Q. This grass is taking over my ornamental beds. It trails along the ground and roots in where it touches the ground. It pulls up easily, and Round Up kills it, but I can't spray that everywhere because it will kill the plants I am growing. Can you tell me what it is and how I can get rid of it? There is a lot to pull out at this point.
A. The writer enclosed a sample with her question that turned out to be crabgrass (Digitaria spp.). While we may think of crabgrass as a lawn weed, it easily invades ornamental beds or any garden space that is not covered with desirable plants.
One of the most effective cultural controls is make sure that beds are densely planted in order to crowd out germinating weed seeds. Consider filling those empty areas with perennial and/or annual flowers so that weeds are shaded out before they have a chance to grow.
Crabgrass is a warm-season annual that germinates from seed as the soil warms in spring. Whether in lawns or ornamental beds, pre-emergence herbicides are the easiest way to control it. These products prevent the seedlings from germinating and developing normally.
The timing of pre-emergence herbicide applications is critical because most of these products do not control crabgrass that has germinated and is actively growing. They should be applied 10-14 days before expected germination. Crabgrass begins to germinate when soils are moist and the temperature in the top inch of soil is 55-58 degrees at daybreak for four or five consecutive days. There is a saying among turf managers that you should apply crabgrass control when forsythia blooms. This is an example of a phenological indicator because the temperatures and day length that push forsythia into bloom are close to those that push crabgrass seed to germinate.
Preen (trifluralin) is one of the most widely available pre-emergence herbicides for home gardeners to use in shrub borders and flower beds. There is a long list of established plants that safely tolerate the use of Preen. Be sure the plants in your garden are on the list before applying it. Also be sure to remove any growing weeds prior to application and water or lightly rake it in after application. Like many pre-emergence herbicides, trifluralin breaks down on exposure to sunlight. After treatment, you should avoid digging or otherwise disturbing treated areas because that disrupts the chemical barrier in the soil, which could allow some crabgrass seed to germinate.
As for the large, established plants in your beds now, hand weeding is the best option. You are correct that Round Up (glyphosate) will injure or kill the desirable plants in them. And because the crabgrass will die with the first frost, it is almost a waste of money to spray it now. The biggest benefit is that you are removing the seeds that will cause you grief for years to come. A single crabgrass plant can produce 150,000 seeds!
Send questions to Sandy Feather by email at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.