Hmm. It seems to be considered relatively benign (C-value:1) in western Montana. Thesium - Montana Field Guide Fish Creek Provincial Park is just south of Calgary, Alberta. I like the bit about how they were just looking for something for the dogs to do when they're not busy sniffing out zebra mussels on boats.
The specific environs probably influence just how successful these invasive parasitic plants are. The farther north, the colder and the more limited the species. Although Montana and Calgary AB are in close to the same Lat/ Long. Alberta may have declared these Thesium to be flora non grata due to particularly susceptible biota in the distribution area. The inclination to disregard the parasitic pest may be due to ranching and agribusiness in Montana not being adversely impacted, or perhaps a greater biodiversity in those areas keep it from becoming a problem.
