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:

1. In a knotweed stand, virtually no light hits the ground.  This prevents other plants and bugs, etc., from living in that patch.
2. New trees cannot grow in a knotweed patch.  This reduces woody debris, for example, in the stream.
3. 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.

We launched our new website!  Please take a look: Green Shoots.

This presentation at the Upper Midwest Invasive Species Conference, 2014, summarized some of our control work on invasive knotweed at Southwood Nature Preserve in

the Twin Cities Metro area.  It also briefly discusses use of our foam herbicide system by the Northeast Iowa RC&D where they have had success treating a large infestation of greater than 25 acres.  Finally, Washington State

University did some testing this summer comparing our foam herbicide dispenser to traditional spraying techniques.  So far (and these results are very preliminary), the foam herbicide has performed better than the spray even though half as much herbicide was used on the plots where foam was applied.  The full presentation is online at: How to Kill Japanese Knotweed.

As soon as we get data from Washington State University on their testing of the foam system, we will provide a link or other information.

We just added a new page with testimonials about our products.  Here it is: Testimonials (link).  They will give you an idea about what people are doing with our foam herbicide dispensers!

We launched our new website: Green Shoots.  We have several new products on the website: our Large Foam Herbicide Dispenser; Foaming Agent; and Aquatic herbicide.  If you are a professional or have a large piece of property with invasive weeds, you may be especially interested in our Large Foam Herbicide Dispenser.

We will also be adding a resource center in the near future.  This will give you a link to our videos and other postings.

As someone working for company that sells herbicide for killing invasive plants, I get push-back from people who don’t like to use herbicides. This is understandable. When herbicides are used almost indiscriminately – killing both weeds and desirable plants, that is a huge problem. Researchers have, for example, found a loss of milkweed in agricultural fields due to increased herbicide use. Lower milkweed populations means declines in monarch butterfly populations. Pleasants et al. 2012.

If herbicides are used with precision, however, I find they can be enormously useful. For quickly eliminating harmful invasive weed trees such as buckthorn, they are indispensable, especially if you are working on a large scale. After reading numerous scientific articles over the years, I feel comfortable using them. But, I certainly respect the decision of those who choose not to use herbicides.

If you decide not to use herbicides, what is the best way to kill a weed tree? It will be tougher, and it will take at least a couple years to accomplish, but it can be done if you have patience and the number of weed trees is limited.

Here are some tips. First, don’t concoct homemade herbicides. Many people think, for example, that pouring salt on a plant is better than using a commercial herbicide. From an environmental perspective, however, you are doing much more harm than good. Herbicides like glyphosate bind tightly to soils and break down relatively rapidly. Salt, on the other hand, is very persistent in soils, and most plants do poorly in soils with high salinity. Read this article from the University of Illinois about homemade herbicides if you are tempted.

Second, be careful if you decide to pull weeds, especially large weed trees. Not only can you strain your back, you can also hurt nearby desirable plants by uprooting them. Also, freshly disturbed soils invite weed seeds. Try to minimize disturbance of soils.

Especially with weed trees, one technique I have used is repeated cuttings to weaken a plant. Timing is critical.  The weed trees shown in the photo above – common buckthorn, for example, were trimmed back to what I call “tall stumps,” i.e., stumps that are above waist height or even taller if possible. I did this in the spring after they had fully leafed out. This is the time of year when the tree has sent energy from the roots to the foliage for leafing out and flowering. It’s also a time when plants are normally vigorously photosynthesizing. By trimming off all the foliage you have not only robbed a tree of much of its stored reserves, you have also harmed the tree’s ability to photosynthesize during a critical time.  (You have also prevented the plant from flowering and creating seeds, which is why I trimmed these trees.)

Most invasive trees will recover. For example, buckthorn will regrow branches after a cut as shown in the photo above. The reason for cutting the tree high is two-fold. First the tree will generally grow new branches as high on the stem as possible. To weaken the tree still further, you should do another cut just below that new growth. Ideally that cut should be made in the same year – after the tree has expended energy in forming new branches. Second, another reason for cutting high and creating a tall stump is to avoid creating a bush. If you cut a stem close to the ground, multiple stems will grow from the stump. You now will have to cut multiple stems rather than just one.

It is surprising how much a well-timed trim will weaken a weed tree. I have noticed that if I top a seed-producing buckthorn it may take several years before that tree produces seeds again. In addition, I often find the sapwood of these topped trees to be discolored. This indicates to me that the tree has been stressed.

The above technique also works well even if you decide ultimately to use an herbicide.  Applying an herbicide in the spring when sap is rising is generally a bad idea.  By trimming and forming a tall stump in the Spring, you have eliminated at least one year of seed production.  You have also weakened a tree ahead of the herbicide application and made the success of that application more likely.