Crab grass, Water grass - Knowledgebase Question

Valmeyer, Il
Question by Craftsman266
July 30, 2009
I sodded my yard 5 years ago but now crab grass or water grass is taking it over, what do I do????? Ive tried crab grass killer and it did nt touch it.

Answer from NGA
July 30, 2009


Proper identification is key to controlling grassy weeds in lawns so you may have to dig up some of the weedy growth and take it to your local cooperative extension office for identification.

The very best 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 then 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 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.

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 and not kill your desirable grass.

Conventional crabgrass controls will not control these perennial grasses. You have three options: either thoroughly dig it out; spot treat it with a non-selective herbicide such as glyphosate sold as Round Up or Kleen Up; or live with it and only look at your lawn through your car windshield. After the weed is dead or removed, the area should be reseeded or resodded.

Crabgrass is an annual and comes 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. 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.

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