Freezing Tolerance of Two Legume Cover Crops for Upper Midwest High Tunnel Conditions.

Winter-annual legume cover crops have potential to provide both soil health benefits and fertility to organic high tunnel systems in the Upper Midwest. Extreme temperature fluctuation in high tunnels during winter, when temperatures can range from 15C during the day to -18C at night, may affect winter hardiness of cover crops. Our objective was to compare legume cover crop survival and plant quality following freezing events for plants grown in either simulated high tunnel conditions or in standard acclimation conditions. Red clover (Trifolium pratense) and hairy vetch (Vicia viliosa) were germinated in the greenhouse, then moved to either a growth chamber set at 20C/-2C day/night for high tunnel conditions or at 2C for standard acclimation conditions. After three weeks of acclimation, 30 plants of each species from both acclimation treatments were frozen at -6, -9, -12, -15, or -18C. Following freezing, high tunnel plants recovered in the same high tunnel acclimation conditions (20C/-2C) and standard acclimation plants recovered in ideal greenhouse conditions (20C day/night). After three weeks, survival, biomass, root nodulation, and percent nitrogen were assessed. LT50 (+/- 1 SE) for clover was -7.7C (0.3C) and -18.0C (21.2C) in standard acclimation and high tunnel acclimation respectively. LT50 for vetch was -11.3C (0.4C) and -19.5C (0.9C) in standard acclimation and high tunnel acclimation respectively. Biomass ranged from 0.000-0.087 grams and 0.000-0.790 grams in clover and vetch respectively. Overall, biomass was greater in high tunnel acclimated plants and at warmer freezing temperatures. Nodule number ranged from 0.0-46.2 nodules per plant. Results of this study suggest that large temperature fluctuation in the simulated high tunnel acclimation treatment did not reduce freezing tolerance for hairy vetch or red clover compared to standard acclimation.