DEFOLIATION AND COLD-HARDINESS OF NORTHERN WHEATGRASS

Freezing temperatures in winter were hypothesized to be a major cause of mortality of tillers following defoliation of northern wheatgrass ( Agropyron dasystachyum [Hook,] Scribn., syn. Elymus lanceolatus [Scrib n. & Smith] Could), Cold-hardiness of northern wheatgrass tillers was determined following a single mowing to a 5-cm stubble height in late May, June, July, or August in 1992 or 1993 in southwestern Saskatchewa n, An unmowed control was also included, Cold-hardiness was determined in early and late winter immediately following mowing by: 1) exposing tillers to controlled temperatures ranging from -3 to -36 degrees C, or; 2) exposing them to -15 degrees C for 0 to 15 days, The LT50 (temp erature at which 50% of tillers died) of northern wheatgrass tillers i n early winter ranged from -29.5 to < -36.0 degrees C in 1992-93, and averaged -24.0 degrees C in 1993-94, In late winter LT50 ranged from - 18.1 to -22.6 degrees C in 1992-1993, and it averaged -22.0 degrees C in 1993-1994, The LDur(50) (duration at which 50% of tillers died) of tillers exposed to -15 degrees C for 0 to 15 days ranged from 8.0 to 1 3.1 days in early winter, and 2.7 to 4.7 days in late winter, Unexpect edly mowed tillers were generally more cold-hardy than those from cont rol, In early winter LT50 was 1.5 to 10 degrees C lower for mowed than control tillers, The hypothesis that defoliation reduces cold-hardine ss of northern wheatgrass was rejected, The degree or duration of cold stress in the field is generally insufficient to reduce tiller surviv al in northern wheatgrass, Late winter through early spring is a criti cal period for tiller survival of northern wheatgrass because cold-har diness declines this time of the year, Maintaining insulating cover ca n moderate soil temperatures and reduce damage to plants from freezing temperatures.