Effects of pre- and post-emergent herbicides on non-target plants native to the longleaf pine ecosystem

Background/Question/Methods

Native grasses and forbs are an important component of the longleaf pine ecosystem, however, little is known about re-establishment of these species in a restoration context. Understanding the effects of herbicides on non-target native plants can advance the use of herbicides as an effective restoration tool, particularly in regard to control of competing vegetation.  The objectives of this study were to determine the effects of several commonly used herbicides on non-target species of grasses, legumes, and composites native to the longleaf pine ecosystem of southwest Georgia.  We assessed the pre- and post-emergent properties of 6 herbicides on 10 species of grasses, legumes, and composites.  For each species, we examined phytotoxic responses to two rates of herbicide: below maximum label rates at three stages of plant growth: 0 days (pre-emergent), 4 weeks and 8 weeks.  Plants were visually rated for leaf damage 4 weeks after herbicide application to assess the phytotoxic effects of the herbicides.  Plants were then harvested, dried, and weighed.

Results/Conclusions

Regardless of herbicide rate or age of plant, legumes were extremely vulnerable to applications of aminopyralid (Milestone) and were also sensitive to hexazinone (Velpar) at the 4 and 8 week stage of growth.  Phytotoxic symptoms ranged from substantial chlorosis and stunting to heavy injury with irreversible damage and death.  Older (8 week) legumes were most tolerant to sulfometuron methyl (Oust) and imazapyr (Arsenal), showing minor, reversible damage.  We observed significant phytotoxic effects on Rudbeckia hirta (composite) when treated with aminopyralid, atrazine and hexazinone, however, this was not the case for another composite, Helianthus angustifolia.  Pre-emergent grasses (Aristida stricta, Sorghastrum secundum, and Saccharum alopecuroides) showed signs of heavy injury with aminopyralid application.  All 8 week old grasses and some (Aristida stricta, Sorghastrum secundum, and Sporobolus junceus) 4 week old grasses were more negatively affected by the highest rate of hexazinone compared to all other herbicides and control plants.  In conclusion, our results indicate that several native species are more sensitive to herbicide application than expected based on the below maximum label rates we used and the specificity implied for the herbicide use.  Hexazonine, used in longleaf pine restoration efforts to control hardwood understory competition, had the greatest negative impact on most legumes, grasses, and composites.  Similarly, grasses in the pre-emergent class were negatively affected by hexazonine, a post-emergent herbicide, and aminopyralid, an herbicide labeled to control broadleaf weeds.