Kathy Zuzek, UMN Extension Educator, Horticulture
In June, the Office of Indiana State Chemist published the 2012 Imprelis Soil and Vegetation Sampling and Analysis Follow Up Study showing 2011 and 2012 levels of aminocyclopyrachlor, the active ingredient of Imprelis, from 11 sites where Imprelis was applied. The level of aminocyclopyrachlor (ACP) was measured in both soil and vegetation. Soil samples were taken to a depth of 4 inches and were then divided into the top 2" and the bottom 2 inches. Vegetation sampled included dead spruce twigs, willow twigs, and honeylocust twigs and leaves on Imprelis-damaged trees. Levels were widely variable in both 2011 and 2012 across the 11 sites. But on average, ACP levels in the 2012 samples of were 10%, 6%, and 23% of the levels measure in 2011 for the top 2" of soil, the bottom 2" of soil, and the vegetation samples respectively.
What does this information tell us? 1) ACP levels still present in plant tissue are higher than levels in soil. 2) ACP levels are dissipating as expected according to the half-life (the time it takes for the chemical concentration to reduce by one-half) of Imprelis in turf environments. Unfortunately, there is no information available as to what the remaining ACP levels in soil and vegetation mean in terms of continuing damage or a timeframe for safe replanting in Imprelis-applied soils.
Here is what is happening to tree and shrub species on Imprelis-applied sites in MN since the last update :
Remember that the effects of Imprelis on non-target species (the injured pines, spruce, and other species growing near or in turf where Imprelis was applied as a weed killer) are the same as the effects seen on targeted weed species. ACP is a synthetic auxin or plant hormone. When applied to plants, ACP causes undifferentiated cell division and elongation in areas of new plant growth, primarily branch tips and root tips. The undifferentiated cell division and elongation results in downward bending of leaves or needles, stem thickening, severe necrosis (tissue death), stunted new growth, calloused stems and leaf veins, and crinkling, cupping and twisting of leaves, needles and stems.
Some species like Colorado blue spruce and cottonwood had a delayed bud break and leaf out this year. Bud break on Colorado blue spruce on an Imprelis-applied site I have been observing occurred in July rather than April (Photo 1).
Among highly susceptible species like eastern white pine, Norway spruce, and white spruce that showed bud and shoot tip injury or mortality shortly after Imprelis applications in 2011, behavior is variable. Some trees continued to decline and died. Others broke bud but the new shoot tips are thickened and often have twisted needles or are completely lacking needles (Photo 2). Others are producing adventitious buds and shoots below dead stems in an effort to put on new growth. It will take time to see if these plants can be managed and restored to worthwhile landscape plants.
The most recent development on Imprelis-applied sites in Minnesota is on honeylocust. As has been seen in states east of Minnesota, honeylocust seems to be a particularly vulnerable deciduous tree species. Many honeylocust have died. Among those that survived, many have dieback in their crowns this year (Photo 4), a more open canopy due to reduced leaf formation, twisted and distorted foliage, and galls or tumors on the trunk (Photo 5) and throughout the tree canopy (Photo 6).