Bob Mugaas and Kathy Zuzek, UMN Extension Educators
Reports continue to be received regarding herbicide injury to white spruces, white pines and a variety of other woody and herbaceous landscape plants. The herbicide in question is Dupont's Imprelis, whose active ingredient is aminocyclopyrachlor. It belongs to a new class of broadleaf weed control herbicides which are similar, but not identical, to existing products such as triclopyr or fluroxypyr, both commonly used for control of more difficult lawn weeds such as clover and creeping Charlie.
Aminocyclopyrachlor is classified as a synthetic auxin or growth regulator type of herbicide. In susceptible plants, the herbicide produces characteristic twisting and curling of the foliage ultimately leading to plant death. Most of us have probably observed these effects on dandelions that we have treated with home lawn weed control products containing 2,4-D and/or dicamba, two other growth regulator type of herbicides but with different chemistries than aminocyclopyrachlor.
Dupont introduced Imprelis to the professional turfgrass management industry this year. It has a track record from various research efforts of providing good to excellent control of some of the more difficult to control lawn weeds (e.g., creeping Charlie, wild violets, clover and Canada thistle). However, what is also being observed in many of the northeastern and midwestern states is significant, unanticipated damage to certain spruce species and white pines (Photos 1 and 2) with a scattering of injury reported on other conifers and broadleaf plants.
When initial reports started showing up around the end of May into June from the northeastern states, the two conifer species most commonly showing injury were Norway spruce (Picea abies) and white pine (Pinus strobus). When injury reports began coming into our own Department of Agriculture and Extension a short time later, most of the injury was on white spruce (Picea glauca), (Photo 3), including its geographical variety Black Hills spruce and white pine, again with a scattering of injury to other conifer and broadleaf plants. Colorado blue spruce (Picea pungens) has also shown some injury but generally much less dramatic than seen on white spruce or white pine.
As one might imagine, this kind of injury has created enormous concern among homeowners and professional lawn care applicators alike. The two biggest questions on everyone's mind is "Will the trees survive?" and if so, "Will their aesthetic landscape qualities be completely ruined and hence still need to be replaced?" Unfortunately, in all but the most severely damaged trees, it's a bit of a wait-and-see situation. In this area, the white spruce and white pine trees observed so far show damage only on the new or current year's growth, but, that is where new buds are formed for next year's growth (Photos 4 and 5). If this year's growth is lost, no new buds will have been formed and next year's growth may be sporadic around the tree and at worst the tree may still end up dying.
However, in some cases, the new growth appears to be setting new buds somewhat normally even though this year's growth itself is often twisted and distorted. For comparison, see Picture 6 for what normal bud set and shoot growth should look like in spruce. If affected shoots remain alive and mature normally the rest of this season, next year's growth may very well be O.K. even though this year's twisted and distorted shoots will still be evident. In more severe instances where the new shoots along with the needles have already begun to turn brown and there appears to be little or no bud development occurring, (Photo 7) shoots are unlikely to survive. Where these symptoms are widespread on the tree, if it does survive, any remaining landscape value would appear to be unlikely. (Photo 8).
A third question on many people's minds is "Is there anything I can do to help save the tree or reduce the damage?" At present there are varying opinions as to what the best cultural practices are to minimize or reduce the impact of current injury as well as reduce the probability of further injury. For now, it seems that preventing any additional stress to the trees would be beneficial. For example, watering to avoid additional drought stress on the tree, while at the same time being careful not to overwater the trees, would be prudent.
Because much is still unknown about this unfolding and expanding situation, the Minnesota Department of Agriculture is currently collecting information from University of Minnesota Extension, DuPont, EPA, commercial lawn care applicators, and other state departments of agriculture to assess the scope of this issue. You can visit the Imprelis page on the MDA's website. See their official statement on the situation at this time and how to contact them with damage reports. Also, there is currently a good summary article by Dr. Peter Landschoot from Penn State University regarding the characteristics of this herbicide and the impacts they're seeing on affected trees. You can access it via Penn State University Extension.
As symptoms continue to develop and further assessment of survival is documented there will likely be other articles on this situation, I'm sure. Stay tuned!