Answer: You can plant a balled-and-burlapped or container-grown tree any time, since they have established root systems. It is best, however, to plant in the spring or fall, so the plant doesn't have to endure severe hot or cold weather right away. Bare root trees should be planted in the spring or fall, since they have limited root systems--the root systems aren't developed enough to withstand harsh weather.
In New York, September or October would be a fine time to plant a tree. The single most important thing is to make sure the newly-planted tree receives adequate water. Remove a circle of sod around the tree at least 2' in diameter, because grass will compete with the tree for nutrients and water. When you prepare the planting hole, you can add a little compost to the backfill soil, especially if it is sandy soil. After planting, create a little basin around the base of the tree to hold water, and mulch with an organic mulch like wood chips to help retain soil moisture. If nature fails to provide a good soaking rain at least once a week, you'll need to water your tree. A deep watering once a week is much better than several shallow waterings.
When planting a container-grown tree, loosen the outer roots a little--they can tend to grow in circles around the inside of the pot, and sometimes keep that shape once planted. You want them to reach out into surrounding soil! For a burlapped tree, either remove the burlap, or at least gently cut slits in the burlap all around the rootball. This makes it easier for the roots to "escape" the burlap and grow into the surrounding soil. Good luck!
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