Vegetative reproduction and the integrated management of Canada thistle

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Montana State University - Bozeman, College of Agriculture


Canada thistle (Cirsium arvense (L.) Scop.) is an aggressive, introduced creeping perennial weed that flourishes in a wide variety of environments. Its deep, creeping root system and colony forming tendencies make it one of the most difficult weeds to control. A strategy that incorporates a better understanding of the biology of Canada thistle into the development of an integrated management plan that includes herbicides and biological control could help reduce the dispersal and impact of this species. The objectives of this work were to: 1) determine how Canada thistle emergence and growth are affected by changes in root size, biomass, burial depth, and soil moisture and 2) compare single and joint impacts of herbicides and biological control agents on Canada thistle growth. Objective 1 was carried out in a greenhouse and in field conditions. To monitor Canada thistle emergence and growth responses, a completely randomized factorial design was used in the greenhouse, and a randomized complete block design was used in the field. Models were developed to predict emergence and growth patterns based on available water, burial depth, and various root metrics. For the exception of available water, the same predictors were used in field conditions to predict Canada thistle emergence and growth. The variables that were manipulated were able to significantly predict the responses measured, and we concluded that available water, root burial depth, and root weight, length, diameter, and volume are indicators of emergence likelihood, emergence time, shoot and root growth, and shoot number. Objective 2 was carried out in greenhouses and field settings. Three herbicides were evaluated with and without insects to determine reduction in Canada thistle root and shoot biomass in the greenhouse. One herbicide was also used at a low rate and evaluated singly and in combination with a stem-boring weevil (Hadroplontus litura (F.)) and a pathogen (Pseudomonas syringae pv. tagetis) to determine effect on Canada thistle response in the greenhouse and field. We failed to reject our null hypothesis of additivity between control agents, and concluded that integrating individual control methods yields greater Canada thistle control than any singular method.




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