What is Root Pruning?
Root pruning is the process of slicing through the roots at the drip line of an established tree that is going to be dug and transplanted. This is done to encourage the growth of new feeder roots along the root ball that will be transplanted along with the tree. A smaller root ball, with many feeder roots, will help the tree acclimate faster to its new spot in the ground.
Established trees that have been growing in the ground have roots that reach out far beyond the branches or drip line - the distance the branches reach out above the ground. These long branches are used by the tree to anchor and support it. However most of the small feeder roots, which bring in food and nutrients to the tree, are likely to be found growing off the main roots at some distance from the tree itself.
Why Do You Need to Root Prune?
When a tree is dug for moving and transplanting, generally the portion of the roots taken, the root ball, is only the circumference of the drip line, sometimes even less. Since the tree or shrub will be dependent on this root ball for most of its nutrients and water, it will need plenty of feeder roots to continue to sustain itself during the shock of being transplanted. To encourage the development of feeder roots closer to the drip line, root pruning, cutting off the long anchor roots, is done.
How to Root Prune a Tree or Shrub
Root pruning involves severing the roots of a tree, all the way around the tree's circumference at the drip line. This can be done by slicing down with a sharp spade, all around.
The larger the remaining root ball, the more feeder roots you will have and the better chance the tree or shrub will transplant successfully. However large root balls are deceptively heavy. This is rarely a job for one person and, for larger trees, you may need to call in a professional.
When to Root Prune
Root pruning does not damage the tree or shrub, but it will stress it. To give the plant some time to recover before being dug, root pruning should ideally take place a year prior to digging and transplanting the tree. Nurseries that grow tree seedlings in the ground will root prune the seedlings the year before they are dug and balled and burlapped. However in a pinch, 2-3 months should allow enough time for the tree to overcome the stress of root pruning and to start the process of developing new feeder roots.
Other Reasons for Root Pruning
Root pruning is also sometimes used to maintain a dwarfed size. Many so called "dwarf" trees are simply slow growers. If a dwarf tree starts to put on more height than is wanted, root pruning will temporarily shock it enough to stop growing tall and put all of its energy into growing new roots.
Another good use of root pruning is to encourage flowering of a fruit tree or slow to bloom vine, such as wisteria. In this case, the plant thinks it is under attack and will set flowers and seed, to propagate itself.
Root pruning is also useful when potted plants have outgrown their container and you do not want to move them to a larger one. Trimming the roots back and repotting with some fresh soil will keep the plants growth in check.