Production and Wind Dispersal of Canada Thistle (Cirsium arvense L.) Achenes

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. Wind blown 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. Concomitantly, seed production in Canada thistle was monitored for 8 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 produce about half the number of seedheads compared to female shoots. 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. This report represents the results of research conducted by the authors and does not necessarily represent the views or policies of the Minnesota Department of Transportation and/or the Center for Transportation Studies. This report does not contain a standard or specified technique. The authors and the Minnesota Department of Transportation and Center for Transportation Studies do not endorse products or manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to this report. ACKNOWLEDGEMENTS We would like to thank Minnesota Department of Transportation (Mn/DOT) and The Center for Transportation Studies for partial funding of this work. We also thank Drs. and the late Robert Jacobson, Mn/DOT, for their helpful guidance and input as members of a technical advisory panel, and especially to Robert Jacobson for his advocacy supporting this work. We would also like to thank Dean Petersen, USDA ARS North Central Soil Conservation Research Lab, Morris MN, for his assistance in managing the Morris trap site; Lizabeth Stahl and Ryan Miller, University of Minnesota Extension; for their assistance in managing trap sites in Welcome and Elysian, respectively; and Drs. Jeanie Katovich and Laura van Riper and students Gina Kawsowski and Andrea Hannan for their assistance in trap maintenance and data collection. We thank Britta Boyum, Matt Carlson, and Andre′ …