How to prevent glyphosate damage to young trees
If you notice the bark on your clients’ trees cracking or splitting, particularly after the winter season, you might just chalk it up to the cold weather, but cold weather may not be the problem. The problem may lie in the way you control the weeds in the landscape.
New research has discovered that split bark—although sometimes occurring naturally on the south and southwest side of trees due to freezing and thawing patterns—may actually be caused by glyphosate herbicides that are applied directly to the tree bark, accidentally sprayed because of drift, or applied too frequently and in overly high doses in the surrounding landscape.
Glyphosate products, including the commonly used turf herbicide, Roundup, do a good job killing weeds in the landscape, but the chemical can also cause irreversible damage on woody plants, said Hannah Mathers, an associate professor at Ohio State University in Columbus, Ohio, and the state extension specialist for nursery and landscape crops.
The problem goes beyond the cosmetic damage of the bark splitting; glyphosate products actually deteriorate the bark structure and destroy the winter hardiness of young trees.
Mathers has been getting the word out to landscapers by traveling around the country to speak about not applying glyphosate products near or around trees. She said that many landscapers are unaware that they are inadvertently causing damage to young trees, but the problem can easily be remedied.
The reason glyphosate is problematic to young trees is because of the wetting agent used in many glyphosate herbicides. This wetting agent is what makes glyphosate easy to take up by the tree. Once the chemical enters the tree, it decreases the tree’s resistance to diseases and weakens the bark structure, making it susceptible to bark cracking and splitting.
“(The chemical) causes a double whammy … the tree can’t heal well, and it doesn’t have the hardiness it had before,” said Mathers. In addition to symptoms of split bark, glyphosate compound uptake in trees can cause other problems, such as witches’ broom (a cluster of twigs that grow on a branch), growth stunting and even death of the tree if it is applied in lethal doses.
“We are not sure how far the glyphosate moves once it gets into the trunk,” said Mathers. “Most applications are not lethal doses, but it does do damage and weakens the tree, and each time you spray there is the potential of the chemical accumulating and building up in the tree.”
The most vulnerable to bark cracking are the young, thin-barked trees that are between 2 and 2.5 inch calipers in diameter.
“Once a tree gets older and has a thicker bark, the glyphosate uptake is not an issue,” said Mathers, since the older barks do not readily take up the chemical.
To determine if a tree can take up glyphosate into its bark (and cause damage to the tree), push the bark up with your thumbnail, if you see green bark, it will take up the herbicide. These are the trees that are most susceptible to damage from glyphosate use in the landscape.
“Bark splitting has always been thought of as cold injury,” said Mathers. Before conducting her research, her agency had received years of data about split bark in warmer areas of the country as well, including Georgia, the Carolinas and California. In these areas, any bark splitting had been blamed on sunscald, but research has shown that glyphosate uptake has also been the culprit.
Prevent glyphosate damage
“Glyphosate is a very environmentally sound product that breaks down quickly in the soil; but it does not break down if it gets into a plant,” said Mathers. Getting into an annual weed is different than the product making its way into a young tree, where the chemical lingers in the tree and causes damage. Research shows that a single, low dose of glyphosate stays in a young tree for one year.
How can you prevent glyphosate damage to young trees on your client’s property? Here are some steps to take:
- Know what kind of glyphosate is safe to use around young trees. There are numerous brands of this herbicide, all with differing levels of adjuvant, or wetting agent. Make sure that you use a brand that does not have an adjuvant load.
- Use correct amounts of herbicide; do not overspray, which can cause drift onto trees.
- There should be a 30-foot buffer between the weeds you spray and young, woody plants. Glyphosate sprayed directly on, or alongside, young trees does the most damage.
- Use shields if you spray glyphosate around young trees.
- Rely more on preemergent herbicides to kill weed seedlings rather than postemergent glyphosate applications to kill the entire weed plant later. Limit the use of postemergent herbicides for rescue treatments or only as the last resort. This will reduce the effect on young trees.
- Adopt an integrated weed management program to reduce reliance on glyphosate.
- Do not use glyphosate for sucker removal. The suckers will take up the glyphosate and transfer it into the tree. Instead, use stump treatments that are registered for sucker removal, such as Scythe (perlargonic acid). Mathers suggests using a sprout inhibitor, such as Tre-Hold Sprout Inhibitor A-112 or Tre-Hold RTU, painted or sprayed on pruning cuts after the sprout is removed. Herbicides that contain glyphosate should be sprayed in the landscape before sucker removal, not after.
- Reduce your use of glyphosate. Research has shown that many tree growers, nursery and landscape professionals are using glyphosate indiscriminately, as much as eight times per season.
Woody Plants Most Susceptible to Bark Splitting
Registered Glyphosate Products
There are currently 45 generic glyphosate products registered for ornamentals. Each has a different wetting agent formulation. To reduce the chances of causing damage to young trees in the treatment area, make sure you select an adjuvant load of “none.” While these products indicate you need to add an adjuvant, Mathers suggests that you use the product without adding an adjuvant.
Trade names with no adjuvant load and are safer to use around young trees include:
The author is a freelance writer from Keene, N.H.