By: Paul Schattenberg

AgriLife Extension experts provide control advice for homeowners, others

DALLAS — Given the recent warm temperatures in North Texas, Texas A&M AgriLife Extension Service experts have put out the word that now is the time to treat for crabgrass.

“With soil temperatures rising to the upper 50s and low 60s, we may see crabgrass germination very soon, especially if we get rain,” said Janet Hurley, AgriLife Extension program specialist at the Texas A&M AgriLife Research and Extension Center in Dallas. “Now is a great time to apply pre-emergent herbicides.”

Hurley said crabgrass germination typically occurs in late winter, but varies from year to year based on temperature, rainfall and location.

“Germination usually begins when the soil temperature at a 2-inch depth reaches about 55 degrees for at least three days,” she said. “Since crabgrass is the first summer annual grassy weed to germinate, we see its initial presence as something of a signal for the application of pre-emergence herbicides.”

Dr. Matt Elmore, AgriLife Extension turfgrass specialist at the Dallas center, recently recommended homeowners and others who take care of turfgrass areas, such as groundskeepers and athletic field managers, apply pre-emergent herbicides in the very near future.

Elmore said when looking for crabgrass seedlings, walk along south-facing slopes, bare areas and areas along sidewalks.

Hurley noted bare spots and weak areas in the lawn are typically where people start to see new crabgrass growth.

Most pre-emergence herbicides must be applied before the target weed germinates or emerges or they will not control them.

“Although these herbicides are commonly used to control annual weeds that germinate at predictable times of the year, they will not control perennial weeds,” Hurley said. “A pre-emergence herbicide is the most effective way to control crabgrass. These herbicides are especially recommended for lawns with a history of crabgrass problems.”

Some of the common active ingredients in pre-emergence products available to homeowners include pendimethalin, dithiopyr and prodiamine. Trade names for these products include Pre-M, Bonide Crabgrass Preventer and Halts Crabgrass Preventer.

“Pre-emergence herbicides that contain these active ingredients will usually provide suitable crabgrass control when applied before crabgrass germination and according to the product label,” Hurley said. “These products mostly control grassy weeds, although they may control some broadleaf weeds as well. Always check to ensure that the product can be safely used on the turfgrass species in your lawn.”

Many pre-emergence products that contain dithiopyr are also labeled for early post-emergence control of small or seedling crabgrass plants, they said.

“Consider using these products if you cannot make the application before germination and check the label to ensure it contains dithiopyr and is labeled for early post-emergence control,” Hurley said.

She noted corn gluten meal is an organic option for crabgrass control, and while research has shown crabgrass control with this product is inconsistent, it may be effective in some situations.

“It is best to select a pre-emergence product that does not contain nitrogen fertilizer,” Hurley added. “While crabgrass preventers with nitrogen may be suitable for northern regions of the U.S., they should not be used in Texas. Warm-season grasses are still mostly dormant when crabgrass germinates and nitrogen fertilizer should not be applied until the lawn is mostly green.”

She also noted that unlike “weed and feed” products that control broadleaf weeds, pre-emergence herbicides are usually safe for use around trees and ornamentals. However, products that contain atrazine are one exception and some are labeled for use over the top of ornamentals.

Pre-emergence herbicides are effective only if they are applied uniformly and are watered into the soil by rainfall or irrigation before the crabgrass or other target weed emerges.

“See the product label for information on the amount of irrigation or rainwater needed,” she said. “After they are watered in, the herbicide molecules remain in the upper layer of soil and control weeds or grasses that germinate from seed for several weeks. If you plan to seed or sod, do not apply a pre-emergence herbicide without first checking the label for the appropriate reseeding and sodding interval.”

Hurley also said it is best not to apply some pre-emergence herbicides before sprigging or sodding.

“If you are planning this type of turf renovation, use other weed-control options,” she said. “In general, apply these herbicides only to well-established turfgrass. Check the product label if you have established turf from sod, seed or sprigs within the previous year. Many pre-emergence herbicides will impede encroachment of existing turfgrass into areas damaged by winterkill, traffic or diseases. It’s also a good idea not to use pre-emergence herbicides if your lawn has been severely damaged.”

Hurley said more information can be found on the Aggie Turf website https://aggieturf.tamu.edu/turfgrass-weeds/large-crabgrass/.

To find which pre-emgergence herbicides are recommended for athletic fields and commercial uses, go to http://schoolipm.tamu.edu/files/2016/02/Pre_emergement_handout.pdf. This handout highlights herbicides labeled for control of grassy and small-seeded broadleaf weeds.

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