Late fall herbicide applications on herbaceous perennial weeds

Mark Renz
Extension Weed Scientist

The fall can be an excellent time to target herbaceous perennials for weed management. Equal to superior control can be seen compared to summer applications.  Why?  It depends upon the weed species, but potential explanations for enhanced control include increased accumulation of herbicide in the perennial roots/rhizomes and reduction of winter hardiness leaving perennial tissue more susceptible to winter-kill.  Many more questions remain with regard to perennials, and future research will hopefully provide more information.  While fall applications have many positive aspects, several questions often arise with fall applications:

1. How late into the season can I apply?  This is both herbicide and weed species specific.  Assuming we are only dealing with herbicides with postemergent activity, many studies have shown improved activity on certain perennials if applications are made in the fall (e.g. Canada thistle, dandelions, quackgrass).  Recently research has also shown that applications following the first frost of the season (temperature drops below 32 F, but leaf tissue is not damaged) will provide a significant increase in control with several perennials.  Researchers at Nebraska have found a 17-39 % and 45-58 % increase in control of Canada thistle and dandelion respectively if dicamba (a herbicide that typically provides fair control) is applied after the temperature dropped below 29 F compared to 5 days before the first frost (Wilson and Michiels 2003).  Unfortunately this elevated control with postemergent herbicides is dramatically reduced after a hard freeze that kills foliage of Canada thistle.  Research by my predecessor found that an application after a hard freeze with Stinger/Transline reduced control of Canada thistle by 36-42 % the following summer.  Thus it appears that a fine window exists for fall applications for Canada thistle and if the initial frost and hard freeze occur on the same date, effectiveness of herbicides may be reduced.  This year may fit that scenario as the weather report for later this week is calling for temperatures in the low 20s for nearly the entire state.  Does this apply for all perennial species? No, I have seen reduced control with a fall application with some perennials, no difference with others, and an improvement if applications are made after a hard freeze with others.  The timing of root bud formation, architecture of above and below ground tissue, and level of soil persistence of the herbicide may all be important factor that influence control.  Additional research is needed to determine what factors are responsible before we can determine which perennials will be susceptible in the late fall.  So to date I would avoid making any late fall applications unless you have information that indicates that this would be an effective treatment for the weed species you are targeting.

2. Does the temperature during application matter? If you are applying a postemergent herbicide, Yes!  A rule of thumb is that herbicide applications should be made when daytime air temperatures are at least in the 50s, preferably higher.  This will result in periods of the day when plants are actively growing, which will help facilitate herbicide uptake through the foliage and movement into the perennial tissue.  Applications when plants aren’t actively growing can limit herbicide uptake or movement, resulting in poor control the next year.  Remember that some weeds will be less sensitive to temperatures than others, and while one species is injured others are not.  This is why many labels say “apply to actively growing plants”.  If you are not sure if a weed of interest has had a killing freeze wait 2-3 days after the frost/freeze as this is the typical timeframe for freeze injury to appear, and then scout the field for signs of injury.  If injury is only minor and the majority of the leaf tissue is healthy, and temperatures increase, treatments should be effective.  If the herbicide has soil activity, it may be taken up by roots in the soil at a later date, but roots need to be actively growing to take up the herbicide, so cold soil temperatures may limit uptake. 

3. How safe is the fall application to desirable plants?  Again this will be herbicide and species specific.  In pastures, desirable annual forbs and grasses have already produced seed, and are likely senesced; therefore injury to these plants would be minimal.  Hopefully some of the desirable perennials have also senesced while the weed is still actively growing.  This provides a perfect opportunity for a selective application with a systemic herbicide that is nonselective, but has little to no soil persistence (i.e. glyphosate).  Herbicides with a significant persistence in the soil can prevent seedling establishment the following year, so use caution.  No general rules exist with regards to which species can tolerate which herbicides and for how long as tolerance to plants is herbicide and rate dependant.  Due to these factors make sure and consult the label, as it will provide detailed information that will prevent injury to desirable plants.

If conditions are correct, however, weeds can be effectively managed in the late fall.  For example, treatments in Lancaster, WI in the middle of November with Milestone provided the best control of Canada thistle of several Milestone treatments made earlier in the fall.  Applications were made on a warm, sunny day, and the plants had several light frosts, but no hard freezes that damaged leaf tissue.  The biggest setback to using this information is that we don’t have control of weather conditions, and waiting for the “proper conditions” may prevent the treatment of populations, leaving them unmanaged for another year.

R. G. Wilson and A. Michiels. 2003. Fall herbicide treatments affect carbohydrate content in roots of Canada thistle (Cirsium arvense) and dandelion (Taraxacum officinale). Weed Science 51:299-304.