Crabgrass is one of the most interesting weeds from a horticulturist’s perspective. This is a warm-season plant that, left untouched, will grow 3 to 4 feet high. However, in the yard, the mower cuts crabgrass to the same height as the turf. So, the plant switches from vertical growth to horizontal growth, spreading out over the turf with its wide blades, choking out the desirable grasses.
Crabgrass is one of the most interesting weeds from a horticulturist’s perspective.
This is a warm-season plant that, left untouched, will grow 3 to 4 feet high. However, in the yard, the mower cuts crabgrass to the same height as the turf.
So, the plant switches from vertical growth to horizontal growth, spreading out over the turf with its wide blades, choking out the desirable grasses.
Crabgrass also needs specific temperatures to germinate.
It needs seven to 10 days of soil temperatures about 55 to 60 degrees Fahrenheit where the seed is located. If it gets four days and the soil then cools, the germination process has to start all over.
Because the seed is usually on the soil surface, it is exposed to more of the vulgarities of weather than dandelions. But that is also why crabgrass may take two months to germinate, as the soil temperatures keep fluctuating.
Spring rains can also hinder germination, as they tend to be cooler than air temperatures.
Come October, crabgrass turns straw brown and dies, leaving nothing but tons of seeds for next year’s crop. And by October, it’s too late to sow bluegrass, ryegrass or fescue to cover the dead areas.
While it is far from absolute, when the forsythia starts blooming, it is time to think that crabgrass may be soon be sprouting.
There are a couple methods for homeowners to control crabgrass. The best method is to make sure you have a thick, lush turf.
Crabgrass needs light to sprout, so if the grass is dense, the seed won’t sprout. Fall fertilizers and winterizers are two of the best methods to achieve that, though it’s too late in the year to put either on the lawn. Remember that, though, come November.
The sure-fire control for homeowners is a pre-emergent herbicide that will kill the germinating seed. There are several products on the market.
Unfortunately, many of the products are also mixed with fertilizer. You may have to search for crabgrass killer without any fertilizer. It’s out there. It’s also something stores can order for you.
Many lawn-care companies offer crabgrass control coupled with small amounts of fertilizer, usually right around a fourth of a pound of nitrogen per thousand square feet. That’s a small enough amount that the nitrogen won’t stop root growth, but you’ll get good crabgrass control.
There are some organic crabgrass controls, usually based on corn gluten. Most recommendations state you need several years of applications to control the weeds, though crabgrass seed will blow in from neighboring yards.
Corn gluten may end up stimulating the grass more than it controls the weeds. Because most fescues and bluegrass are aggressive plants, they’ll choke out the germinating weed seeds.
A word of warning: Most crabgrass preventers, even corn gluten, do a number on most seeds, not just crabgrass. So while they may control dandelion seeds, they’ll also prevent bluegrass from germinating. Always read and follow the directions on the label.
David Robson is a horticulture educator for the University of Illinois Extension. For more gardening information or for your local extension unit office, go to www.extension.uiuc.edu/mg.