Check out our latest guidance on controlling Canada thistle: How to Control Canada Thistle: Seasonal Guide (2021) This document contains our most up-to-date advice on how to control this nasty weed. This is a PDF that can easily be viewed on your smartphone.
This is a prairie restoration that we started a while back. The first thing we did a few years ago was girdle the big pine trees. You can see some of the remaining snags in the photos below. This opened up the canopy but also spurred the growth of lots of small woody species that we had to deal with.
With any of the brush that was over 2 inches OD, we did cut stump treatments in late winter. The small pine trees, of course, we just cut down and did not stump treat. The stump treatments were low enough to the ground so we could come through with a brush mower about a month-and-a-half later without hitting the big cut stumps.
Using the walk-behind brush mower was slick. It chopped most of the brush into small pieces, so we didn’t have to haul it to burn piles. It also shatters the stumps (much like a forestry mower). This will reduce the vigor of the cut brush.
We will have lots of re-sprouts from the brush that we did not treat. We plan to mow again later this spring after the cool season plants (mostly non-native) have started growing vigorously but before the warm season plants (mostly native) are very high. This second cutting will also cut the woody species that have re-sprouted. Then, in the mid-summer and fall we will treat the perennial weeds and the woody plants. Since this area has a native seed bank, we’ll see next year how much of the area we will have to seed.
Controlling a large stand of invasive knotweed can be a daunting task. We just finished this document: Green Shoots Guidance on Controlling a Large Stand of Knotweed. This
document complements our video: Knotweed Control: 3 Simple Steps for the Non-Professional which is intended for those controlling a small knotweed colony.
In our new piece, we synthesize much of the latest research on and our experience with controlling knotweed. The approach solves several problems confronted in controlling a large infestation: First, how to do you ensure coverage of the entire stand when applying herbicide? Second, how do you accomplish the first objective without introducing excessive amounts of herbicide into the environment? Third, how do deal with the few surviving knotweed crowns that are often difficult to kill.
In short, the approach is to make a broadcast application of herbicide to the knotweed colony in the first year. This is done in such a way as to maximize herbicide effectiveness and minimize harm to neighboring desirable species. Thereafter, we use a targeted approach that combines: spot treatments with herbicides; mechanical control – cutting or digging to remove the surviving knotweed plants; and the introduction of competition from native plants.
The three most common species of invasive knotweed in North America are: Japanese knotweed (Fallopia japonica or Polygonum cuspidatum); Sakhalin knotweed (Fallopia sachalinensis); Bohemian knotweed (Fallopia×bohemica).
You see those trees with white flowers in the drone photo below from early spring 2020?
Those are pear trees growing in a southern Indiana plot where oak and pecan trees had been planted in previous years. As the owner of that land will attest, however, those pear trees were not planted as part of his reforestation project.
In fact, as best owner Jerrod Carlisle can tell, those pear trees likely spread from two “dwarf pear trees” included with a “postage stamp orchard” he had bought by mail several years ago.
That postage stamp orchard turned out to be a really bad deal for Jerrod. He soon found the dwarf pear trees produced no fruit any human could eat. Even worse, the pear trees spread into his forest. Only after the timber harvest and some research did Jerrod learn of the full scope of the problem he now faces.
Jerrod found out these were Callery pears (Pyrus calleryana). The story of the Callery pear and why it was brought the U.S. is a fascinating one. Originally, botanists collected its seeds in China to help buck up resistance of pear fruit trees in the U.S. to fire blight. However, John L. Creech, who headed the National Arboretum in Washington, D.C., saw the Callery pear’s potential as an ornamental, and that’s how a variety of this species gained its fame under the name, the “Bradford Pear.”
The Bradford pear arrived on the market in the 1960s with fanfare. According to January 5, 1964 edition of the New York Times: “Few trees possess every desired attribute, but the Bradford ornamental pear comes unusually to [sic] close to the ideal.”
But, why is this “ideal” ornamental trying to take over Jerrod’s forest? If you read that New York Times article, many of the attributes that made it a good ornamental back in the day also make it a formidable invader today.
- The Callery blooms early in spring, and its leaves stay green late in fall.
- The abundant fruit are eaten by birds and squirrels.
- It grows under a wide range of soil and climatic conditions.
- It resists disease and insects.
- Its deep taproots help it survive dry periods.
To tackle his Callery pear problem, Jerrod purchased our Large Foam Herbicide Dispenser Package in December 2019. He started doing cut stump treatments this past winter. As you can see from the area marked in red in the photo below, Jerrod has already made progress. There are no blooming pear trees on the part that he treated.
Jerrod has several goals with his work. He wants to ensure the property serves as good wildlife habitat. Jerrod is a hunter who enjoys spending time out in nature.
But if you talk with Jerrod, you quickly realize he also wants to raise awareness of the threat posed by the Callery pear. As Jerrod noted, Interstate 69 was recently renovated near his home. During construction, the right-of-way had miles of bare soil for extended periods of time. Now Callery pear blanket miles-long stretches of that major highway.
And, all the while, more Bradford pears are coming on the market. On March 30, 2021, I did a search online. Here’s what it produced:
A first step in saving uninvaded natural areas is not paying good money to plant this horrible tree. As Jerrod can attest, you will regret it if you buy it, but you will regret it even more if you plant it.
For further reading:
- Culley TM, The Rise and Fall of the Ornamental Callery Pear Tree, Arnoldia, 2017;74(3).
- Hurley KH, The Detested Bradford Pear Tree Is Coming to a Forest Near You, CityLab, July 2, 2019.
