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.