Now is not the time to put away your lawn mower and tools. Mid-September to mid-October is the best time to groom your lawn and give it a head start for next season, whether you have several acres or 10 square feet in front of a townhouse.

Fall lawn-care tasks include killing weeds, applying lime to sweeten the soil, aerating to combat thatch and compaction, raking and composting dead leaves, and mowing. The weather is ideal for starting a new lawn or overseeding to improve an existing one.

But before you start, take a tip from John Del-Signore, a golf course superintendent for Fairfax County: Use a long tape measure or a measuring wheel, and take the dimensions of your lawn.

Del-Signore, who gives classes in lawn maintenance, says most homeowners have no idea what the square footage of their lawn is. "This is fairly important," he said, "It tells you how much product you need, whether fertilizer or seeds." People who don't know how big their turf area is often overbuy when it comes to lawn care. Seeding is a prime example. Anyone applying more than three pounds of seed per 1,000 square feet of lawn is overseeding.

The goal of any lawn care is thick, lush and healthy-looking turf. Del-Signore says he has run into gardeners who consider lawn care a nuisance, but he always points out that a gorgeous garden and a ratty lawn are incompatible.

"The lawn accents the garden as much as the garden accents the lawn," he said. A beautiful lawn adds to the overall kempt look of a property. Thick turf is the underpinning of all lovely lawns and the best defense against weeds.

When weeds overpower simple pulling, use a spot treatment of liquid broadleaf weed killer. Synthetic chemical lawn weed killers should be used sparingly and only when necessary. There are a number of liquid lawn weed killers on the market, and your local garden center can recommend one. Always carefully follow all labeled instructions.

You can also use a corn-based, all-natural pre-emergent weed control. After a couple of years, this product achieves the same results as weed killer. Always follow labeled instructions and do not apply any type of weed control, natural or otherwise, where you are seeding your lawn.

Older lawns suffer from compacted soil and from thatch, which is dead turf collected at the base of plants. This contributes to water runoff and makes nutrients less available. Now is the time to enrich the soil under your lawn by aerating it with a plug aerator.

You can do this yourself, by renting an aerator from an equipment and tool rental company, or hire someone to do it for you. Go over the lawn at least three to four times -- more often over poor clay soil. The soil should have some moisture for the aerator to penetrate properly, but never aerate wet soil. The best aerators remove plugs of soil, instead of just punching holes.

Once the soil is aerated, it's an ideal time to add soil enrichments. If you haven't applied lime to the lawn yet, now is the time to do so. Get the soil tested at your local garden center or Cooperative Extension Service, to find out what specific nutrients are lacking and what the exact pH is. Del-Signore says he applies lime at a rate of 20 pounds per 1,000 square feet, a good maintenance dose.

Laying fertilizer over the surface of the soil after aerating gives the nutrients the opportunity to work their way into the soil when the grass needs it. Look for a fertilizer with 10 to 20 percent water insoluble nitrogen in a 30-2-8 or similar ratio formulation. You will find every breakdown you can imagine, but basically the first number is nitrogen, the second phosphorous and the last potassium. Fertilize at the pounds per square foot rate recommended on the material.

Once you have fed and aerated the lawn, it's an excellent time to overseed. Pick a cool-season seed, choosing between two styles, compact, turf-type tall fescues or fine-leafed types, such as bluegrasses, fine fescues and perennial ryes. I don't like to mix coarser-textured tall fescues with fine-bladed grasses.

Tall fescues are wear-tolerant, disease resistant and can be mowed three to four inches tall. Fine-textured bluegrass, fine fescue and perennial rye are more prone to disease, but softer to the eye and touch.

For new lawn installations, Del-Signore likes to seed with a machine that slices small grooves in the soil and deposits the seed in them. He cautions that you should always seed in two different directions to avoid the "cornrow" look. If you have a tiny lawn, he says, a pitchfork can become your aerator and a stiff rake your seeder. After punching holes with the pitchfork, rake the soil and seed, then turn the rake over and use it to lightly cover the seeds. Water lightly three to four times a day.

Post-aeration and post-seeding is also the ideal time to apply compost. The simplest way is to use a mower that finely chops fallen leaves. If you are spreading other compost, sprinkle it so it gets down into the aeration holes, but not so much that it covers the blades of grass. If the compost is fine enough, try using a broadcast spreader; otherwise you will need to sprinkle it with a shovel or by hand. In a couple of years, the organic material alone will make the lawn thick and green.

Fall is the ideal time to seed, Del-Signore says, because the days are still warm, the nights are a little cooler, and the ground is still toasty enough. Seeding now will encourage the best root development.

When new turf is about 2 to 2 1/2 inches tall, it can be mowed. Del-Signore says the biggest mistake people make when mowing is to use a mower with dull blades. Dull blades tear the grass unevenly and impede its ability to spring back. Always try to mow in a different direction than the last time.

A common, but incorrect, mowing practice that I have seen used on cool-season turf is to cut it as low as possible once or twice a year for rejuvenation. This doesn't make it stronger, and, if you scalp areas when mowing with too low a blade height, you open bare spots in the turf where crabgrass and broadleaf weeds can get a foothold.

Except for application of synthetic weed killers, all of these lawn-care practices are successful only when combined with rain. That, with sun, is what makes the grass grow. In dry years, you may have to water one inch weekly. Measure one inch by catching one inch of water in a container placed under the sprinkler.

In a year like this, Del-Signore points out, you might just want to wait until some of the hurricanes have cleared out.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.

The season is perfect for raking and composting dead leaves, killing weeds, applying lime, aerating and mowing.