Wait Until Late October to Control Perennial Broadleaf Weeds
While it may be tempting to continue making applications on your typical schedule for controlling weeds in turf, the drought that we are currently experiencing causes two challenges. One is that the weeds, even if green, are likely in a hardened off state that makes translocation of the herbicide and therefore control less effective. The second is that, while completely dormant turf would not be injured by the herbicide application, any green leaf tissue that is stressed will likely be more injured by the herbicide than normal. This is particularly true if a surfactant is added to the tank or if certain combination products, especially those that contain the protox inhibiting herbicides (carfentrazone, sulfentrazone, pyraflufin-ethyl, flumioxazin) are used. This year it may be best to wait until late October or even the first week of November. Hopefully by then the drought conditions will have somewhat improved.
Even if it was not a drought year, fall always has, and will continue to be, the best time of year to control most perennial broadleaf weeds (Figure 1). Fall herbicide applications offer several advantages over springtime applications. Most annual ornamental plants and vegetables have reached maturity and leaves of trees and shrubs are beginning to turn color and fall off the plant. Therefore, the chance of non-target injury due to herbicide drift is greatly reduced compared to springtime applications. Another important advantage of fall broadleaf herbicide applications is that you can get good control of germinated winter annual broadleaf weeds, which have been a significant issue in recent years. Our winters have been comparatively mild for the last several years, which has aided the proliferation of these species, including common chickweed, henbit, purple deadnettle and hairy bittercress. But the major advantage of fall applications is effectiveness and duration of control.
During the fall, perennial weeds begin to store carbohydrates for over-winter and for next year’s growth. When this occurs, the translocation stream is downward. Herbicides applied when the plant is actively translocating carbohydrates underground are also more effectively moved into the roots and storage structures, resulting in better overall control of below ground structures compared to springtime applications.
Make sure that whatever you select contains at least one phenoxy (2,4-D, 2,4-DP, MCPP, MCPA) or pyridinoxy (triclopyr, clopyralid, fluroxypyr) that is in an ester form. You can determine this by looking at the list of ingredients on the label. It is important to select an ester because that formulation is more likely to penetrate the weed leaf tissue and therefore more effectively control the weed when making applications in cooler weather. Amine formulations do not work as well in cool weather.
The best time to apply herbicides is generally around the same time that the last mowing and fertilization of the year occurs (October to early-November in the Midwest). Ideally, air temperatures will be consistently in the 40’s and 50’s and soil temperatures will still be in the 50’s. Another indicator for proper timing is that the grass is no longer growing and being mowed but is still green.
After application, remember that the weeds are not metabolizing as quickly, and you will not see the dramatic burn down, twisting and epinasty that you normally see with an application in warmer weather. However, while it may not appear as though the application was effective, if you return to that spot next spring the weeds will be controlled. While a springtime application may provide as few as 8 weeks of control, a properly timed herbicide application in fall may control winter annual and perennial broadleaf weeds for up to one year.