Weeds in Lawns

Undesirable grass weeds that infest lawns include both annuals and perennials. Annual grasses are species that complete their life cycle within a year whereas perennial weeds persist from year to year. Crabgrass is the most common and troublesome annual grass weed in lawns. Others include foxtails, barnyardgrass, goosegrass, and annual bluegrass.  Perennial weeds include quackgrass, bindweed, nutsedge, brome-grass, and others. Management options differ between the two classes, with annuals generally being easier to control.

Control of Crabgrass and other Annual Grass Weeds

Crabgrass and other annual grass weeds can be treated through both chemical and  cultural methods. Producing a dense, healthy lawn with vigorous grass is the best way to control annual grass weeds and other broadleaf weeds. The proper mowing height and frequency, adequate but not excessive fertilization, and proper irrigation are part of any weed control program and should be practiced throughout the growing season. Crabgrass often invades areas seeded in late spring because of bare soil, frequent watering, and the onset of hot weather, which are ideal conditions for its growth. It also tends to invade established lawns that are mowed to short or are overwatered. Crabgrass likes full sun, thin grass, and water.

Selective annual grass weed control can be accomplished using herbicides (weed killers). Preemergence herbicides provide effective control of crabgrass and other annual grass weeds for several weeks or months, depending upon amount used and products. The effectiveness of these materials is based upon their ability to provide excellent weed control with no injury to lawn grass.

Timing is important for preemergence herbicide application. It is necessary to apply them before annual grass seeds germinate. The best time for crabgrass preemergence herbicide application is two weeks prior to the expected weed seed germination period, which is in the spring when the forsythias bloom – late April in Rhode Island – because crabgrass germinates when soil temperatures are greater than 50o F; other annual grasses germinate as the soils get warmer than 60o F. The herbicides should be watered-in so that the chemical can form a barrier in the soil prior to weed seed germination.

Selective control of germinating annual broadleaf weeds such as chickweed, yellow woodsorrel, and prostrate spurge can also be obtained with preemergence treatments. Some herbicides may be reapplied for extended control; refer to the label for  timing and rates. Check with your local garden supply center for specific product availability; they should be safe for use on established lawns – read product labels. Siduron is the only preemergence herbicide that can be used at time of lawn seeding because it will not prevent grass seed from germinating.

If crabgrass is appearing in mid to late summer remember that it is an annual plant and dies as temperatures drop in the fall. Postemergence crabgrass herbicides need to be applied when crabgrass plants are small; typically crabgrass is noticed too late for these to be effective.

Control of Perennial Grass Weeds

Perennial grassy weeds are considered to be the most difficult weed problems to deal with in lawns. Control options are limited because the weed species are similar to the lawn species. In fact, many perennial grassy weeds are not considered weeds, but are considered desirable grasses when growing by themselves under a different set of conditions. For example, several common perennial grasses, such as tall fescue and creeping bentgrass, when growing in Kentucky bluegrass lawns, are considered “weeds” because they differ greatly in leaf width, color, or growth habit. But there are perennial grasses that are frequent weed problems. One way to distinguish perennial grasses from annuals is the time of the year established plants are present. Perennials will appear as established green grasses in early spring; whereas annual grasses like crabgrass don't appear until late spring or early summer. Likewise, most annuals die off quickly in fall, but perennials do not.

One of the most difficult perennial weeds to control is nutsedge. It is an adaptable weed that makes its home in low ground along water courses or in rich sandy soils where gardening is practiced. The nut-like tubers (found underground) are what make nutsedge difficult to eradicate. If the plant is not completely removed from the soil and the tubers remain they can produce new plants in the lawn. Planting grass seed is about the only means of preventing nutsedge from taking over the lawn. When planted in late summer (August 20 – October 1) the grass plants will spread laterally and the lawn will be dense and mature before most troublesome lawn weeds appear.

Most common perennial lawn weeds can be greatly reduced by improved lawn care. For small lawns or where there are only a few weeds, hand pulling as weeds appear can give good temporary results. Weeds can be pulled more easily after either a heavy rain or watering. The use of herbicides to control perennial weeds is more practical for large areas, more difficult-to-pull weeds, and small patches of persistent weeds. Large areas can be treated with sprays or granules, which give good weed control with minimal injury to lawn grass. Single clumps of weeds can be spot treated. Aerosol "spot weeders" are available for such areas. When weeds infest an area of any size and the populations are high enough throughout the area to be objectionable an overall treatment with a selective chemical that will kill the weeds and leave the grass unharmed may be necessary. Where weed infestation is serious and the turf is poor, consider renovating the lawn. Check the herbicide label for the proper way and most effective time to apply herbicides.

Undesirable Plants as Indicators of Lawn Problems

To minimize weed problems:

• Select the recommended grasses for your location.

• Plant a new lawn in late summer (August 20 - October 1).

• Use adequate fertilizer (and limestone if needed), especially when starting a new lawn.

• Get the soil tested.

• Mow the lawn to a height of no less than 2½ inches to better compete with weeds.

• Control diseases and insects that damage grass and allow weed encroachment.

• Water the lawn deeply and thoroughly, not too little too often.

Adapted from: University of Massachusetts Extension, 1999; University of Illinois Extension, 2007; Tom Kowalsick, Cornell Cooperative Extension, 2001

Useful Websites

Weed Control in Lawns and Other Turf (University of Minnesota Extension)

http://www.extension.umn.edu/distribution/horticulture/DG1137.html

Weed Problems in Lawns (University of Illinois Extension)

http://www.urbanext.uiuc.edu/lawnchallenge/lesson6b.html