Extension Educator, Horticulture
Over the years I have discovered a few catchall names for grassy weeds in the lawn. Some I can't mention. However, I have heard many different species of grasses referred to as "crabgrass" or "water grass". Proper identification, however, is key to controlling grassy weeds in lawns.
First of all the best way to prevent weeds from invading your lawn is through proper lawn care practices which encourage a dense stand of vigorous grass. Weeds get an open invitation anytime the turf is opened due to traffic next to sidewalks, poor growing conditions or improper management practices. For example, lawns mowed higher (over 2 inches) tend to have fewer 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 onset of hot weather, ideal for its growth. Ideally new lawns should be seeded in fall.
If you have a grass which is growing right now with fleshy rhizomes than it is probably quackgrass. Lucky you, quackgrass is one of the most difficult weeds to control in lawns and flowerbeds. Tall fescue can also appear as a weed in Kentucky bluegrass lawns. It is a clump grass with a wide leaf blade. Tall fescue and quackgrass are both cool season grasses so they start to green the same time as Kentucky bluegrass. People often refer to either one of these as "water grass".
A warm season grassy weed is nimblewill. It is a fine-bladed, light green grass that does not green up until warm weather, usually in May. Nimblewill is most noticeable as a straw colored patch now that greens up as the weather warms then goes straw colored early in the fall.
Quackgrass, tall fescue, and nimblewill are perennials which mean the same plant comes back every year, bringing its family of seedlings or rhizomes with it. There aren't any selective herbicides available to home gardeners to kill these perennial weeds. Conventional crabgrass controls will not control these perennials. They must be either dug out or spot treated with a non-selective herbicide such as glyphosate sold as Round Up or Kleen Up. Be sure to read and follow all label directions. In addition be sure to select the correct product. There are combination products now that have a post emergent and pre emergent in them. These are appropriate for driveways and fence rows but not lawns. After the weed is dead, the area should be reseeded or resodded.
Crabgrass is an annual and must come back every year from seed so it is just now starting to germinate. 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 soils get warmer than 60 degrees.
If an herbicide is desired to control crabgrass, it is usually a preemergent herbicide which keeps annual grassy weeds such as crabgrass from emerging. Timing of application is very important, as the weed killer should be applied before the crabgrass emerges from the soil usually in mid April in our area. Do not use preemergent control now if you plan on also reseeding the lawn in the spring.
Check out the Urban Extension website at www.urbanext.uiuc.edu for pictures, descriptions and control measures for weeds in lawns. Or if all else fails you can say, "at least it's green".