Canada thistle is resistant to 2,4-D. Many farmers using 2,4-D have killed the tops only to see new shoots come up from the roots.
Bakke, A. L.
"Controlling Canada Thistle With 2,4-D,"
Iowa Farm Science: Vol. 4
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol4/iss2/4