Forsythias are blooming and thoughts turn to…crabgrass

March 31, 2010 in Extension

Crabgrass is the number one weed problem in Pennsylvania lawns, and can turn your picture-perfect turf into a real mess by late summer. Fortunately, crabgrass can be controlled if you choose the right herbicide and apply it at the correct time of year.

Crabgrass begins as seed that germinates in spring or early summer, makes vegetative growth through the summer, and produces a large crop of seed before dying after the first hard frost in fall. Crabgrass seeds remain dormant over the winter, and the cycle starts over again as the seeds germinate the following spring.

The key to successful crabgrass control involves a combination of chemical and cultural measures. Chemical control involves the use of preemergence herbicides (herbicides that kill germinating grass seedlings before they emerge from the soil). These herbicides act by forming a chemical barrier in the soil prior to seed germination. The barrier effectively prevents grass seedlings from emerging and developing normally. You can find preemergence herbicides in retail garden centers, usually combined with fertilizer as “weed and feed” products.

Most preemergence herbicides have long residual activity in the soil and may prevent germination newly-seeded turfgrasses in spring. Thus, seeding of turfgrasses should be postponed for the amount of time specified on the manufacturer’s label (usually three to four months). Siduron (Tupersan) is the only preemergence herbicide that can be safely used during or immediately following seeding of home lawns.

The timing of preemergence herbicide applications is the most critical component of an effective crabgrass control program. As a general rule, the best time to apply preemergence herbicides is approximately 10–14 days prior to the expected germination period in spring. Crabgrass begins to germinate when the temperature in the upper inch of soil reaches 55 to 58°F at daybreak for 4–5 days. Full forsythia bloom is usually a good indicator of when to apply your preemergence herbicide applications, but in some years they may bloom earlier or later than normal. Normally, preemergence herbicide treatments in Pennsylvania should take place as follows:

Southeastern Pennsylvania — March 15 to April 15
Northern tier and high altitude counties — April 20 to May 15
Other Pennsylvania areas — April 1 to May 1

Depending on the product, time of application, location, and severity of the crabgrass infestation, reapplication of a preemergence herbicide within 60 days may be required for season-long control. Consult product labels to determine if two applications are allowed. Poor control also may occur with late applications.

Successful crabgrass control involves management practices that increase the density and vigor of desirable turfgrasses. A dense turf tends to discourage competition from weeds. Cultural practices for the control of crabgrass are aimed at shading and crowding the young weed seedlings by producing a dense sod. Effective cultural control measures include the proper selection and establishment of turfgrasses, adequate liming and fertilization, proper mowing practices, judicious watering, and insect and disease control.

For more information on control of crabgrass and other summer annual grasses, see: PDF – Control of Summer Annual Grass Weeds in Turfgrasses