So you've just bought a pot-bound plant and you don't know what to do? Gary Johnson, Chad Giblin and I have been testing various techniques to get trees out of their pot-bound states for the last 8 years or so, and here are some of the things that we've found.
The number one problem with planting a pot-bound tree is that they are usually planted too deeply. Trees in containers often have three inches between their uppermost roots and the soil line -- or even more! When a tree like this is transplanted into a landscape without having its planting height adjusted, the roots circling near the top of the container will eventually press up against the stem of the tree and strangle it to death (photo 1).
Photo 1: Pot bound plant's root system 5 years after planting; tree was planted too deep based on media level in pot
When planting a container grown tree make sure that the first large root connecting to the stem (usually about ¼ inch in diameter) is visible after you fill in the planting hole. This will ensure that as the tree gets older and roots and stems expand there will be no compression of the stem.
Roots do not continue to grow in a circle after a tree is planted, even in severely pot-bound plants. Roots which were already circling in a pot-bound plant when the plant was transplanted will not straighten out after planting, but as the roots grow they will grow outwards, not in a circle (photo 1). Right now, with the research we currently have, it is not clear how circling roots will affect a tree if they are only present below the stem. Yes, they look ugly, but looking ugly doesn't mean they're not doing their job.
Most of the common techniques that the extension service recommends for pot-bound trees will not really do that much. Scoring the sides of pot-bound root balls with a razor knife or butterflying the root ball with a shovel just doesn't work that well. If you're really serious about not having any circling roots then you need to use a technique called a box cut. A box cut is performed by cutting the root ball into the shape of a box by using a pruning saw (photo 2).
Root balls treated using the box cut method generally had good looking root systems after 5 years in the ground (Photo 3). Circling roots were drastically reduced, but not entirely eliminated.
In terms of what we're actually recommending -- Right now we are recommending that you check the planting depth of all container grown plants before planting. Only in rare cases do we think that you'll find them planted at the proper depth. After removing the media from the top of the root ball to correct planting depth look for circling roots. If you don't see any circling roots thicker than a pencil then it probably isn't worth your time to do anything besides planting the tree, being careful to make sure that the uppermost root is planted at the soil surface. However, if you see any circling roots with a diameter greater than a pencil, we recommend using a box cut on the root ball.
On a final note, none of the trees we purchase ever need to be pot-bound. There are many different containers out there that will all but eliminate circling roots, such as Smart Pots, Superoots, and Root Trappers (photo 4). If we demand that companies provide trees planted in these containers then, someday soon, we may never need to worry about circling roots again.