Invasive plants such as spotted knapweed (Centaurea maculosa) have overrun vast areas of the United States to the detriment of native plants and wildlife. Managers increasingly use broadleaf herbicides to suppress plant invaders, assuming that suppression will relieve the impacts of invasion. However, this assumption has seldom been tested. Furthermore, herbicides are known to have negative side effects that may worsen the problem. Researchers at the Rocky Mountain Research Station evaluated the effects of a common herbicide treatment on grassland plants in western Montana to determine if and when suppression of spotted knapweed may relieve impacts of this notorious invader.
They found that an aerial application of the herbicide, picloram, not only suppressed spotted knapweed, but also increased abundance of native grasses, relieving the impacts of knapweed on this functional group. However, in areas where there was little knapweed to start with, the treatment depressed native forb abundance and diversity, mimicking the negative effects of the invader. In addition, areas with high densities of knapweed prior to treatment were invaded by the exotic grass, cheatgrass (Bromus tectorum), following knapweed suppression.
Results show that the effects of spraying depend on initial levels of invasion. Broadcast applications typically cover large areas that contain varying invasion levels. More selective application methods such as spot spraying may work to better focus suppression on the target weed species, maximize release of native species, and localize side effects. Findings also show that spraying intervals of more than 10 years improve restoration success by allowing time for native forbs to recover. Additional steps such as seeding with competitive natives may be needed to promote recovery of forbs and reduce the risk of secondary invasion following broadleaf herbicide applications.