Herbicide testing in nursery environment

Over the course of several years of monitoring new plantings on landowner properties throughout the state, reforestation staff encountered a wide range of herbicide prescriptions, with varying results. Often, staff encountered trees showing a high degree of stress, or even mortality, which appeared to be linked to the herbicide application.  The decision was made to test some herbicide treatments in our nursery to see how stock reacted to the chemical at various rates and application timings, under more controlled conditions than what is typically found on outplanted stock.  We selected Dupont Oust XP (Sulfometuron Methyl) as our test chemical, since it is the most widely used pre-emergent herbicide currently in use in Wisconsin.

Prescriptions:  Thirty two nursery bed rows at our Hayward site containing a winter wheat cover crop were planted with 15 plants each of 25 various conifer, hardwood and wildlife shrub species. Oust XP was applied either 20 days prior to planting, 3 days prior to planting, or within 3 days after planting, in an attempt to duplicate common practices we find in our reforestation monitoring. Several rows were also left untreated as controls.  Five separate rates were tested; .5, 1, 2, 3, and 5 (oz Oust XP/ac).  Seedlings were assessed in late July for signs of damage, and graded on a 4 point scale, (0=dead, 1=healthy, 2=damaged (seedling impacted but will recover in a reasonable period of time), and 3=fading (seedling damaged to appoint that it will not recover). In June of 2014 the experiment was repeated at the Wilson Nursery on a smaller scale, using fewer tree species, and utilizing 8 bed rows instead of 32.  Rates applied were 1, 2, 3 5, and 9 (oz. Oust XP/ac).  There were two trials planted, one sprayed prior to planting, and one sprayed immediately after planting.  Seedlings were assessed on the same scale as the Hayward planting in late July 2014.

The 4-point grading scale was found to be a poor tool for assessing the variation between the plots. Amongst the ‘healthy’ trees, there appeared to be quite a bit of variability from one treatment to the next.  Because of this, the trees at both sites were assessed again at the end of the growing season, this time the height being measured to the nearest inch.  It was hoped that this data would reflect the degree to which species were possibly being suppressed by the herbicide, even if no outright damage was evident.

Results:  As expected, the highest rates of herbicide showed the most damage in the initial assessments, especially among the thin-barked shrub species.  This is what is often observed anecdotally in field visits as well.  Surprisingly, some of the species that frequently exhibit high mortality in the field, such as the pines, proved to be almost impervious to the chemical under controlled conditions.   Red and jack pine in particular seemed to do quite well, even at more than double the recommended rate, and in total disregard to the label warning against planting red pine within 3 months of site treatment.  Even at the higher 9 oz./ac rates in the Wilson trials, both red and jack survived at much higher rates than expected, despite being treated at more than 4 times the label recommendation, with little or no waiting period.  They were notably smaller than the controls and the lower dose pines after two growing seasons, but survival was still better than expected.

Conclusions:  The study results confirm that herbicide damage is likely a cause of poor survival of some species in mixed species plantings, especially in regards to thin-barked shrub and hardwood species.  The data also seems to indicate that herbicide is likely not the main cause of poor survival on conifer plantings.  This would suggest that other causes are the issue, and reinforces the need for careful stock handling during the transportation and planting process.


By Roger Bohringer, roger.bohringer@wisconsin.gov, (608) 485-1425