Winter Control of Invasive Plants
by Mark Peterson, PhD Weed Scientist (retired) 

Woody Brush

By late November the time for foliar brush treatments with herbicides has passed.  Most of these species have lost their leaves and won’t take up the spray very well.  Even though honeysuckle (Lonicera spp.) hangs on to its leaves into early winter, they have usually turned yellow and are not that active at absorbing herbicides. Removal by pulling can be effective as long as the ground isn’t frozen, and you can get the roots out.  This can be useful for very small honeysuckle and helps keep previously cleared areas clean. However, I’ve personally found that some other species, such as burning bush (Euonymus alatus), don’t pull out well, even when small.

Basal bark applications are a method that uses herbicides that are more “oily” and able to penetrate bark.  Look for ester formulations and mix them with diesel fuel or basal oil.  Basal oil is nicer to work with (not as smelly) but diesel is obviously easier to come by.  Triclopyr ester formulations (e.g. Garlon® 4) are the most recommended herbicides for this application.  Mix to a 1-5%, volume to volume solution (I prefer the higher concentration) of triclopyr product in diesel or basal oil.  Apply with a backpack sprayer or other low-pressure (20-40 psi) sprayer.  Conventional basal applications require that the stems be thoroughly wetted 12-15 inches from the ground.  This means that treatment will be less effective if snow is covering the base of the tree or shrub.  There should be some visible runoff from the stem.  Low volume basal, as the name suggests, uses less spray on the stem and does not require treatment to the ground.  However, higher concentrations of herbicide are needed (20-30%).  Mixing triclopyr with aminopyralid (Milestone®) can increase effectiveness but preparing the mixtures can be tricky and requires the use of compatibility agents to get the oil-based triclopyr and the water-based aminopyralid to combine.  See the individual product labels for further instructions if you want to try this combination.  Glyphosate is not a good candidate for basal applications since it’s not great at penetrating bark.  Some research reports moderate success with basal treatments for honeysuckle but in one case I’ve read anecdotal evidence that they work better in late winter, say March, rather than December or January.  This technique is limited to stems less than 6 inches and ensure that all stems on an individual plant are treated.

Hack and squirt (sometimes called “frill”) applications can work well on woody brush.  A hand axe or other sharp tool is used to make horizontal cuts to the stems and a herbicide solution is applied to the fresh cuts. Water-based herbicides such as glyphosate or triclopyr amine will work for these applications since you’ve opened up the bark.  The use of basal oil is not recommended in this situation, and you should try not to overfill the hack.  About 1-2 ml of undiluted product per hack is sufficient. This method works well year-round, but it’s best to avoid late winter/early spring when sap is running in woody species as it may ooze and push the herbicide out of the cut. 

The above techniques are fine if you don’t mind having dead trees and brush standing in the treated areas. Otherwise, you can cut off the plant at the base and use a cut-stump treatment on the freshly cut surface.  A 25%solution of triclopyr ester in water or diesel/basal oil, a 50-100% solution of triclopyr amine, or a 50-100% solution of glyphosate will work well for cut-stump applications.  Be sure to apply right away after cutting or the herbicide will not penetrate well.  As with hack and squirt, cut-stump treatments are best avoided in late-winter/early-spring when sap is running.

As you can tell from the photos, many times applicators use a spray dye in their treatments.  This helps you to know what stems or stumps you might have missed.  It can be purchased at your local farm store or found at online vendors.

Mechanical treatments can be useful in the winter.  Forestry mowers that chew up invasive brush and shatter the crowns will beat back serious infestations but follow up treatments of spring regrowth are required. Unfortunately, most of us don’t have access to these specialized machines that are most often used by companies that work on land clearing of larger areas. 

Herbaceous Winter Annuals and Biennials

Winter annuals germinate and establish in the fall, resume growth in the spring, and set seed in late spring or early summer.  Non-native examples include yellow rocket (Barbarea vulgaris), purple deadnettle (Lamium purpureum), and common chickweed (Stellaria media). Biennials usually germinate in the spring, produce a low-growing rosette the first season, overwinter in that form, and then shoot up a flowering stalk in the second spring.  This growth form includes garlic mustard (Alliaria petiolata), poison hemlock (Conium maculatum), and many invasive thistles.

Sometimes during the winter months, we have short periods of milder weather that may present opportunities for foliar applications of herbicides on these species that are not completely dormant.  Late winter probably has the most potential for temperatures warm enough to melt snow cover and allow leaf uptake.  Make sure the forecast includes a string of warmer days to give the applications time to work.  You can pull or dig out plants in small areas, but make sure the roots are pulled out.  Foliar applications of glyphosate, 2,4-D, or triclopyr+2,4-D premixes using hand sprayers can be effective.  Spray to wet and use a 1-5% solution of glyphosate, 1% solution of 2,4-D (I prefer ester formulations in cold weather), or 1-1.5% solution of a triclopyr+2,4-D premix, such as Crossbow®.  Glyphosate treatments will benefit by mixing in ammonium sulfate (AMS). Your local farm store will have AMS products that provide instructions for mixing.  Research in Ohio (1) has shown that applications of 1% glyphosate + AMS can be effective against garlic mustard when daytime air temperatures are down to around freezing.  Additionally, the researchers found that the herbicide application at this time had little effect on non-target herbaceous species present.

A recently updated fact sheet from Penn State Extension (2) provides some excellent information on winter invasive plant control and I recommend you look it up if interested.  Several of the photos in this article have been borrowed from this publication.

Winter really is a great opportunity to clear out invasive species.  Many treatments work well, it’s easier to maneuver through the woods, and you don’t have to worry as much about those nasty ticks!  Best of luck in your control efforts.

References

(1) Frey, M.N., C.P. Herms, and J. Cardina.  2007. Cold Weather Application of Glyphosate for Garlic Mustard (Alliaria petiolata) Control.  Weed Technology, 21:656-660.

(2) Gover, A. 2023.  ‘Tis The Season – Winter Invasive Plant Control.  Pennsylvania State University Extension.  https://extension.psu.edu/tis-the-season-winter-invasive-plant-control