Based on weather forecasts, we may have reached the end of the ideal fall spraying season to control perennial weeds.
As a rule of thumb, this is when daily highs and lows are greater than 60 and 40˚F, respectively. Many people prioritize other activities, such as harvesting crops during this window, which is understandable. Because most of us have yet to receive a hard freeze, our most difficult perennial weeds, such as Canada thistle (Figure 1), still appear healthy. This may leave some folks wondering whether a fall herbicide application is still worthwhile.
To kill a perennial weed you typically must kill the roots and not just the above ground foliage. Most perennial weed species send more resources to their roots during late summer and early fall to prepare for winter survival. Translocated herbicides, such as those belonging to the synthetic auxins (group 4), also tend to move to the roots with these resources more than they do at other times of the year. Examples of synthetic auxins include 2,4-D (2,4-D amine, LV4, LV6, Freelexx®), dicamba (Clarity®, Overdrive®), picloram (Tordon®), aminopyralid (Milestone®), and clopyralid (Stinger®, Transline®). The ALS inhibitors (group 2) are also translocated to roots in higher quantities during fall. These include chlorsulfuron (Telar® XP), metsulfuron (Escort® XP), and imazapic (Plateau®, Panoramic®).
Most herbicides also work best with ample soil moisture, which many of us received in September. Light frosts prior to and following the application typically won’t have a negative impact on herbicide performance. However, a hard frost can damage plant tissue, which reduces the weed’s ability to transport herbicide to the roots. Inspect the leaves prior to application. If they’re severely damaged, then avoid spraying.
Having adequate soil moisture also helps herbicide performance.
What Studies have Shown
There is limited information on the topic of mid-fall herbicide applications. In a western NE trial evaluating dicamba applied at 0.5 lb per acre (equal to 16 oz Clarity® per acre) to Canada thistle and dandelion, Wilson and Michiels (2003) reported significantly greater control of both species when applications were made 1 to 10 days after the first frost (<27˚F) compared to 5 to 11 days before the first frost. It’s important to note that the first frost occurred in late September or early October of their three-year study.
It’s not clear how these results may differ in a year such as 2026, where it’s late October and many places haven’t received their first hard freeze. Applications of relatively inexpensive herbicides may still be worth attempting. Since we’re in a cooler time of year with minimal foliage remaining on susceptible plants and trees, this might also be a good time to apply dicamba or other volatile herbicides, such as 2,4-D ester (LV4 or LV6), with minimal risk for offsite movement.
There are still a lot of uncertainties with spraying perennials in mid-fall. If the sun is out, the daily highs are mostly above 50˚F for several days before and following the application, and the weeds still appear healthy, some applications may still be worthwhile. However, expensive herbicides applied to large swaths of land may be too risky this late in the season.
For more information on any topics from this article, see SDSU Extension’s Weed Control in Pasture and Range or Noxious Weed Control.
References
Wilson, R.G. and A. Michiels. 2003. Fall herbicide treatments affect carbohydrate content in roots of Canada thistle (Cirsium arvense) and dandelion (Taraxacum officinale). Weed Sci. 51:299-304.
