Increasing Cold Hardiness of Container - Grown Douglas-Fir Seedlings

Containerized Douglas-fir (Pseudotsuga menziesii [Mirb] Franco) seedlings, grown for 6 months in a greenhouse then hardened in five different ways, were out-planted at 3,500 ft. elevation. Those hardened under an eight-hour short-day test condition, or in a lath house (half-sunlight), suffered the least winter frost damage; other seedlings subjected to two field hardening treatments at the high-elevation site (those hardened under open-field or shaded-field conditions) as well as seedlings held in a cold-room were killed or injured by repeated winter frosts. The short-day seedlings were also shown to be hardiest at time of planting. The two-month short-day greenhouse treatment applied in late summer sufficiently induces cold hardiness in Douglas-fir container stock of high elevation origins to allow early fall planting.