Over-the-Counter Pre-emergence Herbicides for Crabgrass Control

Homeowners are faced with a barrage of products to choose from for crabgrass control every spring. Many of these products are available at different garden centers and suppliers (Ace Hardware, Home Depot, Lowe's, MFA, Orscheln's, Wal-Mart, local nurseries, etc.) around the state.

Several product lines offered to consumers include the Bayer Advance Lawn Product line (http://www.bayeradvanced.com/pg/pg_lp.html), the Scotts Lawn Care Products (http://www.scotts.com), with the Scotts web site also carrying a link to the Ortho Lawn Care line of products.

Many other good product lines include Ace, Club Green, Green Light, Howard Johnson, Lesco, Real-Kill, Spectracide, Sta-Green, and Vigaro. These are available and Vigaro. These are available through some of the various outlets listed above and others.

Crabgrass is a summer annual grassy weed species. It is a coarse-textured grass that germinates in the spring and grows well throughout the heat of the summer. Its wide leaf blades, heat tolerance and prostrate growth habit make it an eyesore on the lawn and allow it to smother nearby turfgrasses. During the summer, crabgrass will produce seed heads even at low mowing heights.

Crabgrass plants will be killed by the first hard frost in the fall, and will drop their seed heads. In the spring, the new crabgrass seedlings emerge around the previous year's plant, unless this open space is re-seeded during the fall with a desired grass and a pre-emergent herbicide is applied to kill the germinating crabgrass seedlings.

Pre-emergent herbicides are so named because they must be in place before crabgrass seedlings and other weeds begin to emerge. As a general rule, crabgrass may begin to germinate when daily high temperatures begin to reach 70^oF or above. In southern Missouri this may occur as early as mid-March; in central and northern Missouri this may not be until late March or early April.

Highest crabgrass emergence begins to occur as daily high temperatures reach 80^oF. Our general rule is for areas south of Interstate 70, application should be made by March 30; for areas north of I-70, application should be made by April 15. A natural guide, specific to each year's fluctuating weather patterns, is to have your preemergent herbicide in place before the yellow blooms of the forsythia have all dropped.

For more information on over-the-counter pre-emergence herbicides, visit the MU Extension Website at: http://muextension.missouri.edu/explore/agguides/hort/g06750.htm

Pre-emergent herbicides will not kill crabgrass that has already emerged. A pre-emergent herbicide barrier must be present at the soil surface to kill the crabgrass seedling when its first root contacts the soil. Therefore, it is imperative that the pre-emergent be applied at the right time and watered down into the soil surface either by light irrigation or rainfall.

Many effective products are available, almost all of which are combinations of fertilizer and the pre-emergent herbicide (or crabgrass preventer) in the same bag. These combinations are an effective way to fertilize your lawn and control your crabgrass with one application. Both need to be watered in to be activated and effective.

Crabgrass preventers containing Dimension (dithiopyr), Pre-M (pendimethalin), Barricade (prodiamine) or Ronstar (oxadiazon) are excellent choices for the control of crabgrass, other summer annual grassy weeds and some summer annual broadleaf weeds. Be sure to check the label (active ingredient statement) where herbicide information is listed. Make sure you are purchasing the product you desire.

You will need to measure the area you wish to treat and determine the total square footage of your lawn. Many of these products are marketed for various square footages, whether 5,000 or 10,000 square feet. Simply purchase the size bag of the product that best fits your needs.

When applying weed control products, read the label directions carefully. Do not over apply, either by excessive overlapping or by applying more material than recommended to a specific area. Applying too much product could result in damage to turfgrass roots.

Determine the effective application width of your rotary spreader and space out each spreader pass to ensure uniform coverage with minimal overlap. We also recommend you apply one-half the rate required in two directions. This allows better distribution of the particles and avoids striping. Do not spread crabgrass preventer products into flower or garden beds; they can restrict rooting of new plantings.

For further information on lawn weed control, you can refer to MU Extension Guide G6750 (http://muextension.missouri.edu/explore/agguides/hort/g06750.htm) or purchase a copy of "Turfgrass and Weeds," IPM Publication Guide 1009 from Extension Publications at (800) 292-0969.

Brad S. Fresenburg, University of Missouri