Controlled-environment experiments were conducted on Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings during their 1st year. Hardiness of foliage was assessed by visually estimating injury after freezing tests.Germinants (1 week) were unable to attain any freezing tolerance under 8-h days at 2 °C even after 9 weeks but were killed whenever ice formed. Their ability to supercool increased by 5 °C during this treatment. However, seedlings older than 3 weeks (1 to 2 cm of epicotyl) could develop true hardiness under the influence of either short days (less effective) or low positive temperatures, independently of lignification, bud setting, or entry into rest. Ability to acclimate increased gradually with age and was inversely related to growth and maturation, apparently because the latter processes had higher temperature optima. Photoperiod affected growth and bud formation only above about 15 °C but influenced hardiness at 1 °C. The optimum photoperiod for inducing hardiness was longer at low light intensities than at high ones, presumably because of a minimum requirement for photosynthesis.Interruption of the long inductive dark period with 15 min of red light (650 nm) caused a small decrease in hardiness and bud set and an increase in growth. This effect was not reversed, but was enhanced by an isoenergetic burst of far-red (FR) light immediately after. FR interruptions alone had no effect. Night frosts (−7 °C) caused significant dehydration and rapidly increased hardiness only if both the warm, short day and chilling "stages" had been supplied first and the daily supply of light continued.