In the last post, I talked about wintertime being an ideal season to kill an invasive tree, shrub or or other woody weed. Why? During winter woody perennials such as trees, vines, and bushes are dormant, i.e., at rest, but their above ground vascular systems in their stems are still fully alive. If you cut through the outer bark, you will reach the live inner parts of the weed tree, vine, or shrub. Part of that live inner bark, the phloem, will readily absorb herbicide and will translocate the herbicide to the roots or rhizomes of the plant.
In spring and early summer, the situation is very different. Sap will be rising from the roots to feed the above-ground sinks such as leaves, flowers, etc. If you treat an invasive honeysuckle with weed killer in spring, for example, the top of the shrub may die, but the roots will likely survive and you will eventually get nasty re-sprouts near the base. Instead of 5 or 6 stems to treat, you might now have more than 20. The upshot, therefore, is avoid using brush killer in early spring.
What about late winter? How long is winter and dormancy for purposes of applying a weed killer to an invasive tree, vine or shrub?
The answer depends on where you live. “The timing of . . . [the] release from dormancy is synchronized with local climates and is highly heritable.” Yordanov 2014.
Temperature appears to be the key thing to watch. Bud-break in temperate woody species is “almost exclusively dependent on high temperatures.” Busov 2016. If buds don’t seem to be swelling and nights are still cool, the woody perennial should still be largely dormant.
(Note: If you want to get a sense of what bud break looks like, look at the time lapse photos in this article by Sivadasan et al. 2017. Sivadasan and colleagues divide budbreak into five stages. The earliest stage with the bud still closed and with no protruding leaf tips, is probably the only feasible time to do the herbicide treatment.
In south-central Minnesota in locations around the Twin Cities (i.e., pretty far north in the United States!), I have done cut stump treatments on invasive common buckthorn (Rhamnus cathartica) as late as early April with excellent results. Those treatments were done when daytime temperatures were warm, but nighttime temperatures were still often near or below freezing. My guess is that those treatments were successful because the trees were still dormant (or maybe just emerging from dormancy). However, in years when springtime comes early, a weed killer treatment in my area in early April might fail.
In most other parts of the country, the emergence from dormancy of invasive woody perennials will typically be much earlier. Someday we’ll certainly have data that is specific to each invasive species in a region. We already have it for crops such as almonds, pistachio, and walnuts, thanks to a citizen science project the University of California at Davis. Right now, however, determining the end of the winter herbicide application season will have to be an educated guess. Consider daytime and nighttime temperatures, bud stage, and possibly even the start of the pollen season.
I really enjoy doing invasive plant control work in late winter. The sun is stronger and ticks and bugs still shouldn’t bother you. You can also see really well because the bushes and trees still won’t have leaves. It is also particularly satisfying to know that you will be opening the canopy to native species that are just waiting for those first warm days of spring!
Updated: January 21, 2021
When it comes to killing woody invasive plants, winter is generally the best time. This may surprise you. Herbicide labels may confuse you because labels often say the herbicide should be used when the target weed is “actively growing.”
However, cut stump and basal bark treatments work great in winter as long as the plant has an above-ground stem with live inner tissue. Weed trees and bushes all have live inner bark. Therefore consider winter treatment for invasive woody species such as Asian bittersweet, buckthorn, honeysuckle, kudzu, privet, tree-of-heaven, etc.
There are a number of advantages to working in winter (or late fall):
- Effectiveness: The greatest success I have had controlling woody species is in late fall, winter, or very early spring (before sap starts flowing up the stem to the branches). This is true for others too. Reinartz 2002.
- Ease of Movement: The absence of growing plants makes it surprisingly easy to move through dense growth.
- Comfort: Removing invasives can be a lot of work, especially if you use hand tools. In winter, you can stay warm without being sweaty and uncomfortable.
- No Mosquitoes or Ticks: This is a godsend. If it’s above freezing and there’s no snow, ticks can be out but at very reduced numbers.
- Identification of Targets: As long as you can identify the invasive by the bark and structure of the plant, identification is much easier. Without leaves blocking your view, you can see so much better.
- Cold Temps: I usually do not work when it’s below about 20 degrees F. If it gets below that temperature, water-based herbicides may freeze, especially around the nozzle. Plastic containers also become more fragile.
- Deep Snow: It is difficult to do either cut stump or basal bark treatments if the snow is more than a few inches deep. You can remove snow around the base, but this can be time-consuming.
For cut stump treatments, cut the stump as close to the ground as possible (1 to 2 inches) above ground level. (I find that failed cut stump treatments often result from cutting the stump too high.) Brush off any debris on the stump face. (Dirt will neutralize herbicides such as glyphosate.) Then apply the herbicide immediately after cutting (within 5 minutes or so).
How late in winter can you apply? Read our other post on applying brush killer in late winter/ early spring.
Good luck! I hope you are able to get out on a nice warm winter day to remove some invasive plants. It can really be enjoyable!
You have an unwanted tree growing vigorously in the wrong place – in the middle of a flower bed or in a native grass planting or next to a building foundation. You know you need to remove it. Winter is actually a great time of year to do this. Herbicides work fine in the winter if applied correctly. See Reinartz 2002.
If you simply cut the tree or bush down, you will soon get something far worse in the spring: a multi-stemmed bush like the one shown below on the right side.
There is a simple way to prevent this from happening: use the Green Shoots Foam Herbicide System to precisely apply a concentrated herbicide to the stump immediately after cutting to kill the stump. So here is what you do.
- Cut back the tree or bush – If it is a big tree or bush, I typically cut off the upper branches first and leave a tall stump anywhere from knee to shoulder height. Clear out the branches so you have room to work.
- Prepare the Foam Herbicide Dispenser according to the instructions. Be sure to read the herbicide label.
- Cut the stump close to the ground – try to cut it within 2 to 4 inches of ground level.
- Immediately apply the foam herbicide to the cambium layer of the stump. This is a thin layer of live inner bark just inside the outer bark.
The Green Shoots Foam Herbicide System has several advantages over standard herbicide sprays:
- Precision – This is critical when you are applying to the narrow ring of live inner bark less than 1/10 inch thick. With a liquid spray, the application is so imprecise – you may miss your target. At the very least, you will waste herbicide trying to hit the target.
- Less Drip – The foam herbicide sticks to the target and will slowly soak in. Liquid sprays will bead and drip off the target.
- Visibility – the foam is visible for some time after the application. This helps in identifying what part of the target you have treated.
For more information about the Green Shoots Foam Herbicide System, visit the Green Shoots website.