• Know Your Lawn Grasses
• Dealing with Shady Sites
• Seeding and Sodding Lawns
• Watering, Mowing & Fertilizing
• Thatch & Lawn Renovation
• Weed Problems
• Grubs & Other Insect Pests
• Managing Home Lawn Diseases

Weed Problems in Lawns

Managing Grass Weeds in Lawns

Grass weeds in lawns are classified as either annual or perennial. Management options differ between the two classes, with annuals generally easier to control.

Crabgrass and other annual grass weeds are common problems in home lawns that can be treated through both chemical and nonchemical methods. Proper lawn care practices to encourage a dense stand of vigorous grass is the best way to prevent weeds from invading. For example, mowing height can have a big impact; lawns mowed higher (over two inches) tend to have less problems with annual grasses such as crabgrass. Close-mowed lawns tend to open up, allowing weeds like crabgrass to invade. Light, frequent watering also favors crabgrass. Crabgrass often invades areas seeded in late spring because of bare soil, frequent watering, and the onset of hot weather, which is ideal for its growth.

Herbicides (weed killers) are also available to manage annual weeds. Preemergence herbicides prevent annual grass weeds such as crabgrass from emerging. Timing of application is important, as the weed killer should be applied to soil before the crabgrass emerges from the soil. Crabgrass will germinate when soil temperatures are greater than 55 to 60 degrees F. for 7-10 consecutive days, and continues until soils reach 95 degrees F. Other annual grasses germinate as the soils get warmer than 60 degrees.

For northern Illinois, late April to early May is the suggested time for applying a preemergence crabgrass herbicide. If the spring is very warm, consider late April. In cold, "late" springs, these materials could be put down well into May. Using forsythia blooming as a guide is not dependable. Many preemergence crabgrass herbicides are available in combination with lawn fertilizer, so the crabgrass prevention and spring fertilization can be done at the same time. Preemergence herbicides include benefin, benefin/trifluralin, bensulide, dithiopyr, oxadiazon, pendimethalin, prodiamine, and siduron. Some herbicides may be reapplied for extended control; refer to the label for timing and rates.

Core aerifying or dethatching should be done based on label instructions. One of the management problems associated with preemergence herbicides is seeding or overseeding practices. Except siduron (Tupersan), preemergence annual grass weed killers will also damage germinating desirable grass seed. Oftentimes siduron is combined with starter fertilizer.

If crabgrass plants are appearing in lawns in mid to late summer, remember that they are annual plants and die as temperatures drop in fall. Postemergence crabgrass herbicides need to be applied when crabgrass plants are small; typically crabgrass is noticed too late for these to be effective. The suggested strategy to avoid crabgrass next season would be to improve the lawn through cultural practices and consider a preemergence herbicide in spring.

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, when growing in Kentucky bluegrass lawns, are considered weeds because they differ greatly in leaf width, color, or growth habit. Tall fescue is more coarse and grows in distinctive clumps when it occurs with Kentucky bluegrass. Creeping bentgrass, a very desirable turf species for golf courses, becomes a weed in bluegrass lawns because it appears as patches of finer grass, usually lighter in color. Zoysiagrass, a warm season turf species, appears as patches of thick grass, dormant (straw- colored) for much of spring and fall in Kentucky bluegrass or other cool-season grass lawns.

There are additional perennial grasses that are frequent weed problems. Quackgrass, a coarse species with thick underground stems (rhizomes) can be a major problem in lawns. Nimblewill, a creeping warm season species, often appears as light colored patches in lawns.

One way to distinguish perennial grasses from annuals is the time of the year established plants are present. Perennials (other than nimblewill and zoysiagrass) will appear as established green grasses early in 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.

Removing these weed patches by hand is one control option. It's important to get all of the plant, as many have underground or above ground stems (rhizomes or stolons). These stems enable these species to spread quite readily, so if broken or cut, they regrow.

Selective chemical control is not an option with most perennial grass species. Unlike selective herbicides used on annual grasses (e.g., crabgrass), nonselective herbicides used to control perennial weed grasses may also damage the lawn species. For this reason, spraying over the lawn is not suggested unless the problem is severe enough that all grasses need to be killed and the lawn reestablished. Using a nonselective herbicide, such as glyphosate, patches of the undesirable species can be spot treated. After weeds and portions of lawn hit with spray die, reseed with desirable grass species. Treating in mid-August is generally thought of as the best timing (late July to early August to control nimblewill and zoysiagrass), as late August into early September is the most favorable time for reseeding.

If resodding the area afterwards, there is a longer period of time to treat the weeds. Keep in mind the weed species needs to be actively growing to be controlled by glyphosate,however. Allow 10 to 14 days to determine if weeds have been completely controlled.

Undesirable Plants As Indicators of Lawn Problems

Plant	Potential Underlying Problem
Crabgrass	Mowing too short, watering too often
Ground Ivy (creeping Charlie)	Shade, poor drainage
Clovers	Low fertility
Knotweed	Soil compaction
Moss	Shade, poor drainage, poor fertility