The two most important things you will want to do when planting a tree that has been grown in a plastic nursery pot are to root prune aggressively prior to planting, and to dig a hole of the correct depth and width.
Root pruning? What’s root pruning you say? Root pruning cuts or removes tree roots that are poorly formed so that the tree can have a long and healthy life. When trees are grown in plastic nursery pots it is common to remove them from the pot and find heavily matted roots on the outside of the root ball. As the tree grows inside the pot, roots hit the side of the pot, turn and start circling. And circling. And circling, and getting more and more root bound. If left uncorrected at planting time, this root bound condition will severely limit the future growth vigor and life expectancy of the tree.
Fortunately, it is very easy to root prune a potted tree. Traditional instructions have recommended making four vertical cuts into the outside of the root ball from top to bottom to cut through circling, matted roots, and cutting an X across the bottom of the root ball. For many years we have recommended much more aggressive root pruning
We recommend making 1″ deep vertical cuts into the outside of the root ball every 2 to 3 inches, continuing all the way around the root ball, and making six to eight 1″ deep cuts in a star pattern across the bottom. Then rough up the outside of the root ball, and plant no deeper than even with the ground level. This method works pretty well when potted trees are moderately root bound when they come out of the pot.
For severely root bound trees take an old pruning saw and cut off the outer ½” to 1″ of the root ball side and bottom. I know this sound a bit crazy, but it is now being recommended by Dr. Gary Johnson from the University of Minnesota’s tree research program. Dr. Johnson has extensively researched why trees fail or die in urban landscapes. Poor roots, poor planting techniques and poor soils/sites are the biggest culprits.
Using a hand saw to cut slabs of matted roots off the outside of potted tree root balls gives the tree a chance to grow new roots that fan outwards like the spokes on a bicycle wheel. This very aggressive root pruning does set the tree back in its visible growth the first season, but every year after that the tree will grow much better than if root pruning had not been done. The tree will grow faster, be able to absorb more nutrients and be better anchored.
You may need to temporarily stake the tree for one year only to help it stand vertical, and root in quickly. DO NOT leave stakes, ropes and guy wires on the tree more than 12-18 months, or the tree will envelope your staking materials in new wood, trapping these foreign objects in the trunk of the tree.
I am also sharing this blog with the Northfield News.