Extension Educator, Horticulture
For some people the picture perfect lawn looks like a putting green and for some the perfect lawn is a nonexistent one. None of the ideas are wrong–just different. This time of year grassy weeds appear in lawn areas. These weeds have a different appearance than bluegrass so some people may choose to control them.
Crabgrass, nimblewill and quackgrass are several of the problem grasses popping up in lawns.
Crabgrass is the most common grassy lawn weed. Crabgrass stems generally grow flat and branch freely. They will often send down roots where each joint comes in contact with soil or moist grass. Seed heads of crabgrass form in late August and resemble a chicken's foot. Seeds germinate when soil temperatures reach 55 degrees F. for at least three consecutive days in the spring and continue to germinate until soils reach 95 degrees F. in summer.
If crabgrass plants are appearing in lawns now (mid to late summer), remember that they are annual plants and die as the temperatures drop in the fall. Applying postemergence chemicals for crabgrass control should be applied when the plants are very small. In most situations it is too late for these to be effective. For control of the annual grassy weeds such as crabgrass the best option now is frequent mowings and applying a preemergent crabgrass control in mid to late April next year.
Quackgrass and nimblewill are perennials. Individual plants and their seeds will come up next year. Quackgrass is a coarse, cool season grass that spreads extensively by long white underground stems.
Nimblewill is a fine textured, light colored warm season grass. It is late to green up in spring and quick to go dormant in fall.
Perennial grassy weeds are particularly difficult to control. Removing these weeds by hand is one control option. It's important to get all of the plant since many have underground stems. If stems are broken or cut, they will regrow.
Selective chemical control is not an option with most perennial grassy weed. Herbicides used to control perennial weeds will generally also damage the lawn grass. For this reason spraying glyphosate (Round-Up) is an option only if the problem is severe enough and all the grasses (including the lawn grass) must be killed in an area and the lawn reestablished. Weeds must be actively growing when using glyphosate and allow 10 to 14 days to determine if weeds have been completely controlled and before reseeding.
According to Tom Voigt in the U of I publication Illinois Homeowner's Guide to Pest Management, proper lawn care practices that encourage a dense stand of vigorous grass are the best way to prevent weeds from invading. For example mowing height can have a big impact on weed problems. Lawns mowed at 2 inches to 2-1/2 inches tend to have fewer problems with annual grassy weeds such as crabgrass. Closely mowed lawns tend to have open spaces which allow grassy weed seeds to germinate.
Light frequent watering also favors crabgrass. Crabgrass often invades areas seeded in late spring because bare soil, frequent watering and the onset of hot weather are ideal for its growth. Seeding a lawn in late summer (August 15-September 15) can help lessen this problem.