Noxious & Invasive Species in Grasslands

Program Contact: Mark Heitstuman, Director
(509) 243-2009 • mark.heitstuman@wsu.edu

Noxious weeds are biological invasions. They invade ecosystems where they have never been before, and cause dramatic changes in these ecosystems. They contribute to the loss of habitats and biodiversity by affecting the processes of succession.

Grasslands face the greatest threat from invasive weeds. Land dominatedStarthistleStart by invasive weeds has accelerated soil erosion and surface runoff. Silt accumulation in the Snake River tributaries is a major negative factor in native salmon recovery programs. Soil disturbance by grazing, rainfall, fire, flood and erosion make the SE corner of Washington susceptible to weedy invaders. The presence of invasive weeds can change an ecosystem dramatically. Successful invasions of weeds form monocultures that become climax communities. Predation by alien species has been ranked as the second largest threat to biodiversity by the Nature Conservancy.

YellowStarThistlePlantEarly detection of new invasive species and a subsequent aggressive control/eradication program may help contain or eliminate some weedy invaders before they can cause harm to our aagricultural and natural areas. Monitoring of these sites is essential to long term control. A prime example of new weeds in Asotin County are the scattered infestations of rush skeletonweed, whitetop, leafy spurge, Japanese knotweed, hawkweeds spp, spotted knapweed, musk thistle hybrid, common bugloss, Mediterranean sage, and Dalmatian toadflax. These weeds should be aggressively treated where ever they are found. Weeds that are “landscape” problems need a different outlook. Yellow starthistle dominates tens of thousands of acres NE of a line drawn from the NW corner to the SE corner of the County. Sulfur cinquefoil has a strong grip on the SW corner of Asotin County. A line needs to be drawn between areas that are infested and areas that are not. Yellow starthistle and sulfur cinquefoil should be chemically treated in “new” areas or in areas that can be carefully monitored (someone’s horse pasture, for example). Other areas with a “landscape” problem need biological controls (yellow starthistle) or a program that includes wide spread use of herbicides (sulfur cinquefoil). These areas need to be carefully monitored.

When considering herbicide control it is important to take into consideration the time of year, the maturity of the plant, and the rainfall/moisture conditions. Herbicides should be applied when they are the most effective. For effective translocation of herbicides in perennials, the plant needs to be transporting sugars to its root system. This occurs just before first bloom and on those weeds with fall rosettes, just before freeze-up. On annualStarthistleFieldWeb weeds, herbicides should be applied on actively growing plants. Drought conditions or long dry spells may decrease the effectiveness of herbicides. Plants adapt to drought conditions by forming thicker cuticles with greater wax contents, by slowing their biological processes, by developing a smaller leaf/root ratio, and by adopting a vertical leaf orientation so that herbicides may roll off. Adding surfactants to the herbicide spray will help correct these problems. Spraying after a rain, early morning, or late evening will allow the plants’ stomates to open thus assisting in the uptake of herbicides. A surfactant should be added to the spray mixture before treating any hairy or woolly weeds. Surrounding vegetation should be assessed prior to and after herbicide application.