Planting trees in a planter box might be your preferred method if you have limited outdoor space or want a tree to flank each side of your entrance way. It's doable, so long as the tree doesn't outgrow its container. Plant bare-root trees and balled-and-burlapped trees during the dormant season, which runs from late autumn to early spring. Container-grown trees can be put into planter boxes any time of year, but it's best to plant them in the fall or spring, to avoid the hottest temperatures.
Find a suitable planter box for the tree. Make sure it is large enough to fit the current root ball and allow for expansion. It needs to be at least the same size as the nursery container the tree came in.
Line the bottom of the planter box with a fiberglass or mesh plastic screen to keep the soil inside the container. Add 2 to 4 inches of broken pottery or gravel on top of the screen if you're worried about drainage. However, if you have a loose, well-draining soil and drill holes in the planting box, you're all set.
Add soil mixture to the bottom of the planter box so that the tree will sit evenly with the top of its roots in line with the top of the container. Test it out a few times to make sure you've laid down enough soil. Figure that watering will pack the soil down a couple of inches.
Water the tree thoroughly. Remove the packing material such as burlap, the planting container or plastic, depending on the type of tree you purchased.
Wiggle the roots gently if they are packed tightly. This will loosen them up and encourage them to spread better.
Place the tree in the planter box. If needed, have someone hold it in place. Shovel in the soil mixture around the root mass, adding water every few inches to eliminate air pockets. Continue until the planter box is filled.
Water the container until the water dribbles out the drainage holes at the bottom of the box. Add a liquid transplant fertilizer solution to the water, such as vitamin B1, to promote new roots and a strong, healthy tree.