In the Tokachi region of Japan, soil frost depth has been decreasing. Outbreaks of volunteer potatoes caused by overwinter survival of unharvested potato tubers have become a severe nuisance. Some farmers have started snow plowing (. yukiwari in Japanese) to deepen soil frost and thereby freeze tubers to death. However, the system relies on farmer's intuition and experience and sometimes they fail to completely kill tubers. To establish a reliable volunteer potato control, we implemented farm testing of the vertical distribution of unharvested tubers and effects of soil frost control on the survival ratio. The control of volunteer potato by attaining adequate soil frost depth using snow plowing and accumulation termed as soil frost control. Furthermore, we compared tuber survival by soil frost control based on estimated soil frost depth by numerical soil temperature model and soil frost control based on farmers' voluntary effort. Most unharvested tubers are distributed at 0-0.15. m depths. The target soil frost depth was set as 0.30. m based on the relation between soil frost depth and tuber killing depth: the depth at which the soil temperature decreases to the critical temperature of -3. °C from a previous study. Soil frost control remarkably decreased survival to less than 0.5% of number of unharvested tubers if soil frost depth exceeded 0.30. m. Consequently, the target soil frost depth was confirmed as appropriate for almost complete tuber elimination. Soil frost control based on the estimated soil frost depth enhanced soil frost penetration more deeply and killed more tubers than soil frost control done by farmers' voluntary effort, thereby suggesting advantages of decision-making based on the estimated soil frost depth. Soil frost control is attracting attention and as a reliable method that can greatly reduce the outbreak of volunteer potatoes. The strategy minimizes climate change related risk.
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