Canada Thistle Seed Movement

Project summary:

Canada thistle is designated by the State of Minnesota as a Noxious Weed, and is present in all parts of the state with the exception of heavily forested areas. Control is mandated for the Minnesota Department of Transportation (MnDOT), other state agencies, and private landowners. Complaints regarding the movement of Canada thistle seeds onto new lands are an important enforcement issue; Mn/DOT is particularly concerned with the potential for Canada thistle seeds to move from state and federal lands onto private lands. Weed laws and enforcement are based upon the premise that Canada thistle spreads by wind-distributed seed, although it is not clear to what extent this premise is valid. The goal of this research project was to characterize the potential for Canada thistle to spread in Minnesota by studying the movement and viability of seed. Based on previous research, the researchers hypothesized that the degree of seed movement is likely far less than current perceptions. This information was expected to provide the basis for a re-evaluation of enforcement guidelines and help improve long-term management recommendations. Information on seed dispersal is also critical to inform the public on the actual degree of seed movement, and a better understanding of seed movement of Canada thistle would improve management strategies, enabling state agencies and private landowners to optimize thistle control while minimizing harm to native non-target plants and animals. Quantity, quality and dispersal distance of wind-blown achenes and pappi were determined during the peak time of dispersal over twelve site-years in Minnesota during 2006 and 2007. Approximately twice as many pappi than achenes were trapped, and windblown pappi tend to travel near the ground. Most achenes fell near the parent plants and their population density declined exponentially with increasing distance. Over 80% of pappi collected did not have an achene attached. Concurrently, seed production in Canada thistle was monitored for eight site-years in Minnesota. Averaged across years and locations, female Canada thistle shoots that flowered produced an average of 38% empty, 17% shrunken, and 44% normal achenes. Male shoots produced about half the number of seedheads compared to female shoots, and a low level of hermaphroditic expression was observed in male shoots. Though seed production by female Canada thistle is extremely variable, when sufficient pollen is available, Canada thistle clearly has the potential to generate significant contributions to seedbanks to maintain invasive stands. However, dispersal by wind is for the most part, local. Long distance dispersal of significant numbers of healthy achenes would be a rare event.