Evergreen Tree Potting Instructions
Planting in Pots
Although potting is generally not as good as planting directly in the ground, potting can work well if done right. If you choose to pot your trees, they will need the equivalent of an inch of rainfall per week, and the pot must have holes to allow for drainage. Covering the holes with a rag or sponge before filling will keep dirt from falling through the drain holes while still allowing proper drainage.
The other trick is keeping the pot dirt from heating up and cooling off too much. The pot should be white or very light in color, or must be protected from direct sunlight. Lightly colored mulch or woodchips on top of the pot dirt will also help to reflect sunlight and keep the pot cooler. A dark pot getting hit directly on the east or west side by sunlight in summer [or on the south side in the winter] will result in damage to the roots or withdrawl of the roots from the side facing the sun [see photos]. In general, avoiding wild temperature fluctuations will result in a much healthier tree.
The photos on the left [above and below] show vigorous root growth in a black pot, while the photos on the right [above and below] show poor root growth due to overheating from direct sunlight from the West. The white spots are vermiculite, an amendment added to the soil at transplanting time to increase airflow within the soil.
The advice given thus far is even more important during the late fall and winter months. Nearly all evergreen trees need winter weather for normal growth, and should therefore be set outside. Heeding the "no sun on the pot" rule, set them on the north side of your house or garage and allow them to get snowed in. Avoid putting them directly under a drip-edge or other location where an icefall could occur ["Honey, somebody crushed our little trees this winter, but there's no evidence left behind of who did it!"].
For seedlings and plug seedlings, the pots should be at least 6-10 inches deep, and for transplants and plug transplants, at least 10-16 inches deep. Although the roots can be trimmed, it is better for the pot to be a little too big than a little too small. For short term potting [such as a wedding or special occasion], you can use smaller pots. If you intend to grow them in pots for a year or more, consider doubling the pot size you have in mind. Larger pots reduce the changes of a blow-over due to strong winds.
Lastly, please understand that since potting introduces additional variables and risks as opposed to planting in the ground, we cannot guarantee your trees if you pot them.