corn gluten meal also prevent crabgrass, but they take several years of spring and late-summer applications to be the most effective.

By the latter part of September, temperatures will have cooled enough to begin using broad-leaf herbicides on dandelions, plantain, creeping Charlie and other perennial weeds, which spring back from the same roots year after year.

Because perennial weeds are storing nutrients in their roots now for next year's growth, they'll take in herbicide more readily in the fall. On really tough weeds such as creeping Charlie, you can add a second herbicide application two weeks after the first.

If you prefer not to use herbicides, manually dig out perennial weeds. And remember, the weeds you remove this fall won't be around to produce seeds next year.

If your soil is hard or you have a thick buildup of thatch (more than 1/2 inch), your lawn will benefit from core aeration in September. Aeration takes small plugs out of the soil, which allows water, fertilizer and oxygen to penetrate below the surface and encourages good grass growth. (The small cores of soil should be left on the surface of the grass, so they break down and top-dress the soil.) You can rent an aeration machine (it's hard work) or hire the job out.

If the thatch is really thick, rent a dethatching machine, also known as a "vertical mower." (Again, this is hard work you may wish to hire out.) The machine slices through the grass, bringing up lots of thatch, which will have to be raked up and added to your compost pile.

Aerating and dethatching may be done on the same day, but they will dry the soil rapidly, so be sure to water the lawn once you're done.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota.