Managing Annual Weeds in Lawns

Published: April 1, 2011

When caring for your lawn and trying to keep it 'weed free', keep in mind that perceptions of a perfect lawn may still include a few weeds. We often receive questions across the state about identifying various weeds. The most common question following that weed identification is, "What can I buy to kill it or get rid of it?" A better question to ask about weeds is, "Why is it there?"

Weeds have been defined as plants out of place – plants growing where they are not wanted. Weeds disrupt turfgrass uniformity and aesthetics. While some individuals dislike weeds and try to do what they can to control them, others grow and mow weeds as the green canopy in their lawns.

The old 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 management practices that give lawns a fighting chance against weeds, such as planting the appropriate grass for our location and then maintaining good turf density with fall over-seeding. Proper mowing, fertilizing, and irrigation are additional practices that provide an excellent defense against weeds. Lawns, with good density, mowed tall (3 to 4 inches) can provide up to 80 percent control of annual weeds alone. The height of the mow has the greatest effect on reduction of annual weeds.

Tall fescue mowed at 3.5 inches.

Tall fescue mowed at 1.5 inches - notice large amounts of crabgrass.

By following good fertility and irrigation practices, essentially all annual weeds can be controlled, and a great reduction in perennial weeds, like dandelions and plantains, can be seen. Single fertilizer applications in the fall or early spring can improve turfgrass density sufficiently to provide 50 percent control of crabgrass. Optimum watering practices with less frequent "deep-soak" watering encourages a deep, healthy root system and maintains a drier soil surface where weeds get their advantage. Frequent light sprinkling encourages shallow-rooted weeds and weed seed germination.

A good alternative to synthetic fertilizers or the use of crabgrass preventers, is the use of corn gluten-based organic fertilizers. Corn gluten is a by-product of ethanol production that is composed mostly of plant proteins. It serves as an excellent organic fertilizer with an extra kick. As corn gluten breaks down, it releases a natural allelochemical (toxin) that works like a crabgrass preventer. Corn gluten organic fertilizers can provide up to 60 percent control of annual weeds, including crabgrass, when applied at 10 to 15 lbs per 1,000 square feet in late March or early April. Combining this type of product with a taller mowing height and dense lawn can provide nearly 100 percent control of annual weeds without applications of synthetic fertilizers or pesticides.

Crabgrass, goosegrass, foxtails, and barnyardgrass are summer annual grassy weeds. They tend to be course, textured grasses that germinate in the spring, growing well throughout the heat of the summer. Knotweed, spurge, purslane, and annual lespedeza are the more common summer annual broadleaf weeds. These broadleaf weeds also emerge throughout the spring and can persist during the entire summer. Summer annuals (grasses and broadleaves) mature by late summer and begin to slow-down in growth. They die off with the first hard frost of fall. In spring, new weed seedlings emerge around the previous year's plant, unless better turfgrass competition exists or pre-emergence herbicides are used. Pre-emergence herbicides control both annual grasses and broadleaf weeds. Check product labels for the weeds controlled.

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°F or above. In southern Missouri, this may occur as early as mid-March; in central and northern Missouri, this may not occur until late March or early April. Highest crabgrass emergence begins to occur as daily high temperatures reach 80°F. A soil temperature of 55°F in the top inch of soil for five consecutive days will provide crabgrass seed germination and provides the optimum timing for applications of crabgrass preventers. A natural guide, specific to each year's fluctuating weather patterns, is to have pre-emergent herbicide in place before the yellow blooms of the Forsythia have all dropped.

Pre-emergent herbicides will not kill annual weeds that have already emerged. A pre-emergent herbicide barrier must be present in the soil surface to kill seedlings 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 by light irrigation (0.25 to 0.5 inches) or rainfall. Many effective products are available, almost all of which are combinations of fertilizer with the pre-emergent herbicide in the same bag.

Many over-the-counter products are available at several garden centers, 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 summer annual broadleaf weeds. Products are usually designed for 5,000, 7,500, or 10,000 square feet.

Prior to applications of lawn care products, measure the area being treated to determine the total square footage of your lawn. This will enable the homeowner to purchase the proper amount of product required to treat the lawn. Before applying weed control products, always read the label directions carefully. Applying too much product can result in damaged turfgrass roots and environmental concerns. 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 to apply one-half of the rate required in two directions. This provides better distribution of particles and avoids striping. Do not spread crabgrass preventers into flower beds or vegetable gardens; they will restrict rooting of new plantings.

For more information, refer to IPM Guide 1009 – "Turfgrass and Weeds," through the MU Extension website @ http://extension.missouri.edu/.