A replicated field trial was conducted to determine the effective rates of tebuthiuron for control of individual tall larkspur plants in the mountains near Emery, UT, Cedar City, UT, and Yampa, CO. The size of larkspur plants was measured, and tebuthiuron was applied to the base of each plant at 0.1, 0.21, and 0.5 g product/1,000 cm2of foliar crown cover. Tebuthiuron at 0.21 g/ 1,000 cm2rate controlled 62% of the plants. Tebuthiuron at 0.5 g/1,000 cm controlled 78% of the plants (2.5 g for an average-sized plant of 5,000 cm2). The high clay content of the soils at Emery or high organic matter content of soils at Cedar City or Yampa did not adversely affect efficacy.
Environmental concerns of using pesticides on public lands have greatly reduced the use of herbicides to control tall larkspur (Delphinium barbeyi Huth). An alternative method of control used ammonium sulfate placed at the base of individual plants. The objective of this study was to determine the mechanism by which fertilizers kill tall larkspur. We hypothesize the salt from the fertilizers kill the plant. We applied ammonium sulfate, ammonium nitrate and sodium chloride at equivalent salt concentrations and evaluated their effect on tall larkspur plants. There was no difference among treatments in larkspur mortality (P > 0.10). The high rate of all treatments (ammonium sulfate 400 g plant'1, ammonium nitrate 264 g, and sodium chloride 180 g, at equivalent salt concentrations) killed greater than 70% of larkspur plants. We conclude the salt in fertilizers kills tall larkspur, not the nitrogen. It is necessary to place the fertilizer or salt at the base of the plant to concentrate it in the root zone, rather than broadcast it. At the end of the study, bare areas left around the dead tall larkspur plants were only 13% of the original size of the tall larkspur plants at Yampa Colo. and Cedar Ut., and 46% at Emery Ut., indicating the surrounding vegetation was quickly filling in the vacated space. The relative cost of materials per plant for both ammonium sulfate and nitrate was 12.9¢, and 2.6¢ for salt.
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