Contribution of Soil Spray Deposit from Postemergence Herbicide Applications to Control of Canada Thistle (Cirsium arvense)

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Abstract
Foliage-only, soil-only, and foliage plus soil applications of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide}, and metsulfuron {2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoic acid} were used to determine the contribution of soil spray deposit to the control of Canada thistle [Cirsium arvense(L.) Scop. ♯ CIRAR] under greenhouse conditions. Herbicide reaching the soil contributed significantly to the suppression of secondary shoot growth, particularly when pots were watered from above. In subirrigated pots, the herbicides showed activity if they were sprayed on moist soil, but not if they were sprayed on dry soil. These results suggest that it is important to shield the soil during postemergence applications of herbicides in greenhouse experiments in order to avoid overestimating foliar activity.