Watering Trees FAQ suggests that you should stop watering trees in the fall until all the leaves fall. Once that occurs, give your conifer trees a good deep watering before the winter. Any time it warms up unexpectedly in winter for a long period, it is OK to give them a drink then too as they don't shut down like deciduous trees. You just don't want them to have any growth that is un-hardy enough for winter by watering them too close to winter.