As winter stretches on, you may find yourself with brown lawn areas that you swear were healthy green turf a few months ago. If so, the culprit is likely crabgrass. This grassy, warm-season weed can't take the cold, but it will most likely return.
Unfortunately, once crabgrass has germinated and begins to grow, there are no herbicides available to homeowners or commercial applicators that can kill it without harming the turf.
The best way to manage crabgrass is to get ahead of it with a pre-emergent herbicide. Dithiopyr or Prodiamine are two pre-emergent herbicides that can be used to prevent crabgrass seeds from germinating. Pendimethalin, which is sold under a variety of names, also works well for controlling mature, actively-growing crabgrass and is safe when used at the recommended rate.
The downside is that there is a narrow window of herbicide application for best results. That window depends on where you live in Florida. The general rule of thumb is to apply pre-emergent herbicides in early February for South Florida, mid-February for Central Florida, and early March for North Florida. You want to apply the pre-emergent herbicide for four or five consecutive days before temperatures reach 65 to 70°F.
Note that you should not use pre-emergent herbicides on areas where you plan to put down new sod or grow turfgrass from seed. A good general rule is to wait two to four months between the last use of pre-emergent herbicides and the planting of new sod, but it is always important to consult product labels before using any product on your lawn.
Unless palms are listed on the product label, you should also avoid using pre-emergent herbicides around the root zones of palms as some decline may occur. Also, you shouldn't use a pre-emergent herbicide paired with a fertilizer ("weed and feed"). The right time of year for applying a pre-emergent lawn herbicide is the wrong time for lawn fertilizing.
An important part of preventing crabgrass and other weeds from taking over your lawn is maintaining healthy turf. Choosing the right type of grass for a site is the first step in insuring a healthy lawn. Be sure to mow your lawn to the proper height and mow often, so that you are only removing 1/3 of the leaf blade. Thick, healthy turf minimizes the spaces where weeds can become established and blocks sunlight that weed seeds need to germinate.
Also on Gardening Solutions
Special thanks to Dr. Chris Marble, Assistant Professor in Ornamental and Landscape Weed Management at the Mid-Florida Research and Education Center.