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An Imprelis Update

Kathy Zuzek, UMN Extension Educator, Horticulture

The History of Imprelis
In October of 2010, a new broadleaf weed killer by the name of Imprelis became available to turf professionals. Key to DuPont's release of this herbicide was its effectiveness at very low concentrations, its low toxicity to humans and other mammals, and its effectiveness on difficult-to-control turf weeds such as creeping Charlie, wild violets, clover and Canada thistle.

During the spring of 2011, damage to ornamental plants in landscapes where Imprelis had been applied began to appear in the eastern half of the United States including Minnesota. Damage to new growth of plants became visible within a matter of weeks after an Imprelis application and included twisting and/or browning of shoot tips, leaves, and needles (Photo 1).

K. Zuzek

Photo 1: Brown, twisted, & drooping shoot tips from Imprelis applications

As the summer progressed, impacted shoots and their associated leaves, needles, and buds often died on the most susceptible species. In some cases, entire trees or shrubs died. Broadleaf and conifer species of ornamental plants were impacted but conifer species were impacted much more severely. The most seriously impacted species from states east of Minnesota were Norway spruce (Picea abies) and eastern white pine (Pinus strobus) trees. The most seriously impacted species in Minnesota were white spruce (Picea glauca) and eastern white pine, but noticeable damage to Colorado blue spruce (Picea pungens), Norway spruce, Austrian pine (Pinus nigra), red pine (Pinus resinosa), and arborvitae (Thuja occidentalis) were also common. A wide range of damage was seen among all of these species. Some plants showed little injury throughout the 2011 growing season, others died quickly, and the symptoms on others became more severe as the growing season progressed. The number of plants damaged, the level of damage, and plant mortality was highest among white spruce and eastern white pine in Minnesota.

In August of 2011, sales of Imprelis were stopped but damage to trees and shrubs in landscapes continues to be an issue. DuPont initiated a claims resolution process to compensate customers with damaged or killed plants. The deadline for claims submission was February 1, 2012 and DuPont is now processing claims submissions. Lawsuits have also been filed against DuPont.

How Imprelis Works
Aminocyclopyrachlor is the active ingredient in Imprelis and is classified as a synthetic auxin or growth regulator type of herbicide. Imprelis and other growth regulator herbicides are absorbed by roots and leaves and then move via the xylem & phloem to two of the meristematic regions responsible for new plant growth: shoot tips and root tips. In susceptible plants, growth regulator herbicides produce characteristic twisting and curling of the shoot tips and their foliage and plant mortality often follows. Herbicides are meant to kill and eliminate targeted plants such as weeds. When they impact non-targeted plants such as conifers in the case of Imprelis, problems arise.

Imprelis does not bind well to soil particles and is a very water-soluble compound. It is also a very stable compound in soil. Because it is active over a long period of time in soils and because its low adsorption and high solubility allow it to move downward in soil once applied to turf, it appears that Imprelis was able to reach the root systems of ornamental plants. Absorption of the herbicide occurred through the roots of these plants, followed by movement through the plant to new growth areas, and injury occurred. It is still not known why particular spruce and pine species are so susceptible to Imprelis.

K. Zuzek

Photo 2: Branch with active bud-break

K. Zuzek

Photo 5: Branch with no bud-break

Will Trees Damaged by Imprelis Recover?
The answer to this question depends on the severity of damage. Trees that showed minimal damage (such as a small amount of injury at shoot tips in the upper branches of a tree) during the spring of 2011 and had little additional decline during the growing season will probably survive. These trees have active bud-break and are putting on new growth throughout most of their crowns this spring (Photo 2). Most broadleaf trees and shrubs and some of the conifer species that were impacted by Imprelis are in this group. Retaining these trees and shrubs in the landscape will depend on how badly damage impacts their appearance. Care of these plants during the recovery period should include irrigating the plants during dry periods to minimize moisture stress, avoiding over-watering that can cause additional stress, and avoiding fertilization for a minimum of one growing season to avoid excess growth that can compound herbicide injury.

Trees that showed damage last spring (Photo 3), further decline (dieback of shoots, dead needles and buds, yellowing of the tree canopy) during the 2011 growing season and/or the winter of 2011-2012 (Photo 4), and limited or no bud-break throughout the tree crown this spring (Photo 5) will probably continue to decline and then die. Even if these trees do not die, they will be of little or no aesthetic value in the landscape. In Minnesota, the majority of trees in this group are white spruce and eastern white pine.

K. Zuzek

Photo 3: Initial symptoms on white spruce in July, 2011

K. Zuzek

Photo 4: Symptoms on the same white spruce in April, 2012

DuPont's Compensation Plan
If you are a property owner with Imprelis-damaged plant materials and you and your lawn care professional are part of the claims resolution process that DuPont enacted, you can access information on the claims process here. Compensation will cover removal and disposal of impacted trees, replacement costs or direct payment for removed trees, care of replacement trees , two-year warranties for all replacement trees and for any other trees impacted by Imprelis over the next two years, maintenance of impacted trees as they recover, and additional compensation for inconveniences associated with Imprelis.

Removal and Disposal of Plant Materials and Soil containing Imprelis
Listed below are some of Dupont's recommendations for the disposal of plant materials and soils containing Imprelis. These recommendations are for individuals who had Imprelis damage to plant materials but are not part of the claims process against DuPont.

    Disposal Recommendations
  • No tree debris should be left on site.

  • DuPont recommends disposal of tree and excavated soil materials in solid waste landfills that will accept such waste.

  • The Imprelis label prohibits the use of grass clippings for mulch or compost.

  • Under no circumstances should tree material be used for mulch or compost or disposed of in facilities that would turn it into compost or mulch (e.g., recycling).

  • Trees that are cut down may be used for lumber, firewood, or to fuel various wood burning processes if such usage is otherwise consistent with state, regional and local regulations. Open burning is also an alternative if approved under local regulations.

  • If not landfilled, excavated soil should be disposed of in locations where it will not impact any other plantings through direct application or runoff. With the property owner's consent, excavated soil may be used on site as long as it is applied well away from desirable plant root.
Recommendation for planting replacement plants:

  • To address potential effects of Imprelis remaining in the soil, activated charcoal (that serves to deactivate any residual herbicide) should be applied to the backfill soil in accordance with manufacturer's instructions:

  • For liquid applications, apply at a rate of 1 pound of activated charcoal in each gallon of water uniformly to sides and bottom of hole dug for tree planting as well as to the complete root ball of the tree to be planted. Also, spray the burlap covered root ball if burlap is left in place. Thoroughly coat to the point of run-off the surfaces of the hole and the root ball.

    For dry applications, during tree planting, apply activated charcoal at a rate of 7 to 14 lbs/1000 cubic feet of soil and thoroughly mix with the clean soil. Use this soil as the new backfill soil when planting the tree.
  • Excavated soil should not be used as backfill. New soil, of a similar nature as the existing soil, should be used to backfill around the root ball. No fertilizer should be added to the backfilling mixture.

  • The replacement plant should be watered according to nursery recommendations.

  • The property owner should ensure adherence to best management practices consistent with the geographic area in which it is performing this work, taking into account any unique environmental and climate conditions, and any state, regional or local ordinances. Information on selecting, planting, and care of trees in Minnesota can be found here.
Based on the degradation rate of Imprelis in soils and on recommended planting times for Minnesota, a good additional recommendation would be to not plant susceptible species into Imprelis-impacted landscapes until September of 2012.
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