In mid- to late summer, some homeowners may see crabgrass taking over their lawns and head to the garden center. Unfortunately, according to a turfgrass specialist in Penn State's College of Agricultural Sciences, little can be done about crabgrass in summer. The time to kill crabgrass is early spring.
"Crabgrass is a summer annual grass weed," explains Peter Landschoot, associate professor of turfgrass science. "That means the plant starts growing in the spring or early summer and is killed by the first frost in fall. During its life cycle, a crabgrass plant can produce thousands of seeds, which will infest your lawn and germinate the following year."
Landschoot says that your first line of defense against crabgrass is to use good cultural practices -- such as proper fertilization, mowing and irrigation -- to thicken the lawn and crowd out the weeds. "This should help to cut down the amount of crabgrass in the lawn, but good cultural practices may not eliminate the problem," he says. "Where crabgrass is stubborn, homeowners can apply a 'pre-emergence' herbicide designed to kill the plant before it starts to grow."
Most pre-emergence herbicides are marketed as weed-and-feed products; that is, herbicide and fertilizer rolled into one. The herbicide and fertilizer are granulated, allowing homeowners to use spreaders to apply herbicide while fertilizing at the same time.
Landschoot warns homeowners not to grab the first weed-and-feed product they find at the garden center. "There are two types of weed-and-feed products: one for crabgrass and another for broadleaf weeds, such as dandelions," he says. "In addition, crabgrass control must be used in early spring, while broadleaf weed controls are most effective in late spring or fall."
If homeowners cannot distinguish between the two weed-and-feed products, Landschoot strongly suggests asking a garden center employee or the manager for help.
"A common backyard rule of thumb is to apply crabgrass herbicide when forsythia blooms begin to drop off the plant," Landschoot says. "But that doesn't always work, so it's better to go by date."
Landschoot recommends the following general periods for successful crabgrass herbicide applications in Pennsylvania: Southeastern Pennsylvania: March 15 to April 15. Northern tier and high-altitude counties: April 20 to May 20. Other Pennsylvania areas: April 1 to May 1. Landschoot also says that most herbicide applications will not provide 100 percent control, but they will eliminate most of the crabgrass plaguing a lawn for the remainder of the growing season. Landschoot warns against applying a pre-emergence crabgrass herbicide in the summer or fall. The herbicide is not effective once the crabgrass is visible in the lawn. If you miss your spring treatment, let the frost kill the plants in fall, then consider making an application next year.
"Most pre-emergence herbicides also kill lawn grass seedlings, so don't apply seed to your lawn for three to four months after using a pre-emergence product," Landschoot says. "It's better to overseed in September if you have a major weed problem."
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