A common ingredient in residential landscape herbicides may damage or kill woody plants. 
“Cracking, splitting and separation of bark from the underlying wood usually associated with sunscald in the West may actually be caused by glyphosate,” said Heidi Kratsch, Utah State University Extension ornamental horticulture specialist. “This type of injury was once thought to be largely associated with alternating warm and freezing temperatures during the winter. It now appears that glyphosate can weaken the bark structure, making it susceptible to the freeze-thaw injury we commonly see on the south and southwest side of the trunks of susceptible trees.” 
Glyphosate is a non-selective herbicide used as a broad-spectrum, post-emergent weed killer, she said. When used too closely to trees and shrubs, glyphosate can be taken up by these non-target plants and transported to active, growing tissues such as leaves and roots.
“We’ve known for some time that glyphosate drift can cause stunted, distorted shoots, chlorosis and often death of woody plants,” Kratsch said. “But now it has been associated with a condition we in the West are all too familiar with – sunscald.” 
Suspicion that there was more to the story of bark-splitting arose when reports of sunscald-type damage started coming from mild-winter states like Georgia, the Carolinas and California, she explained. Hannah Mathers, a researcher and Extension nursery and landscape specialist at Ohio State University who heads up national research on the effect of glyphosate products on woody plants, recommends that consumers use glyphosate products with caution. 
Studies show that the chemical glyphosate itself is not what causes symptoms, but a surfactant, or wetting agent, in some glyphosate products, Kratsch said. The surfactant helps spread the chemical on target plants and enhances uptake.  On product labels, the surfactant can be identified as “adjuvant load.”  Mathers’ study recommends using glyphosate products with no adjuvant load around sensitive plants.
Kratsch and Salt Lake County Extension horticulturist, Maggie Shao, suggest the following products registered for use in Utah that contain no adjuvant load: Campaign, Fallow Star, Glypro, Landmaster BW, Rodeo and Roundup Custom. 
Woody plants most susceptible to glyphosate include: Pyrusspecies (especially Callery pear), Prunus species (especially Yoshino cherry and Kwanzan cherry), red maple, Norway maple (especially ‘Emerald Queen’), Japanese maple (especially var. dissectum), paperbark maple, mountain-ash, serviceberry, sycamore, crabapple, dogwood (especially Kousa dogwood), hawthorn and magnolia (especially ‘Butterflies,’ ‘Elizabeth,’ ‘Sawada’s Cream,’ ‘Yellow Bird’ and ‘Yellow Lantern’).
“Risk of damage can be minimized by using glyphosate products at least 30 feet from woody plant materials and using it early when weeds are still seedlings,” Shao said.   “Glyphosate should be used as a last resort; emphasis should be on use of pre-emergent herbicides to catch weeds before they germinate.  Glyphosate should never be used to control root suckers.”
Glyphosate products have been shown to increase levels of shikimic acid in plants. This acid reduces the level of phenolic compounds, which are natural substances found in woody plants that protect against attack by pathogens, Kratsch said. Research shows that the more glyphosate the plant takes up, the higher the levels of shikimic acid.  Glyphosate products accumulate in plant roots where they can be stored for years, causing injury long after the original herbicide application. Glyphosate and all herbicides have benefits and risks; following the label will maximize the benefits and reduce the risks.

By: Dennis Hinkamp - Sept. 15, 2008