Answer: The initial watering serves to settle the soil and eliminate any possible air pockets around the roots. In general, for new plantings the soil needs to be kept moist but not sopping wet during the growing season and on into the fall (for a full year) because the roots will continue to grow while the soil remains relatively warm.
Note that when temperatures drop, the soil tends to stay moister longer after watering or rain than it would in hot weather. Fall plantings thus usually need watering much less often than spring and summer plantings.
In addition, once the ground freezes, there is no need to water at all since it would not penetrate the soil.
Between fall and winter rains, snow melt, and cold weather, it is highly unlikely you would need to water in December. Depending on the snow this winter and the rain next spring, you may not need to water until a dry spell in early or mid summer. The best way to tell if you are in doubt is to dig down a few inches and feel the soil. Do this regularly so you can monitor the situation.
Good luck with your trees.
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