I attended a wonderful conference in Coeur d’ Alene, Idaho, put on by Steve Manning and his group at Invasive Plant Control from December 10-12, 2019. One theme: rodent eradication! Dickie Hall, Habitat Restoration Project Director, talked about the complete eradication of rats on the Island of South Georgia. The island is a treeless tundra not far from Antarctica. This is a huge win for many bird species that nest on South Georgia (and two species endemic to the island). The South Georgia Heritage Trust sponsored the restoration. Dickie also gave a talk on the planned eradication of mice on Gough Island. The mice eat Albatross chicks and other birds and have devastated avian populations. I won’t even link to the videos on YouTube which you can easily find if you are so inclined.
Dan Tompkins gave another great talk on making New Zealand predator free by 2050. This is a massive undertaking to eliminate alien predators (rats, opossums, and stoats) that kill all sorts of native wildlife including the kiwi.
In our work focused on terrestrial invasive plants (not growing on isolated islands) total eradication is nearly impossible. Nonetheless, these talks were inspiring. And, anyone who has been on a restored native prairie knows the beautiful sights, sounds, and smells of being in a native landscape. Perfection is not required!
My talk focused on precision herbicide applications using our Precision Electronic Dispenser. The key thing to remember about the Green Shoots® products is that they all reduce off-target harm from herbicides. Traditional high-pressure spray from backpacks inevitably causes off-target harm. Native plants are highly valuable, especially mature native perennials that may have been alive for as long as 30-40 years. You should do everything you can to preserve the health of those native plants when you are spot spraying nearby them!
Henry Grabar wrote an excellent piece in Slate on knotweed: “Oh, No, Not Knotweed! What I think is so important about this article is that it lays out the environmental consequences of knotweed – not just the potential for property value losses that are so widely reported about in the United Kingdom. Knotweed is especially devastating to waterways. The article notes the three key problems with knotweed identified by Chad Hammer at the University of New Hampshire:
- In a knotweed stand, virtually no light hits the ground. This prevents other plants and bugs, etc., from living in that patch.
- New trees cannot grow in a knotweed patch. This reduces woody debris, for example, in the stream.
- Erosion! Patches of knotweed have very little organic matter on the ground and the ground can erode easily.
Especially with flood events becoming more common – and water washing live, viable knotweed debris downstream – the problem will only get worse.
Knotweed can be controlled! The above photos show that an aggressive treatment can virtually eliminate it. The key is to keep after the straggler plants that will inevitably persist after the initial treatments. Here is our YouTube video: How to Kill Knotweed: 3 Simple Steps for the Non-Professional.
This is an impressive 2018 study on treating Chinese privet (Ligustrum sinense) with herbicide: Enloe et al., The Influence of Treatment Timing and Shrub Size on Chinese Privet (Ligustrum sinense) Control with Cut Stump Herbicide Treatments in the Southeastern United States. Note also that the study has good advice for treating many different kinds of woody invasives that are prone to lateral sprouting – tree of heaven being another prominent one. Here’s specifically what I like about the research they conducted.
First, the researchers used good technique for doing cut stump treatments. They cut the stump low to the ground – about 1 inch above ground level. Anecdotally, I have found that cutting the stump low to the ground makes a difference in the success of the treatment.
The researchers applied the herbicide immediately after making the cut – within 30 seconds. It is not known how quickly woody plants seal off wounds to live tissue; however, research does indicate that plants can react to physical wounds very quickly – within minutes. Why take a chance? Treat quickly after making the cut.
Second, the investigators did good follow-up. They didn’t just look at whether the stumps had re-sprouts. They also looked at sprouting from lateral roots within a 30 cm radius. The researchers checked the stumps at 6, 12, and 18 months after treatment. Most of the treated privet stumps likely died within the first 6 months in this study. However, as the authors note, that is not necessarily the case with different species of invasive woodies.
Third, the study reached important conclusions that really provide guidance to those in the field. It found that November treatments were more effective than April ones – 95% vs 87% mortality. The study found that glyphosate outperformed triclopyr in the November treatments – 97% to 91% control. The authors note that this suggests that “glyphosate is more effective than triclopyr for controlling species prone to lateral root sprouting.”
Finally, the authors found that lower than labeled concentrations of herbicide were effective at controlling privet. This conclusion may not be applicable to all species, but it does suggest that you should be safe using the lowest rates recommended on the label for cut stump and cut stem treatments.
Spring and fall are excellent times to attack non-native thistles such as Canada thistle (Cirsium arvense). Here’s a photo of a very young Canada thistle rosette taken in early spring in a small prairie area that we are restoring.
I used our Large Foam Herbicide Dispenser with the mesh brush attachment to wipe foam herbicide on the rosettes. One nice thing about early spring is that the plants are well spaced and you can easily isolate the target weed. Thus, you can avoid harming desirable native plants.
As shown below, you can get a perfectly targeted application of the foam weed killer by wiping the foam directly onto the leaves of the thistle.
In terms of herbicide rates, most labels for glyphosate suggest rates that are pretty high for wiping. In some cases, the recommended wiping solution may have 20% active ingredient. In my opinion, this is too high. (That high rate may be suggested because conventional wiping applications with sponges or fabric wipers result in a lot of drippage.) Foam herbicide reduces the drippage to a minimum. Using foam herbicide, I have had success with rates that are about 4% to 8% active ingredient.
In terms of timing, in the spring make sure you treat the Canada thistle in the rosette stage before it starts to bolt. Once it is bolting, it will be hard to control with herbicide. In the fall, wait until you have had a frost or the weather has become pretty cold in your area. As long as the thistle leaves remain green, you can treat them.
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!