Q. Help! My lawn has been overrun with crabgrass this year. I used a pre-emergent herbicide in April, but it did not seem to do much. Do you have any suggestions?

A. Crabgrass is a summer annual weed that grows from seed every spring. It would be a waste of time, energy and money to spray crabgrass this late in the season. It will die when we have our first frost in the fall. The best thing you can do now is to mow regularly to remove the seedheads before they mature. Bag and dispose of your clippings, rather than composting them. Like most annual weeds, crabgrass is prolific seed producer. Also, anything you can do to encourage a thick, healthy lawn will go a long way toward crowding out the crabgrass. A combination of cultural and chemical controls will give the best result.

Annual grassy weeds such as crabgrass are best controlled with pre-emergence herbicides that are applied before the seed germinates in the spring. These products prevent crabgrass seeds from germinating but have no effect on established plants, so it is critical to apply them at the proper time. Crabgrass seed germinates when temperatures in the upper inch or so of soil reach 55 to 58 degrees for four or five consecutive days. This usually occurs in mid-April in our area. Another rule of thumb is to have it in place when forsythia blooms.

Crabgrass continues to germinate in our area until mid- to late July, when we get into very hot, dry weather. Pre-emergence products include Dimension (dithiopyr), Halts (pendimethalin), Barricade (prodiamine) and Team (benefin and trifluralin). They are often sold in combination with fertilizer, such as Step 1 in the Scotts Turf Builder program. Organic gardeners can use corn gluten, which does not provide the same level of control as these other products at first but will work if you use it for a few consecutive years. Pre-emergence herbicides should be watered in after application to activate their seed-killing properties and protect them from breaking down on exposure to sunlight.

These products may not perform as expected for a number of reasons, including improper timing of application, improper rate, wrong spreader settings or practices such as core aeration that can disrupt the protective barrier created by the herbicide. This year, heavy rain through June and early July and cool weather so far in August have extended the germination period for crabgrass seed right into August. Although the effective life of pre-emergence herbicides varies from one product to another, the frequent rain likely affected all of them.

There are a few products that can be used to control crabgrass after it germinates, but they are most effective on very small plants, with just two or three leaves. Mature crabgrass plants that have grown all summer would laugh at them! These include Ortho Weed-B-Gon Crabgrass Killer for Lawns (calcium acid methanearsonate) and Weed Hoe (monosodium methanearsonate).

In addition to herbicides, cultural practices that encourage a thick, healthy lawn are critical to winning the war on weeds of all kinds. Start with a soil test to see what needs to be done to adjust the soil pH and fertility. Soil test kits are available from your local Penn State Cooperative Extension office. In Allegheny County, kits are $12 or $9 for each additional kit for separate soil tests. You can send a check, payable to Penn State Cooperative Extension, to PSCE, 400 N. Lexington St., Pittsburgh 15208. Please write Attn. Soil Test Kit in the lower left corner.

If you have lawn areas that are very different -- for example, if your front lawn is exposed to full sun while mature trees shade your back yard heavily -- order two kits and test those areas separately. You should take separate tests for flowerbeds or vegetable gardens. The lab's recommendations are based on what you tell the lab you are growing in a given area.

The cost of the kit includes the testing. Your only other cost is for the postage to send the sample to Penn State's Agricultural Analytical Laboratory in University Park.

Other cultural practices that help your lawn outcompete the weeds include raising the mowing height to 2 1/2 to 3 inches. In addition to shading out germinating weed seeds, a higher lawn has a deeper, more extensive root system that makes it more drought-resistant.

If you water your lawn during hot, dry weather, it is important to water deeply and infrequently. Apply 1 to 1 1/2 inches of water once a week, rather than watering a little bit every day. This also encourages a deep root system. Shallow, frequent watering actually works to crabgrass' advantage because it weakens the lawn and creates openings for more crabgrass seed to germinate.