Even though the cool weather in July and August has been a curse for farmers, it has been a blessing for anyone with a cool-season lawn. The rain and cool temperature has caused many homeowners to mow their lawn almost twice a week versus only twice for the entire months of July and August last year. The lush green lawn is a homeowner's paradise and it didn't even cost a huge water bill. However, after a closer look, one also notices that it was a very good year for a lot of weeds. Crabgrass germinated much later than in past years, but once germinated, it quickly made up for lost time. Most of the crabgrass is now producing seed and nearing the end of its life, the first hard frost. Therefore, try to keep seed production to a minimum by mowing regularly and make a mental note about applying a preemergence herbicide next spring in heavily infested areas.
The perennial broadleaf weeds have had a very productive year and the time for control is almost here. Fall (mid September November) is an excellent time to selectively control these weeds with a herbicide. Control of broadleaf weeds in the fall gives the turfgrass an opportunity to fill in those open areas that appear after weed control. This results in a denser turf which culturally helps prevent weeds from becoming established. Fall is also the best time for other cultural practices such as aerifying compacted areas. Fertilizing later in the fall, when the top growth of the cool-season turfgrasses has nearly stopped, will favor rhizome growth resulting in a denser turf. Anything that will promote proper turfgrass growth will ultimately help minimize weed populations. For those lawns that are mainly weeds, the time for complete renovation is now. Renovation means killing all existing vegetation and starting over.
Controlling broadleaf weeds. There are a number of postemergence herbicides available for selective control of broadleaf weeds. Most of these products are two-, three-, and even four-way combinations of 2,4-D, and either 2,4-DP, dicamba, MCPP, or triclopyr. These combinations have taken most of the guessing out of broadleaf control because broadleaves not controlled by 2,4- D are usually controlled by one of the other herbicides. For example, 2,4-D provides excellent control of dandelion and plantain, but provides poor control of white clover or red sorrel. Dicamba, on the other hand, provides excellent control of white clover and red sorrel and only fair control of dandelion or plantain. By applying a combination of these two, all four weeds will be controlled. This doesn't mean that a combination product will successfully control all of those unwanted broadleaf weeds in your turf. A few broadleaf weeds are just naturally hard to control with the current herbicide choices, and secondly, the wrong combination product may have been chosen (i.e. 2,4-D + MCPP instead of 2,4-D + MCPP + dicamba). So it is still important to identify your problem weeds before you try to chemically control them.
Finally, one needs to consider seeding restrictions because these herbicides are selective only in established lawns. Do not apply broadleaf herbicides within 4 to 6 weeks before seeding and not until the new grass has been mowed several times. Therefore, if overseeding is currently being considered, one should avoid using these products.
This article originally appeared in the August 26, 1992 issue, pp. 141-142.