Unlike Deciduous plants, Evergreen Conifers are always in leaf, and continue to Photosynthesis during the Winter months, albeit at a much reduced rate. This means that it continues to transpire and lose water through its leaf. Water with which, through contact with the soil it can replenish itself. This is why a newly winter-planted bare root conifer can die of drought stress before the Spring comes along. At that time it has such negligible root activity that it can’t pick up any moisture from the soil to counter the drying action of the moving air.
And even a container-planted conifer will stand a better chance from April onwards.