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Examining the unpredictable nature of yellow toadflax




Krick, Nicholas Jon, author
Beck, Kenneth George, advisor
Nissen, Scott Jay, committee member
Ward, Sarah M., committee member
Meiman, Paul J., committee member

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Yellow toadflax (Linaria vulgaris Mill.) is an aggressive creeping perennial forb that was introduced to North America in the 1600's. It is now naturalized throughout the United States and Canada and is a serious weed in the Intermountain West. This plant threatens cropping systems, rangelands, and natural areas. Colorado State University Weed Science has conducted research on yellow toadflax control for more than 20 years yielding variable and inconsistent results. Short term success has been achieved; however, recommendations for acceptable long term control are unavailable. Dramatic site-to-site variation has been observed, but the source of that variation has not been determined. Yellow toadflax is an obligate outcrossing species that exhibits much genetic and phenotypic variation and may inhabit a wide range of ecosystems. The success of managing yellow toadflax might hinge on application timing; therefore, an observational study of root bud phenology was conducted for 2 years at two sites in Colorado. This study showed that yellow toadflax exhibited a pattern of root bud development and that pattern had an important relationship to flowering. Root buds were present throughout the growing season, but their numbers fluctuated. Following bud emergence in the spring, bud numbers declined until they reached their lowest counts around the full bloom/seed set growth stage. Following this growth stage, bud numbers increased suggesting the root system was a demanding carbohydrate sink. This observational study supported results from herbicide field trials. Identical herbicide efficacy trials were conducted at five separate locations in Colorado where four rates of chlorsulfuron and imazapyr were each applied in September 2008. Plants were harvested from these same sites and were subjected to a common garden experiment and an ALS enzyme bioassay. Analysis of field experiments 1 year after treatment (1 YAT) showed site variation using low herbicide rates (40 g ae ha-1 chlorsulfuron and 127 g ae ha-1 imazapyr), but most variation was overcome by increasing herbicide rates. Chlorsulfuron applied at 94 g ae ha-1 controlled more than 76% of yellow toadflax at all sites; whereas, 380 g ae ha-1 of imazapyr was necessary to overcome site to site variation, but control was ≥73% at four of five sites. Evaluations 2 YAT showed that yellow toadflax recovered at two sites. The common garden study and ALS enzyme bioassay revealed that these populations were susceptible to herbicides on a whole plant level and on a mechanistic level; confirming that herbicide resistance is not responsible for spatial variation. It appears that yellow toadflax recovery was largely driven by length of growing season and the growth stage of a population at which applications occurred. Lower elevation sites had a higher percentage of shoots flowering at the time of application and were more difficult to control. Comparing results from the field experiments to the pattern of root bud development, applications at the lower elevation sites occurred when bud numbers were decreasing or at their lowest counts. Higher elevation sites were in a more advanced growth stage, which resulted in better control likely based on the phenology of root bud development. It is likely that more buds present or developing on a plant resulted in better control because bud are strong sinks and herbicides were likely translocated to those tissues and had a greater negative effect on their root systems. Managers can utilize what has been learned though these experiments to improve their programs and management success. Management can be improved by increasing herbicide rate, but more importantly by targeting populations at their most susceptible growth stage. Although a rate increase of herbicide is necessary to overcome site variation, this should result in less overall herbicide use (decrease due to fewer tank mixes, fewer application events, and elimination of high rates of ineffective compounds), reduced costs for managers, and will decrease environmental exposure to herbicides.


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ALS inhibiting herbicide
invasive plants
linaria vulgaris
yellow toadflax


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