FROST HARDENING SPRUCE CONTAINER STOCK FOR OVERWINTERING IN ONTARIO

Difficulties overwintering container stock in northern Ontario led to the development of the ''extended greenhouse culture'' hardening regim e for spruce seedlings. Laboratories to measure shoot frost hardiness and evaluate terminal bud development were established to monitor nurs ery crops being hardened using this regime. information on frost hardi ness and bud development provided by these laboratories has been used by nursery managers to determine readiness of container seedlings for overwintering. Since 1982, over 200 stock lots have been monitored by these operational laboratories. This database can be used to determine the importance of nursery cultural factors and seed source on frost h ardening. The database shows large differences between nurseries in ap proach to hardening seedlings which were reflected in levels of freezi ng damage, winter desiccation and overwintering success. Rates of fros t hardening (i.e., the interval between terminal bud initiation and at tainment of a -15 degrees C level of shoot frost hardiness) of crops p roduced in north central Ontario failed to show significant seed sourc e effects. The rate of frost hardening was faster in crops producing f ewer needle primordia in terminal buds.