University Park, Pa. -- Crabgrass is the No. 1 weed problem in Pennsylvania lawns and can turn your picture-perfect turf into a real mess by late summer. But crabgrass can be controlled if you choose the right herbicide and apply it at the correct time of year, according to a turfgrass specialist in Penn State's College of Agricultural Sciences.
Crabgrass begins as seeds that germinate in spring or early summer, die after the first hard frost in fall and produce large seed crops that remain dormant over the winter. The cycle starts over again as the seeds germinate the following spring, said Peter Landschoot, professor of turfgrass science, so good crabgrass control must combine chemical and cultural measures.
"Chemical control involves the use of pre-emergence herbicides to kill germinating grass seedlings before they emerge from the soil," he said. "These herbicides act by forming a chemical barrier in the soil that effectively prevents grass seedlings from emerging and developing normally. You can find pre-emergence herbicides in retail garden centers, usually combined with fertilizer as 'weed and feed' products."
Most pre-emergence herbicides have long residual activity in the soil and may prevent germination of newly-seeded turfgrasses in spring, Landschoot said. So seeding or reseeding your lawn should be postponed for the length of time specified on the manufacturer's label (usually three to four months). Siduron (Tupersan) is the only pre-emergence herbicide that can be used safely during or immediately after home-lawn seedlings.
"The timing of pre-emergence herbicide applications is the most critical component of an effective crabgrass control program," he said. "As a general rule, the best time to apply them is approximately 10 to 14 days prior to the expected spring germination period. Crabgrass begins to germinate when the temperature in the upper inch of soil reaches 55 to 58 degrees Fahrenheit at daybreak for four to five days."
Seeing forsythia plants in full bloom is usually a good indicator of when to apply your pre-emergence herbicide applications, but they may bloom earlier or later than normal in some years. Normally, pre-emergence 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 your crabgrass infestation, you may have to reapply a pre-emergence herbicide within 60 days for season-long control," Landschoot said. "Check the product labels to determine if two applications are allowed. Poor control also may occur with late applications."
Successful crabgrass control also involves using management practices that increase the density and vigor of desirable turfgrasses, he noted.
"A dense turf tends to discourage competition from weeds," he said. "Crabgrass-control practices aim at shading and crowding young weed seedlings by producing a dense sod. You can do that with the proper selection and establishment of turfgrasses, adequate liming and fertilization, proper mowing practices, judicious watering and insect and disease control."
A free publication on the control of crabgrass and other summer annual grasses is available online at http://pubs.cas.psu.edu/FreePubs/pdfs/uc078.pdf. Landschoot and other Penn State turfgrass faculty also post timely lawn and turf information to the "Penn State Turfgrass Blog" at http://psuturf.blogspot.com/.