COLUMBUS, Ohio -- Glyphosate products, such as Roundup, may be a killer on weeds, but researchers are finding that the product may also damage landscape and nursery woody plants.
Hannah Mathers, an Ohio State University Extension nursery and landscape specialist, said that glyphosate applied improperly or in too high of a dosage is causing a phenomenon known as split bark -- where, through the tree's uptake, the chemical is deteriorating the bark structure and destroying the winter hardiness of the plant. The cosmetic damage makes the plant unsaleable, and is costing the landscape and nursery industries millions of dollars per year in damaged product.
"The economic cost to the U.S. nursery industry from bark cracking is conservatively estimated at $6.6 million a year. That's roughly 2.5 percent of finished inventory," said Mathers, who also holds an appointment with the Ohio Agricultural Research and Development Center. "Add to that the conservative estimate of $14 million in landscape tree failures, and we've got a national phenomenon that has been happening for several years but only now are people taking seriously."
Mathers is teaming up with the agricultural company Monsanto to help develop a glyphosate product that is safer to use for weed control around landscape and nursery woody plants. She has been leading national public research on the impacts of glyphosate on woody plants, as well as educational efforts on which glyphosate products to use and how to properly apply the chemical.
"For a long time, industry felt that split bark was an environmental problem, driven mainly by cold temperatures. But we were receiving reports of split bark in warmer parts of the country, such as Georgia, the Carolinas and California. Why would it be a cold issue if split bark is happening in those areas?" said Mathers. "Winter temperatures are part of the problem, but only because glyphosate weakens the bark structure enough to cause the trunks to split under stress. There's a large body of national research that supports those findings."
Mathers said that the first step in controlling split bark is education: recognizing that glyphosate could be a contributing factor, which glyphosate product to use, and using that product properly.
"The first thing I tell nursery and landscape professionals is to use glyphosate only when necessary," said Mathers. "We want to stress pre-emergent glyphosate applications to kill weed seedlings, rather than a post-emergent application that kills the entire weed plant. It reduces the impact on woody plants, as well as saves money. Adoption of integrated weed management programs with reduced reliance on glyphosate can cut herbicide expenses and application labor by up to 50 percent."
In situations where glyphosate is required, users should pay attention to which product they apply. Research has shown that it's not the glyphosate itself that is causing split bark, but the surfactant found in some glyphosate products that is causing the problem. A surfactant is a wetting agent that allows for easier spreading of the chemical, and increases uptake of the chemical in woody plants. Surfactants are known as adjuvant loads on glyphosate product labels.
"When glyphosate use is necessary, use a glyphosate product around woody plants that has no adjuvant load," said Mathers. "Products that have a full adjuvant load are the worst around ornamental plants because of the increased potential for uptake of the glyphosate by the surfactant into the bark."
Fourteen registered glyphosate products contain no adjuvant load. They include: Backdraft, Campaign, Expert, Extreme, Fallowmaster, Fallow Star, FieldMaster, Glypro, Landmaster BW, Land Star, ReadyMaster ATZ, Rodeo, Roundup Custom and RU SoluGran.
Mathers also encourages nursery and landscape practitioners to apply glyphosate products properly. A Horticultural Research Institute-funded project conducted last year found that many growers and nursery/landscape professionals were using glyphosate indiscriminately -- making applications (one quart per acre) as frequently as eight times a season, or approximately every 2.5 weeks; removing suckers with glyphosate products; and applying product so close to woody plants as to increase uptake through drift exposure.
"Glyphosate should not be used to remove suckers, there should be a 30-foot buffer between the weeds you are spraying and the woody plants, and glyphosate should not be applied so frequently," said Mathers. "The formulations for glyphosate have changed over the years. I don't think people realize that the glyphosate they use now is more potent than older products they are used to. Plus, more generic brands are now available and they are cheaper to come by, so users are getting more lax in their applications."
Mathers said that glyphosate with surfactants are dangerous for woody plants because it takes years for the plant to break down the chemical once it's taken up. Research has shown that one single low dose of glyphosate stays in the plant for at least a year.
"Just imagine what kind of damage you are doing to a woody plant when you apply glyphosate two times a month," said Mathers. "Glyphosate injury is also difficult to diagnose because symptoms may not be present for up to two years after glyphosate absorption."
In addition to split bark, other symptoms include witches broom, stunting, loss of apical dominance, individual dead limbs, chlorosis and death.
Woody plants most susceptible to glyphosate uptake include: Pyrus species, especially Callery pears; Prunus species, especially Yoshino cherry and Kwanzan cherry; Crab apples; Sycamore; Serviceberry; Hawthorn; Mountain Ash; Black Gum; Paper bark maple; Japanese maples, especially variety dissectum; Norway maple, especially ‘Emerald Queen'; Red maples; Dogwood, especially Kousa dogwood; Magnolias, especially Magnolia ‘Elizabeth'; and the yellow magnolias such as Magnolia ‘Butterflies', ‘Sawada's Cream', Magnolia ‘Yellow Bird' and Magnolia ‘Yellow Lantern'.
Specifically, glyphosate uptake leads to an accumulation of a type of acid called shikimic acid that results in a reduction of phenolics -- plant compounds that serve a variety of roles in plant development and survival including defense against pathogens. Research has found that the more glyphosate is taken up by the plant, the higher the shikimic acid levels. In addition, glyphosate stays within the plant for years, being stored in the roots with sugars in the summer and fall, and then translocating to areas of the plant where growth takes place in the spring and continuing to cause injury.
Mathers said that until safer glyphosate products are developed, a change in weed management practices in the nursery and landscape industries is required to control the split bark phenomenon.