除草劑嘉磷賽影響樹木健化強度

除草劑嘉磷賽影響樹木健化強度

Glyphosate can damage trees and affect hardiness

Production

The economic cost to the U.S. nursery industry of bark cracking is conservatively estimated at $6.6 million annually (or 2.5% of finished inventory). This estimate doesn’t include $14 million in landscape tree failures due to bark cracking. The estimates continue a pattern of increased severity and frequency of bark cracking since 2004. At the same time, consumer preference for faster-working glyphosate products was driving production of various surfactants to break down the cuticle of plants and increase rate and amount of glyphosate uptake.

In 2005, researchers at Ohio State University speculated that bark cracking was not solely related to cold injury, but that the absorption of glyphosate into thin or pigmented bark was also increasing with the use of new surfactants. Glyphosate is the most common herbicide, but little literature exists regarding its safe or correct use around trees. Horticultural Research Institute funded research at Ohio State, which is evaluating glyphosate’s role in crown dormancy disruption and woody tissue hardiness in relation to species, time and frequency of application and glyphosate formulation.

Use with caution

Generally, pre-emergence herbicides should be the backbone of a nursery or landscape weed-control program. Achieving satisfactory weed control generally requires repeated applications of one or more pre-emergence herbicides, but at least two applications (spring and fall) are recommended in nursery fields and landscape settings.

Post-emergence herbicides can be used with extreme caution and forethought. Post-emergence herbicides are the big guns in your weed-control program, so limit their use to rescue treatments. Always avoid contacting green bark of ornamentals with post-emergence sprays. Be especially careful with newer glyphosate formulations that contain surfactants to increase penetration into green tissue. Glyphosate formulations with increased surfactants should be avoided in nursery production due to an increase in incidence of injury problems.

Never use glyphosate for sucker removal. Scythe (pelargonic acid) is the only broad-spectrum post-emergence labeled for sucker removal. At OSU we have identified many risky grower and landscaper practices such as reducing pre-emergence applications and making applications (1 quart per acre) as frequent as eight times a season (every 1 1/2 weeks), removing suckers and adventious shoots with glyphosate, applying glyphosate shortly after mechanical removal of suckers and repeated drift exposure of glyphosate  to trees.

Sucker removal

Herbicides containing glyphosate should be sprayed before sucker removal, not after. It is best to remove the sucker within a few hours of spraying. Using systemic herbicides such as glyphosate after mid-July is not recommended. Fall is when plants accumulate stores in the roots and systemic post-emergence applications are recommended for killing stumps and perennial root systems. After mid-July use only Scythe for sucker removal and any post-emergence treatments.

The best way to remove suckers is with a sprout inhibitor such as Tre-Hold Sprout Inhibitor A-112, Tre-Hold RTU or other formulations containing naphthaleneacetic acid (NAA) to inhibit the development of adventitious shoots. Mechanical removal of suckers, another common practice, has been shown to increase sprouting, as it creates a wound response in the plant and initiates more shoots. Paint or spray the NAA products onto pruning cuts after sprouts are removed.

New products are confusing, not better

Roundup (glyphosate) has been labeled for ornamental use for more than 30 years. It controls weeds by inhibition is of 3-enol-pyruvylshikimate-5-phosphate (EPSP) synthase. It has been used extensively due to its numerous attributes: limited soil activity; non-volatile, broad-spectrum, systemic, low environmental impact; ability to translocate throughout the plant; ease of use; and low mammalian toxicity. Roundup went off of patent in 2000, opening the market to generics. In 2007, there were 45 generic glyphosate products registered for ornamentals. Each uses surfactants with varying loads. Research at Ohio State University to study glyphosate products found a significant reduction in trunk cold hardiness with the use of newer (increased surfactant) formulations.

Nursery growers adopted standard practices of glyphosate applications before 2000 when Roundup Original was used. Trying to find a product like the original Roundup is like entering a maze. Roundup originally was sold with isopropylamine salt. Today, you find glyphosate products labeled with four salts: isopropylamine (ipa), trimethylsulfuronium, diammonium and potassium. There are glyphosate products with three kinds of surfactants and full partial or no load, nonionic, cationic (polyoxyethylene tallow amine) and transorb surfactant system. Concentrations are also different. Products are labeled using acid equivalents or as active ingredient, making it very difficult to compare products for activity. In short, buying glyphosate has become one of those “mystifying experiences” -- where you begin to wonder if Amelia Earhart and other incidences in the Bermuda Triangle are not somehow linked to glyphosate.

Consumers tend to want a faster-working product, so makers of glyphosate products focused on changing surfactants to improve plant cuticle breakdown.

Sub-lethal dosing

Glyphosate is broken down within the soil, but once in the phloem of a plant, it may take years to break down. Exposure to an ornamental through green bark is considered a sub-lethal dose and can injure the plant. Absorption of glyphosate is possible with thin bark or bark with pigment. Injury symptoms from sub-lethal doses of glyphosate may not be present up to two years after absorption of the glyphosate.

Several symptoms occur when a woody plant is exposed to a sub-lethal doses of glyphosate: witches broom, stunting, bark cracking or splitting, loss of apical dominance, individual dead limbs, chlorosis and death. Glyphosate injury is difficult to diagnose for two main reasons: the amount of time the herbicide is contained within the plant and the numerous symptoms that may occur. You can minimize the risk of glyphosate injury by calibrating spray equipment, using correct amounts, limiting use, reducing drift and overspray and using shields when spraying around thin-barked trees.

How glyphosate works

Inhibition EPSP synthase by glyphosate leads to an accumulation of shikimic acid, depletion of aromatic amino acids and change in phenolics in plants. Pools of phenolic compounds derived from aromatic amino acids are reduced. Phenolic compounds are plant-based phytochemicals. Phenolic compounds, which are synthesized primarily from products of the shikimic acid pathway, serve as defenses against herbivores and pathogens. Glyphosate kills plants by blocking a step in the shikimic pathway.

Glyphosate is not metabolized in higher plants. It is rapidly translocated to the roots where it is stored along with sugars flowing to the roots in summer and fall. In spring when the flow of sugars is reversed, glyphosate can be translocated to meristematic regions to cause glyphosate carryover injury.

A direct relationship exists between the accumulation of phenolic compounds and increased cold hardiness in some plant species. Recent studies have shown that low temperatures cause a direct increase in flavonoids, phenololin and tannins in woody plants. If you apply glyphosate for one or two years even in single, low doses up to eight times a season, you’re severely compromising cold hardiness development in roots, crowns and trunks.

In 2005, when Ohio State first proposed bark cracking was not just an environmental issue, few people agreed. Researchers now can prove glyphosate is a major factor in bark cracking. Ohio State is helping Monsanto to explore development of a glyphosate formulation with increased safety around nursery trees.

What to use

When glyphosate is necessary around ornamental plants, use a product that has no adjuvant load. These products indicate you need to add an adjuvant, but don’t. The products that have a full load of adjuvant are the worse around ornamental plants with green bark as these already have a surfactant (you have no idea what) that will increase their uptake into your plants.

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- Hannah Mathers

Hannah Mathers is associate professor and state extension specialist nursery/landscape, Ohio State University; http://hcs.osu.edu/basicgreen.

July 2008