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Washington State University Extension

Garden Tips

CONTROLLING ANNUAL GRASSY WEEDS IN YOUR LAWN

GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published February 28, 2016

It would be wonderful if new weed-free lawns could stay that way forever. Right? However, weeds “get their foot in the door” when lawns are subjected to stress from the environment or poor management practices, including watering, mowing, and fertilization. However, weeds will eventually show up in lawns as they age, even in ones that are well maintained.

Much has been accomplished in the field of weed science for the control of broadleaf lawn weeds, like dandelions and clover. Whether chemicals are utilized or removal by persistent pulling or digging is employed, most broadleaf weeds in lawns can be controlled relatively easily.

Grassy weeds are a different story. It is difficult to pluck out most grassy weeds from a lawn interspersed with regular lawn grass. Using chemicals to control grassy weeds is difficult because many chemicals that will kill the grassy weeds will also kill lawn grass. Before you can begin to consider using chemicals to control grassy weeds in your lawn, you need to understand how grasses grow.

There are two main types of grassy weeds, annual and perennial. Annual grasses die and come up again from seed every year. Crabgrass and annual bluegrass are the two most common annual grasses that cause problems in our area lawns. Excessive watering, frequent shallow watering, and consistently mowing a lawn too short are practices that make it easier for crabgrass to get started in a lawn. Excessive watering and compacted soil are conditions favorable to annual bluegrass. Correcting these problems and making a lawn as healthy and dense as possible with proper maintenance makes it more difficult for both of these annual grasses to persist in a lawn.

Chemicals are available that can provide help in managing these two weedy annual grasses in lawns. Preemergent herbicides chemically prevent seed germination and are applied before the seed of the annual grasses have the opportunity to germinate and grow.

Crabgrass seed germinates in the spring and preemergent herbicides or “crabgrass preventers” are only effective if the application is made prior to seed germination. The right timing for an application of a crabgrass preemergent herbicide is when the soil temperature at a depth of one inch is greater than 55 degrees for at least a week. This typically occurs when the yellow-flowering forsythia bush has been in full bloom for a week or two.

While some annual bluegrass seed germinates in the spring, most of the seed germinates in early to mid-fall, growing rapidly during mild winter and early spring weather, flowering in the spring and summer and producing lots of seed. Preemergent herbicides applied before crabgrass germinates will not persist long enough to prevent most of the seed from germinating. However, preemergent herbicides applied in the spring for preventing crabgrass will discourage early germinating annual bluegrass seed. For effective control of annual bluegrass, a preemergent herbicide should be applied in mid-August.

There are also postemergent herbicides available to kill seedlings of crabgrass and annual bluegrass if a preemergent materials are not applied at the right time. However, these chemicals are only effective if the plants are relatively young and small. These materials have the potential to injure your lawn grass if not applied correctly, so be sure to read the label and follow the directions before use.

Garden Tips, WSU Extension, Benton County, 5600-E West Canal Drive, Kennewick, WA 99336-1387, 509-735-3551, Contact Us

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