Chemically resistant Russian Thistle continues to crop up in Union County
By Garrett Christensen on Friday, May 3rd, 2024 in More Top Stories Northeastern Oregon News
UNION COUNTY – Russian Thistle, also known as Salsola Kali, or even commonly as Tumbleweed in its final stage of life, is a hardy, and physically stubborn, weed found across the Western United States. While not the most noxious or dangerous weed present in Oregon, it’s natural resistance to both herbicide treatment and burning in its later growth stages still make it a persistent pest for farmers, landowners, and county officials. Even more unfortunate for those working to abate it, some variations of Russian Thistle are growing more resistant to Glyphosate herbicides, such as Roundup.
According to Union County Weed Department Supervisor Brian Clapp, reports of Glyphosate resistant Russian Thistle in Union County began around 2019 and 2020, with the county beginning official treatment of the variety in 2022. How the Glyphosate resistant variation reached Union County isn’t confirmed, though there is some speculation that seeds gradually blew in from areas further West. Generally, Russian Thistle is spread throughout Union County, though is more common on the Valley floor.
On its own, Russian Thistle is already somewhat resistant to common herbicides, especially in its later growth stages. As explained by Clapp:
“Russian thistle, when it’s less than three inches tall, and preferably even shorter than that, is not that hard to kill. You can kill it with a wide variety of things. Once it gets taller than that, once it starts hardening up and it’s more than six inches, there’s really very little that kills it.”
The plant’s growth cycle also presents another issue for treatment. Typically, Russian Thistle tends to grow rapidly during periods of spring rains, generally in-between the standard spraying cycles for the county and farmers. When beyond the six-inch mark, common herbicides such as Roundup and Weed Master lose effectiveness. Patches of the weed potentially being more resistant to Glyphosate by default make abatement even more difficult.
The need to potentially use more potent herbicides also presents another complication when it comes to what landowners can legally and safely use to treat it. While Patches of Russian Thistle in right of ways and along railroad tracks can generally be treated with a degree of impunity, those within fields effectively have no viable treatment beyond a certain point of growth due to necessary restrictions on herbicides when spraying crops. Paraquat can more effectively neutralize Russian Thistle in its later stages, but the chemical itself is significantly more dangerous and, as such, often not viable.
Mowing can also be used to eliminate mature weeds but, again, this is not always viable and generally doesn’t prevent the spread of seeds. Overall, the most effective means of eliminating Russian Thistle, especially as it continues to grow resistant to Glyphosate, is simply to abate it as early as possible when common herbicides still have an effect and before treatment becomes impractical due to spring rains and agricultural work. As stated by Clapp:
“Any place where you know from last year that you had Russian Thistle, go out there and when you see little plants start popping up, especially when they’re less than three inches, make sure and hit them.”
Within Union County, the weed is often treated seasonally, with the Weed Department assisting landowners when possible. However, Russian Thistle’s status as an Agricultural Class B weed limits resources at times. As the weed is not considered state priority, grants are not always available to provide treatment funds.
