Spring is the Time for Crabgrass Preventers

When caring for your lawn and trying to keep it “weed-free,” the saying that “the best defense is a good offense” holds true. Weeds are opportunistic and invade weakened lawns; thus, the best weapon to fight weeds is a dense, healthy stand of grass. There are several good management practices that give lawns a fighting chance against weeds, such as planting the appropriate grass for a particular location, re-seeding bare areas in the fall, proper fertilizing and correctly mowing and watering. The height of mowing influences competition against weeds such as crabgrass - the higher the cut the lower the crabgrass infestation. This also holds true for many other weeds.

Optimum watering practices involve less frequent “deep-soak” watering that encourages a deep, healthy root system and maintains a dry surface area where weeds get their advantage. However, our clay-type soils do not allow us the luxury of infrequent watering, since puddles and run-off will occur with as little as ½ inch of irrigation or rainfall. So, if we water frequently to get the right amount of water absorbed in the soil, while avoiding puddles and runoff, these frequent light waterings will encourage shallow-rooted turfgrasses and weeds while promoting weed seed germination. So, try to keep from over-watering in the spring when we usually receive sufficient rainfall and try to maintain a drier soil surface through the early spring.

Crabgrass is a summer annual weedy grass species. It is a course-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 in the lawn and allow it to smother nearby bluegrasses or fescues. 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 70oF 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 80oF. Our general rule is that for areas south of I-70, application should be made by March 15; 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 pre-emergent herbicide in place before the yellow blooms of the Forsythia have all dropped.

Pre-emergent herbicides will not kill crabgrass that has already emerged. A pre-emergent herbicide layer 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 with the pre-emergent herbicide (or crabgrass preventer) in the same bag. These combinations provide effective ways to fertilize your lawn and control your crabgrass with one application. Both need to be watered in, to be activated (effective).

Many over-the-counter products are available to you at several garden center, hardware stores, farm centers and nurseries. 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.

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. It is also recommended that 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 will restrict rooting of new plantings. The fertilizer supplied in these crabgrass products is usually designed to provide 1 pound of nitrogen per 1000 square feet. This is an adequate amount of nitrogen for good spring growth. You should not need any additional fertilizer during the rest of the spring or summer. Excessive spring and summer fertilization favors shoot growth over root growth, requiring more mowing and resulting in a less stress-tolerant lawn during the summer. The optimum time to fertilize your lawn is during the fall. For further information on lawn weed control, you can refer to MU Extension Guide G6750 (http://muextension.missouri.edu/xplor/agguides/hort/g06750.htm) or purchase a copy of “Turfgrass and Weeds,” IPM Publication Guide 1009 from Extension Publications @ 800 292-0969.

Brad Fresenburg, UMC Extension/Reasearch Associate, Turfgrass Research Center, (573) 443-4893