Herbicide Caution: Glyphosate Use May Damage Woody Plants

As we wrote yesterday, Kohl Construction proposes to apply herbicide in an effort to eradicate two invasive species on its property: Japanese knotweed and multiflora rose. Here is a news release from Ohio State University Extension with cautions about the use of glyphosate, found in a number of popular herbicides.


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Use Glyphosate Properly to Protect Woody Plants


Candace Pollock
(614) 292-3799


Hannah Mathers, OSU Extension, OARDC
(614) 247-6195

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.

See also:

Japanese Knotweed and Multiflora Rose: Is Herbicide the Answer?
We have already discussed how the commonly used herbicide glyphosate (Rodeo and Roundup) has been shown to be highly toxic to amphibians. Two more scientific papers shed light on the other questions.

First up is this article on Japanese knotweed by R.H. Shaw and L.A. Seiger (2002):

…To be effective, Japanese knotweed control probably will need to be undertaken on a watershed-wide basis…

Once established, F. japonica is very difficult to eradicate and removal efforts may have further adverse impacts on the soil or other plants…

In areas where F. japonica has been introduced, it is found primarily in moist, unshaded habitats… Fallopia japonica
requires high light environments and competes effectively for light in
such situations. It is found primarily in open sites, and its growth
and abundance are depressed in shady sites (Beerling, 1991b; Seiger,
1993). Consequently, it is unable to invade forest (Beerling, 1991b,
Seiger, 1993)…

…environmental and financial costs associated with ineffective
chemical-based control measures… Japanese knotweed is certainly a
plant for which classical biological control is the only long-term,
sustainable solution.

Even if Kohl tries to eradicate knotweed from its entire
parcel, this is far short of a “watershed-wide basis”. And while Bruce
Young disputes the notion that removal of invasives can disturb the
wetland and its buffer zone, just such a risk is mentioned by Shaw and
Seiger. We also note that Kohl’s proposed condo development calls for
extensive tree-cutting. The resulting increase in light penetration on
the property may well facilitate the spread of Japanese knotweed.

Let’s now turn our attention to multiflora rose, and this paper by J.W. Amrine, Jr. (2002):

Where plants have become well established, a huge seed bank develops
that can continue to produce seedlings for at least twenty years after
removal of mature plants…

Even if not deliberately spread, its [rose seed chalcid’s] range will
increase by birds. Eventually, multiflora rose will be reduced to low
levels, occurrence of RRD will become minimal, as in California,
Wyoming and Utah, where it originated, and problems for farmers and
rosarians alike should be greatly reduced.

…Based on the longevity of the seed bank, eradication might be a 20-year
project or more, far longer than the year or two (or even 10)
contemplated in Kohl’s latest written proposal.
It seems unreasonable and unrealistic to ask a developer and the
Conservation Commission to implement and monitor such a lengthy
program, especially when non-compliance with wetlands agreements has proven to be a widespread problem in Northampton.

contrast to a relatively short program of toxic
herbicides–extraordinary or not–the long-term, low-risk,
environmentally gentle solution to these invasive species appears to be
bringing the ecology back into balance, allowing and in some cases
encouraging the natural predators of knotweed and multiflora rose to
feast on their abundance. Additionally, preserving mature trees will
help control knotweed.

Gazette Reports on January 22 Kohl Condo Hearings; Pictures of the Latest Proposal; Conservation Staff Report; HYLA Critique

[Dr. Bryan Windmiller:] …the applicant proposes to mitigate the impacts of buffer zone
disturbance by disturbing even more area of inner buffer zone and
forested wetland itself.  This mitigation effort will, in fact, only
worsen the impacts to the wetlands bordering Millyard Brook…

The intended use of herbicides by the applicant to control Japanese knotweed (Polygonum cuspidatum)
will moreover result in the pollution of the wetland with herbicides
and their toxic surfactant agents.  The commonly used herbicide
glyphosate (Rodeo and Roundup) has been shown to be highly toxic to
amphibians, for example, in numerous papers by Rick Relyea and
colleagues (see summary at: http://www.medicalnewstoday.com/articles/22159.php).   Japanese knotweed and multiflora rose (Rosa multiflora)
are furthermore difficult to eradicate, even with herbicides.  To do so
will require significant doses of herbicide applied many times.

the end, such schemes are likely only to result in further degradation
of the wetland system.  How long will the applicant continue to remove
exotic species and replace dead shrubs and trees that are planted in
the mitigation areas?  Three years?  Five?  Ten?