Seasonal patterns of frost hardiness and the effects of photoperiod, water stress, and low temperature on hardiness development were examined in shoots of 1st-year seedlings of western red cedar (Thuja plicata Donn), yellow cedar (Chamaecyparis nootkatensis (D. Don) Spach), and white spruce (Picea glauca (Moench) Voss). Under natural conditions, spruce hardened in two stages starting in early August, first at a slow rate in response to shortening photoperiod, then at a faster rate in response to decreasing temperatures. Western red cedar and yellow cedar started hardening in late October to early November when the photoperiod was already less than 11 h and daily maximum temperatures less than 15 °C. Seedlings of these two species attained a greater degree of frost hardiness when exposed to persistent subfreezing temperatures. Under controlled conditions, 4 weeks of short photoperiod at warm temperatures (9 h light:15 h dark, 20:15 °C) had little effect on hardiness of the two cedars but increased hardiness in spruce to about −15 °C. Water stress increased hardiness in spruce under long photoperiod (18 h light: 6 h dark, 20:15 °C) but only marginally in western red and yellow cedar. Low temperatures (7:3 °C, light:dark, 9-h photoperiod) increased hardiness in all species. Exposure to 2 and −3 °C (light:dark, 9-h photoperiod) increased the rate of hardening in western red and yellow cedar but not in white spruce. The ability to deharden in white spruce was related to chilling requirements and not necessarily exposure to warm temperatures. The induction and maintenance of hardiness in the three species appear to be controlled by different environmental signals. Key words: photoperiod, low temperature, water stress, dormancy, dehardening.