Quackgrass and crabgrass look somewhat alike, but they are very different. For one thing, quackgrass is a cool-season perennial (note its ability to grow in the high mountains) where crabgrass is a warm-season annual. They have different ways of spreading and the short-term solutions for eradication are different. Long-term solutions to crabgrass and quackgrass, however, are remarkably alike.
It is a cold-hardy perennial weed, which means it will come back year after year. In fact, it stays green year-round in some climates. It's one of the first grasses to come out of dormancy each spring. It is not a native plant. It reproduces from both seed and long underground stems (rhizomes) that dive as much as six feet below the surface.
Yes, various weed killers will set it back—as will repeat mowing to a very low height in spring. In both cases, timing and repetition are everything. Quackgrass relies heavily on the food stored in its roots. The earlier in spring you mow it to the ground and/or spray the leaves, the less it can build a food supply in the root zone. Expect to spray or mow repeatedly to keep the roots in a state of starvation.
In addition to fighting quackgrass, however, remember to encourage desirable lawn grasses. There’s a good chance the lawn grass will out-compete quackgrass. You may not eliminate it altogether, but it will become less and less prominent.
Note: Under different naming systems, quackgrass may be listed as Elymus repens, Agropyron repens or Elytrigia repens.
Crabgrass is an annual that grows from seed during the warm season. Northerners don't see it until late May.
The name "crabgrass" is actually an umbrella for numerous related species. Some are native, others are introduced. The two of greatest concern in lawns are the non-native smooth crabgrass (Digitaria ischaemum) and hairy crabgrass (Digitaria sanguinalis).
Crabgrass is opportunistic, taking over in bare spots where nothing else thrives. It is not fussy about moisture, pH, or fertility, so it volunteers where lawn grass is weak. In the north, crabgrass has its hay day when cool-season grasses struggle under summer sun and drought.
Crabgrass dies in winter, leaving areas of flat brown blades in the lawn. Crabgrass reproduces from seed each year. Each crabgrass plant produces thousands of seeds and the seeds can be viable for years.
Crabgrass eradication has inspired an entire product category, the pre-emergent crabgrass killers. While the chemical-based versions are effective, they are not without serious drawbacks. Some pre-emergents affect all grass seed for up to 12 weeks after application. Therefore, desirable seeds are inhibited along with the crabgrass.
Corn gluten is a pre-emergent sometimes used as part of organic lawn care programs. It doesn’t inhibit grass seed, but it has limited effectiveness on crabgrass. Post-emergent grass killers are also effective, particularly when used before the plants go to seed.
What Are Post-Emergent Grass Killers?
Post-emergent grass killers are herbicides used to kill unwanted plants that have already grown above the soil (as compared to just being seeds or roots in the soil). There are both synthetic and organic options available. Note that while an herbicide may kill quackgrass or crabgrass, it will also kill the surrounding vegetation, so use it carefully.
One way to discourage crabgrass is to bag the clippings during August or September when the plants go to seed. This will keep fresh crabgrass seeds off the lawn.
Defensive Lawn Care Is the Best Offense
Both quackgrass and crabgrass are opportunistic weeds that thrive where lawn grass is weak or non-existent. Therefore, healthy turf is the best deterrent to either of these weeds. It is especially important to promote deep roots since this is a leading defense against the impact of summer drought. Drought creates openings in the turf where crabgrass, quackgrass, and plenty of other weeds can thrive.
Good lawn care begins, always, with a soil test. Be sure to ask for the amount of organic matter with your soil test reports, even if it costs a bit extra to get this report. The test may recommend lime or sulfur to reach the correct pH (6.5 to 7.2). It may advise compost to build organic matter. Finally, the soil test reports will suggest the right amounts of nitrogen, phosphorus, and potassium for healthy turf.
Quackgrass. University of Maryland Extension
Managing Crabgrass in Home Lawns. University of Illinois Extension