Sept. 5, 2002

DALLAS - Now is the best time to apply pre-emergent herbicides for controlling annual grassy and broadleaf weeds in lawns and landscapes, say experts from Texas Cooperative Extension. In the northern parts of Texas, these winter weeds start germinating in early September; in southern areas of the state, in mid- to late-September.

"Many individuals wait too late to apply their pre-emergent herbicides and then wonder why they still have problems with winter weeds in the lawn," said James McAfee, Extension turfgrass specialist at Texas A&M Dallas.

"Once these annual weeds germinate and start to grow, most of the pre-emergent herbicides will have no affect on the weed," he said. "Application of a pre-emergent herbicide is the most effective method to control the winter annual grassy weeds. If winter annual grassy weeds are allowed to become established in the lawn, there isn't a real good post-emergent control.

"While post-emergent herbicides such as Roundup and Finale can be used to control winter grassy weeds, these products should only be used once the warm season turfgrasses are completely dormant," said McAfee. "In many areas of Texas, these grasses do not go completely dormant and applying products such as Roundup and/or Finale can cause serious injury to the lawn."

When purchasing a pre-emergent herbicide, read the label very carefully to make sure the herbicide is labeled for the type of turfgrass to be treated, he added. Also, read the label to determine how much herbicide is needed to treat the lawn. Over-applying the herbicide can injure turfgrass plants.

"Once the herbicide is applied, then water the application in very thoroughly. It takes approximately 0.5 inches of supplemental irrigation to move the herbicide down in contact with the top layer of the soil where most of the weed seed are germinating," said McAfee.

"One of the most common reasons for failure to achieve good weed control is not watering the herbicide in thoroughly. Also, following application make sure the material is watered in thoroughly before the lawn is mowed. If the lawn is mowed before the product is watered in thoroughly, then control of the weeds will be disrupted."

Fall is also the best time to apply post-emergent herbicides for winter broadleaf weeds.

"If you wait until spring to try and control the winter broadleaf weeds, they will be a more mature plant that is going to be much harder to control," said McAfee. "For winter broadleaf weeds such as henbit and dandelions, use one of the hormone type herbicides such as 2, 4-D or products that contain 2, 4-D in combination with other hormone-type herbicides such as MCPP and Dicamba.

As with the pre-emergent herbicides, always read the label to make sure the herbicide is labeled for the type of turfgrass in your landscape and that it is labeled for the particular broadleaf weeds you are trying to control, he said.

"With the application of post-emergent broadleaf herbicides, it is important to delay watering for at least 24 to 48 hours following application of the herbicide. Also, delay mowing of the lawn for the same period of time," said McAfee.

He also warned not to apply pre-emergent herbicides in heavily shaded areas of the lawn. Because turfgrasses growing in heavily shaded areas are under stress, the application of a pre-emergent herbicide could damage these plants.

"If you are planning on over-seeding the lawn with one of the cool-season turfgrasses such as ryegrass for winter color, then do not apply a pre-emergent herbicide," said McAfee. "These herbicides will not only affect the weed seeds in the soil, but will also affect the ryegrass seed."