How to eliminate

By: Sandy Feather ©2010
Penn State Cooperative Extension

Sandy's Gardening Columns

Q: My issue is a severe infestation of crabgrass. We have been using the Scott’s 4-Step program for about three years. We have seen general improvement of the yard except for the crab grass issue.  Several of our neighbors also have crab grass issues but ours seems to be the worst. Can you please provide us with your insight as to how we can get rid of this crab grass?
  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. The best thing you can do now is to mow regularly to remove the seedheads before they mature as much as possible. 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 to crowding out the crabgrass. A combination of cultural and chemical controls will give the best result.

Controlling Crabgrass

Annual grassy weeds such as crabgrass are best controlled with pre-emergence herbicides that are applied before crabgrass seed germinates in the spring. These products work by preventing the weed seeds from germinating, but have no effect on established plants. It is critical to apply them at the proper time for optimum control. Crabgrass seed germinates when temperatures in the upper inch or so of soil reach 55 to 58 degrees F for four or five consecutive days. This usually occurs in mid-April in our area. Another rule of thumb is to have your crabgrass pre-emergence herbicide in place when forsythia blooms. Crabgrass continues to germinate in our area until mid – late July, until 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. It does not provide the level of control that these other products provide at first, but if you use it for a few consecutive years, you should see some improvement.

crabgrass seedheads
Seedheads of crabgrass

Pre-emergence herbicides can fail to perform as expected for a number of reasons, including improper timing of application, improper rate, wrong spreader settings or any practices such as core aeration after pre-emergence application that would disrupt the protective barrier created by the herbicide. Also, 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.

There are a few products that can be used to control crabgrass after it germinates, but they are most effective on very small crabgrass plants - those with two or three leaves. The 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).

Finally, 2010 was a great year for crabgrass. Since it is truly a warm season grass, the hot weather favored crabgrass over our perennial, cool season lawn grasses. Hot, dry summer weather makes cool season grasses go dormant unless you water regularly. Even then, they are not growing strongly because they are so heat stressed.

Cultural controls for crabgrass

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 to meet the needs of your lawn. Penn State soil test kits are available from your local Penn State Cooperative Extension office. In Allegheny County, kits are available for $12, with additional kits for separate soil tests costing $9 each. You can send a check, payable to Penn State Cooperative Extension (PSCE) to Penn State Cooperative Extension, 400 North Lexington Street, Pittsburgh, PA 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 have those areas tested separately. You should also 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 kits come with instructions for taking a good sample. The cost of the kit includes the testing. Your only other cost is for the postage to send the sample up to Penn State's Agricultural Analytical Laboratory in University Park, PA.

Other cultural practices that help your lawn outcompete the weeds include raising the mowing height to 2.5 - 3 inches. In addition to shading out germinating weed seeds, leaving your lawn higher helps it maintain a deeper, more extensive root system. This also makes it more drought resistant. If you water your lawn during hot, dry weather, it is important to water deeply and infrequently. Apply one to one-and-a-half inches of water once a week, rather than watering a little bit every day. Deep watering also encourages a deep root system. Shallow, frequent watering actually favors crabgrass because it weakens the lawn, encourages a shallow root system and creates openings for more crabgrass seed to germinate through the growing season.


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