Raspberry root frost hardiness

Protected cultivation of raspberry in soilless system has become more common. The root systems of plants in pots are exposed to lower winter temperatures than the ones grown in soil. As the roots are usually more susceptible to frost than the canes, they may suffer winter injury in areas with low winter temperatures. Frost hardiness of raspberry cultivars has been extensively studied, but root frost hardiness in general is less explored. Therefore, a study was undertaken to determine root frost hardiness with controlled freezing tests in selected raspberry cultivars. In the first experiment, the canes of potted 'Glen Ample' and 'Ottawa' raspberries were pruned to soil level, and the root systems in pots were subjected to test temperatures ranging from -3 to -15 degrees C. The injury was evaluated based on regrowth of the canes after the freezing test. Visible injury to 'Glen Ample' occurred at -12 degrees C, whereas 'Ottawa' had no injury at the lowest test temperature of -15 degrees C. In the second experiment, the root systems of 'Glen Ample, 'Maurin Makea, and 'Muskoka' raspberries in pots were subjected to the test temperatures in a range of -10 to -26 degrees C. In 'Glen Ample' shoot regrowth was impaired already at -10 degrees C. In 'Maurin Makea' and 'Muskoka, a significant decline in regrowth was observed at -18 degrees C. In conclusion, raspberry roots were less hardy than what has been reported for the canes. The roots of the Canadian cultivars 'Ottawa' and 'Muskoka' and the Finnish cultivar Naurin Makea' were more frost hardy and could tolerate almost 10 degrees C lower temperatures than the roots of the Scottish cultivar 'Glen Ample'. As winter temperatures may drop below -30 degrees C in Finland, the growers should take these cultivar differences into consideration and pay attention to protection of the overwintering potted plants.