Extension Educator, Horticulture
You say "water grass". I say "what is it?" Take a poll at your coffee shop and you will get wildly different descriptions for water grass. Numerous grassy weed species are called "water grass". It may not seem like a big deal but proper identification 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 a clear invitation anytime 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. Closely mowed lawns tend to open, allowing weed invasions. Light, frequent watering also favors crabgrass. It 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 grey green colored grass which is growing right now with white 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. This tall fescue is the pasture type and not the more desirable turf-type tall fescues. Quackgrass and tall fescue are both cool season perennial grasses so they start to green the same time as Kentucky bluegrass.
A warm season grassy weed is nimblewill. Its fine-bladed, light green leaves do not green until warm weather, usually in May. Nimblewill is most noticeable now as a straw colored patch that greens as the weather warms then goes straw colored early in the fall. The similar looking Bermuda grass can also appear in lawns.
Quackgrass, tall fescue, Bermuda grass 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 and not kill your desirable grass.
Conventional crabgrass controls will not control these perennial grasses. You have three options: 1) either thoroughly dig it out; 2) spot treat it with a non-selective herbicide such as glyphosate sold as Round Up or Kleen Up then after the weed is dead or removed, the area should be reseeded or resodded or 3) live with it and only look at your lawn through your car windshield.
Crabgrass is an annual and must come back every year from seed. Crabgrass germinates when soil temperatures are greater than 55 to 60 degrees F for 7-10 consecutive days, and continues until soils reach 95 degrees F. So most have not germinated yet. Other annual grasses germinate later as soils warm. If you see a big clump of grass with long leaf blades right now, it is not crabgrass.
If an herbicide is desired to control crabgrass, it is usually a preemergence herbicide which keeps annual grassy weeds such as crabgrass from emerging. Timing of application is very important. The preemergence herbicide for crabgrass should be applied in our area by mid to late April.
Do not use preemergence control if you plan on also reseeding the lawn this spring. Only one preemergence herbicide, siduron (Tupersan) is labeled for application to newly seeded areas. All others have a waiting period. Be sure to read, understand and follow all pesticide label directions.
Check out the University of Illinois turf site or U of I Extension website for pictures, descriptions and control measures for weeds in lawns. Or if all else fails you can say, "at least it's green most of the time".