McKrNzIn, J. S. andMcLEAN,G. E. 1980

Plants of Medicago falcata ’Anik’ were samples to assess their relative cold hardiness during the fall, winter and spring periods from 1974–75 to 1978–79. Precipitation and soil temperature patterns and cold hardiness profiles varied considerably from year to year. Environmental conditions in the fall appeared to exert the greatest influence on the cold hardiness profile and the maximum cold hardiness level in mid-winter. In general, plants started to harden in mid-September, but during one fall hardening period there was a delay associated with the early development of crown buds and the accompanying flush of growth during August and September. During two fall hardening periods, water-saturated soil conditions were associated with a dehardening phase in October. Conditions favoring delayed fall hardening and complete dehardening in the late fall were also associated with a lower level of hardiness in mid-winter. The maximum hardiness level, and the month during which it occurred, fluctuated considerably each year. Plants began dehardening as soil temperatures increased in late winter and early spring during 3 of the 5 yr. In the remaining 2 yr, plants began to deharden prior to an increase in soil temperature.

Plants propagated from single plant selections of the alfalfa cultivars Saranac and Luna (Medicago sativa L.), Beaver (M. media Pers.) and Anik (M. falcata L.) were transplanted into the field in May and subjected to a range of freezing temperatures at 3-wk intervals during August and September. A portable field freezing chamber was used to study the influence of frost on leaf injury in the fall and winterkill following the severe 1977–1978 winter. Leaves of all plants had the capacity to harden during the fall, but the selection from Anik was consistently the most frost hardy. Temperatures ranging from −4 to −5 °C in mid-August caused 50% leaf injury to the selections of Beaver, Saranac and Luna, while in late September, temperatures ranging from −9.5 to −10.5 °C were required to produce similar amounts of injury. In the Anik selection, −6 °C in mid-August caused 50% leaf injury, but in late September −12.5 °C caused less than 35% injury. Thus, the Anik selection appeared to start hardening about 3 wk earlier than all other selections. In all plants, the potential for winter injury during the 1977–1978 winter increased as a result of leaf frost damage during mid-August and early September in 1977. The application of these results to the winter survival of seedling stands of alfalfa is discussed.

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