Scotch Thistle Along the Grande Ronde River

Erin L. Hestir

We observed Scotch Thistle, an invasive weed, along the river as we descended into the more arid regions of the Grande Ronde. Both in Oregon and Washington. What made the appearance of this weed most remarkable is the apparent defoliation occurring - indicative of a likely herbicidal treatment in several areas along the river. The herbicide applied was causing the plant to defoliate in a way that made it appear to shrink or shrivel as it decreased in height. Based on the remoteness of the locations of the weed, and the state of defoliation, we speculated that the herbicide was most likely applied aerially, and recently.

Scotch thistle (Onopordum acanthium) has been listed as a class B designate weed by the Sate of Washington Noxious Weed Control Board, and a class B weed by the state of Oregon. Scotch Thistle is native to Eurasia, and is characterized by sharp spiny leaves that grow up to 2 feet long, and flowers of pink to purple in color. It is found in most counties in Washington east of the cascade mountain ranges, and generally infests wet meadows and pastures, as well as streams, rivers, irrigation canals, and other waterways. Piper (1984) found that the plant is successful in well-drained sandy and stony soils, such as those characteristic of the arid reaches of the Grande Ronde. It is considered a problem in rangelands because it grows quickly in such dense stands that it prevents cattle access to water and grazing. It has been documented to invade areas of disturbance, and there was a definite qualitative correlation between observed grazing increases along the river and the occurrence of the weed.

The herbicides that are effective against Scotch thistle include picloram, dicamba, 2,4-D, dicamba, + 2,4-D, and metsulfuron. I was unable to find any documentation about which herbicide may have been applied to the Scotch Thistle observed along the Grande Ronde.



REFERENCES; accessed on June 28, 2007.