![]() ![]() Regardless of soil moisture, the wind velocity necessary to break Russian thistle stems was 27 m/s (61 mph) on average. We used wind tunnel measurements to convert force required to break plant stems (stem strength) to wind velocities. In contrast, kochia was much less likely to tumble under moderate (average) fall precipitation and Russian thistle was unaffected by soil moisture. Diffuse knapweed was much more likely to tumble when soil is moist. This work has shown that in Colorado, soil moisture after senescence can have a strong effect on the likelihood of tumbling dispersal, although the effect is not consistent among species. Though common, this mechanism of dispersal has received little attention from researchers. Tumbleweeds are a diverse group of often economically important weeds that share a dispersal mechanism. SASKR, Salsola iberica (Sennen & Pau) Botch. CENDI kochia, Kochia scoparia (L.) Schrad. Nomenclature: Diffuse knapweed, Centaurea diffusa Lam. Wind velocities necessary to break diffuse knapweed stems ranged from 16 to 37 m/s (36 to 77 mph). A wind tunnel was used to develop a conversion factor between force and wind velocity. There was a strong difference in diffuse knapweed stem strength between field collection sites that corresponded to observed differences in proportion of plants tumbling. In contrast, diffuse knapweed plants under zero water treatment required four to six times greater force to break compared to plants under the moderate and high water treatments. Kochia, under moderate water treatment, required more than twice the force to break compared to plants under the zero and high water treatments. There were no differences in stem strength among three water treatments for Russian thistle. Bolting rosettes of these species were transplanted to pots and reared in a common garden to determine the affect of postsenescence water on stem strength. Although diffuse knapweed, kochia, and Russian thistle are important tumbleweeds of the western United States, environmental factors contributing to their dispersal are not well understood.
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