By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties
A local landowner contacted me this winter, concerned about his ten acres of young trees and whether they had been damaged by herbicides. In May 2014, a year after the trees were planted, grass was coming in thick, so he hired a contractor to do a release spray. We don’t know what the exact spray mixture was, but the landowner thought it may have been a formulation of glyphosate such as Accord. Glyphosate is known to damage Douglas-fir seedlings (and many other conifers) during the active growing season – the time from when buds begin to swell in spring until resting buds are formed and hardened off in fall.
Affected trees showed several symptoms characteristic of herbicide damage. The least affected trees simply had stunted growth. On many others, the leader and branch tips were droopy and dead. Some seedlings were completely dead.
But the contractor knew the risks, and had attempted to protect the trees. According to the landowner, he had covered each seedling with a section of PVC pipe before spraying around it. This is a common practice which many people assume is sufficient to protect desired plants from spray. Unfortunately, it doesn’t always work. The outside of the pipe (or bucket, or whatever barrier is used) ends up getting drenched in herbicide as the applicator sprays around it. Then, when the pipe is lifted off, the solution drips off all over the plant it was supposed to protect. There seemed to be a pattern where the lower branch tips on many seedlings were the most affected, so it looks like this could be what happened.
So the symptoms are consistent with damage from herbicides, but can we pinpoint them as the culprit? Many months had passed since the spray occurred, making it impossible to detect any residual chemical. We’d have to rule out other possibilities.
Could it be freeze damage? Tender new growth in spring is sensitive to freezing temperatures, and the symptoms from frost damage and herbicide exposure look very similar. But the planting site was not one that would be typically considered frost prone, such as a low-lying area. Instead, the site was mid-slope. Did we have any unusually cold weather last spring? We didn’t remember any, but just to be certain we looked back at temperature records from the closest weather stations. There were no records anywhere close to freezing in May 2014. So, we ruled out frost damage and concluded that unintended herbicide contact was the most likely cause.
This unfortunate situation leaves the landowner with several challenges. First, it’s not clear whether the residual stocking is adequate to meet the landowner’s goals, or to meet Forest Practice standards for that matter. The trees that were not killed should recover, but they lost a crucial year of growth in their race against competing vegetation. And, as the photo shows, the grass is growing back around the seedlings. The landowner may need to consider a repeat herbicide application, this time selecting a chemical that is safe around growing conifers, or that effectively controls weeds in early spring (before bud break).
So what are the lessons learned here?
- Keep records of all herbicide applications – the chemical(s) and surfactants used, spray rate, and date. Ask contractors to provide that information to you.
- Read the herbicide label. Pay close attention to what it says about safe application timing.
- Be cognizant of the phenology (seasonal growth stage) of both desired plants and target plants. This year, many broadleaf species – including common weeds – are leafing out early due to unseasonably warm weather. Douglas-fir budburst responds to various climate cues, and our early spring might not affect its timing in a similar way. To be safe, check your trees, not the calendar, for spring flush.
- Don’t assume that a physical barrier protects seedlings from herbicide exposure.