Controlling Crabgrass and other lawn weeds

Foxtails, barnyardgrass and goosegrass may occasionally be problems, but crabgrass is usually present each year. Crabgrass can easily be identified by the novice by its hairy sheath and leaf. Preemergence herbicides are generally used to control crabgrass. Application of the preemergence herbicide is typically made when the forsythia begins to drop its blooms. As the crabgrass seed germinates, the seedlings contact the herbicide and are killed. Most preemergence annual grass herbicides control the germination of other hard to control weeds such as spurge, oxalis, and annual bluegrass.

If effective preemergence control is not obtained, postemergence controls do exist. Postemergence controls are effective and can be used on a spot-treatment approach to reduce the overall cost and amount of herbicide used. However, using postemergence herbicides means that some level of weed infestation will be tolerated. In addition, if using a postemergence herbicide, make sure the application is made before the grass weeds begin producing seed. Annual weeds spread only by seed so they are efficient seed producers. Controlling these weeds prior to seed production will greatly reduce the possibility of future weed problems.

Annual bluegrass (Poa annua) may be the most common grass weed problem in Michigan. Particularly on high- maintenance turf, annual bluegrass is a very tough competitor. This species produces tremendous quantities of seed in the spring of the year and continues to produce seed throughout the growing season even at mowing heights as low as 1/8". Many golf course superintendents have been forced to learn how to manage for the survival of this weed because it has become the dominate grass on many greens, tees, and fairways.

As mentioned above, most preemergence annual grass herbicides control the germination of annual bluegrass seeds, but since this species seeds germinate throughout the growing season, a late summer application, August 15- September 1, is necessary to provide season-long control. Postemergence control of annual bluegrass is very difficult to achieve. Ethofumesate (Prograss 1.5EC) is the only available postemergence control but results have been erratic. Paclobutrazol (Scott's TGR and Turf Enhancer) and flurprimidol (Cutless) can suppress annual bluegrass and allow other species to make up a higher percentage of the stand; however, once applications have ceased the annual bluegrass begins to take over the stand.

Patches of annual bluegrass can be removed with a sod cutter or sod knife and replaced with clean sod. Make sure to cut out the top 1/2" of soil to remove much of the annual bluegrass seed. Round-up can be used to kill the annual bluegrass but the seed remains and will reinfest the area. Annual bluegrass usually shows up in home lawns where traffic has resulted in compaction of the soil and thinning of the turf. Part of the replacement procedure should include relieving the compaction in the soil.

Other perennial grass weed problems are typically turfgrasses out of place. Examples include creeping bentgrass or tall fescue in a Kentucky bluegrass lawn. There are no selective controls for most perennial grass weed problems. Creeping bentgrass is best controlled by removing the sod infested with creeping bentgrass with a sod cutter and replacing with clean sod. There is a selective control for tall fescue when it contaminates Kentucky bluegrass. The herbicide chlorsulfuron, sold as Lesco TFC, will selectively remove tall fescue from Kentucky bluegrass without injuring the bluegrass. This product is not available to homeowners; consult your lawn care professional.

Quackgrass is a very tough perennial lawn weed without a selective control. Quackgrass spreads through a lawn by underground structures called rhizomes. A sod cutter does not effectively remove a quackgrass problem because the rhizomes will often be below the level at which the sod is cut. The homeowner really only has two options- live with the problem or kill off the lawn with Round-up and reestablish.

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