Answer: Although many trees are grown commercially as container stock, unfortunately it can be a bit difficult to maintain them in pots at home for extended periods.
These particular trees are winter hardy, but, when planted in a container, it is important to protect the rootball from extreme cold. One way to try to do this could be to place the original container inside a second, larger container filled with soil to act as an insulating layer. Additional insulation in the form of mulch could also be placed around the container. Another approach would be to bury the pot up to its rim in the ground. Another method some gardeners have had success with is placing the pot in an insulating container such as a styrofoam cooler and packing additional insulation alongside it. The goal is simply to protect the roots.
In addition, it is important to keep the soil and thus the tree from drying out completely. The soil should be kept slightly moist year round (except when it is frozen).
Mulching or double potting as described above can also help protect the roots from summer heat. This is important because the nursery pots tend to heat drastically in the summer sun and may "cook" the plant's roots as a result. Cooler soil is also likely to stay moist longer on hot summer days. On very hot days and on windy days you may need to water daily or even twice a day.
You may find that the plant has outgrown its current pot and needs to be repotted. This would be done in spring, using a soil mix as close to the original as possible. In general it is not a good idea to move a plant from a small container to one that is far larger than the original. Instead, moving up to a pot that is just a few inches wider across is usually preferred. You would also need to provide adequate nutrients to sustain the plant's growth until it is time to plant it in the ground. To do this, you might use a complete water soluble fertilizer applied at a low rate during the growing season along with a top dressing of compost.
Finally, since holding a tree in a container is stressful for it, in my experience it would be better to plant it in the ground as soon as possible. It is often suggested that evergreens be planted only in spring, but a fall planting would probably be better than trying to hold it over another winter in a pot.
When you plant the tree, make sure to water deeply as needed to keep the soil moist until it freezes for the winter, and then water as needed to keep the soil moist (slightly damp but not soggy wet) for the entire following growing season and through that fall as well.
Good luck with your project!
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