Field trials conducted in the three major peppermint-producing regions of Oregon showed that repeated applications of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at 2.2 kg/ha controlled Canada thistle [Cirsium arvense (L.) Scop.] in peppermint (Mentha piperita L.) selectively. Repeat applications of 2.2 kg/ha controlled Canada thistle better than single applications of 3.4, 4.5, or 6.7 kg/ha. Late May or June applications were more effective than early May applications. The number of Canada thistle shoots was reduced 82% 10 months after repeat applications but had almost returned to the original number 22 months after application. Basipetal translocation of bentazon was demonstrated by observing bentazon injury symptoms on untreated Canada thistle shoots which were attached by a root to bentazon-treated shoots.
