Today’s Pest and Disease post is a little different than some of the other topics that we’ve looked at such as gloomy scale or aphids. Today we are going to look at something that you may not even think of when you’re spraying weeds in your garden: glyphosate damage to trees. In case you’re not up on your chemical names, glyphosate is the active ingredient in products like Roundup and RazorPro. We have been told for years that glyphosate is biodegradable and has no residual impact on the soil that it touches. More and more, people are beginning to realize that may not be the case.
Dr. Hannah Mathers of Ohio State University has led the research on determining if glyphosate is responsible for cankers on trees where the chemical has been applied. The cankers resemble frost cracks that can occur during cold temperature extremes. The difference is that frost cracks generally appear on the south side of the tree whereas “glyphosate cankers” can occur on any side of the tree. The long and short of her research is that glyphosate is accumulating in the phloem of the trees and causing cankers and ultimately death. The death is a slow one as microorganisms move into the canker and set up shop. Glyphosate may not be the cause of death in the end but it is what allows the microorganisms a chance to kill the tree.
So how does the glyphosate end up in the phloem of the tree? It generally occurs one of two ways: the first way is by applying the herbicide around the base of the tree and having it come into contact with the trunk. You may not be trying to spray the trunk directly but if there are weeds at the base of the tree, you may inadvertently spray the trunk. The second way is through killing the weeds that grow within the root zone of the tree. That is, after all, probably why you are spraying herbicides to begin with. When these weeds die, they exude a small amount of the chemical into the soil which can then come into contact with the roots of the trees. The roots take up the small amount of glyphosate into the xylem of the tree but as it is transported throughout the tree, it ends up being stored in the phloem. Research indicates that the glyphosate can build up in the phloem for years and continue to cause problems for the tree for a long period of time.
Dr. Hannah Mathers has found that using glyphosate products that contain a surfactant exacerbates the problem. Surfactants are added to many pesticides to allow the chemical to “stick” to the target pest. The stickiness that results also allows the glyphosate to adhere to the trees if they are inadvertently sprayed. Unfortunately, virtually all of the homeowner versions of glyphosate contain surfactants.
So what can you do to avoid glyphosate damage to trees? MULCH! While I certainly don’t advocate the practice of mulch volcanoes, a 3″-4″ layer of mulch under your trees will keep most of your weed problems at bay. For the weeds that appear, hand weeding is going to be your best option. Also, consider planting perennials under the trees. A groundcover of plumbago (Ceratostigma plumbaginoides) will thrive in sun or shade and will form a dense mat that is almost impenetrable to weeds. If you don’t have to have the perfectly manicured garden, consider planting a nitrogen fixing plant under your trees so that the trees have a free source of nitrogen. Examples include members of the legume family such as peas and beans, alfalfa and clover.
I hope that you’ve gleaned a bit of useful information from today’s post. Take a look around your yard and other landscapes to see if you spot any of the “glyphosate cankers”. Let me know what you find by leaving me a comment below or sending me an e-mail at firstname.lastname@example.org. Happy gardening!