How late is too late to control dandelions and other weeds this fall?

We are now in the prime season for broadleaf weed control starting about September 15, but there is plenty of time in the fall for good, if not even improved, control over earlier applications. Furthermore, applications made now have very little chance of affecting near dormant trees and ornamentals, unlike with spring applications made around non-target species that are leafing out and/or blooming. Nebraska research in agricultural fields reported in 2003 showed 2,4-D and/or dicamba were more effective in controlling dandelions and Canada thistle when applied 1 to 10 days after the first fall frost than when applied 5 to 11 days before the frost (Wilson and Michaels, 2003). Earlier work by Bruce Branham at Michigan State showed that even though early September applications were effective in controlling broadleaves yet that same fall, September applications were less effective long-term than applications made in October and November (Figure 1). More recent work on ground ivy at Purdue shows that herbicides like triclopyr (Turflon) that are very effective on ground ivy, retain their effectiveness when applied regardless of the first frost and as late as early- to mid-November in Indiana (Figure 2) (Reicher and Weisenberger, 2007). However, herbicides with lower activity on ground ivy were most effective from Oct 1 through Nov 1, and efficacy decreased outside of this window. This study showed that broadleaf applications should be effective when made into the first week or two of November, but control may not be seen until spring. However, herbicides that contain carfentrazone (FMC’s Quicksilver, PBI Gordons’ Powerzone and Speedzone) will still give a response in 7 to 14 days even when applied in late October or early November. More recent studies at Purdue were applied in Fall of 2007 used two highly effective herbicides on ground, triclopyr (Turflon) and fluroxypyr (Spotlight), and applied them with and without carfentrazone (QuickSilver) on Sep 15, Oct 15, and Nov 15. QuickSilver had similar effects on both herbicides, dramatically improving short-term control from November applications (Figure 3). Though one might suspect that QuickSilver would decrease long-term control because the quick burndown could limit herbicide translocation, QuickSilver did not decrease long-term control from these herbicides as rated the following May (Figure 4). Similar to our previous work, Turflon applied as late as Nov 15 provided over 90% control by the following May of the very difficult-to-control ground ivy .We repeated this research at UNL in fall of 2010 with the then popular herbicide Imprelis, which is now off the market. The same trends occurred, even on the extremely difficult-to-control Nebraska ground ivy (Kohler et al., 2004) (Figure 5 and 6). The take home message is that broadleaf herbicides can be very effective when applied well into the fall and applications this late will control perennials as well as winter annuals that have already germinated. If quick knockdown is required for immediate customer satisfaction in applications later in the fall, using products or tank mixes that contain carfentrazone will give a rapid burndown while not decreasing long-term control. Zac Reicher, Professor, Turfgrass Science,