Canada Thistle Control by Atrazine

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Abstract
In greenhouse studies, atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine killed Canada thistle [Cirsium arvense(L.) Scop.] when applied as preemergence or postemergence treatments. Postemergence atrazine applications caused more rapid kill when allowed to cover the ground as well as the foliage compared to only foliar atrazine applications. In three field experiments conducted in 1967, 1968, and 1969, Canada thistle was controlled with atrazine and amitrole (3-amino-s-triazole) + NH4SCN. Control resulting from atrazine was as good or superior to amitrole + NH4SCN when both were applied to foliage prior to plowing. Control from 2.2 kg/ha of atrazine applied prior to plowing was 78% for the 3-year average. Atrazine applied preemergence at 2.2 kg/ha resulted in inferior control, whereas postemergence atrazine applications resulted in thistle control similar to treatments applied prior to plowing.