Potted plants — like other plants — grow, and while a 50-foot-tall weeping fig is a glorious sight on a Caribbean island, my living room ceiling won't allow it.
Root pruning slows stem growth, and makes room for new soil and new roots.
How often to root prune any potted plant depends on how fast that plant grows, and any quirks it has about being root-pruned. Angel's trumpet, for example, gets one severe root and stem pruning every year, in fall. Root and stem pruning are needed to stimulate new stem growth, on which is borne those eerily beautiful, pale-apricot-colored trumpets.
Potted fig plants can fruit in pots as long as they get root pruned every year or two so that roots find new soil to promote vigorous, annual stem growth.
In contrast, after 19 years, I have yet to root prune my pony tail palm.
Plants have subtle ways of indicating that their roots need new soil in which to roam. Keep an eye out for plants that dry out too rapidly or send roots creeping out drainage holes. My ponytail palm burst its pot; perhaps it's time to root prune or give it a larger pot.
Root prune any plant that's beginning to look too tall or too crowded in its pot — or give it a larger pot. Exceptions include clivia, which thrives cramped for years in the same pot until eventually bursting it.
The best way to tell whether a plant needs root pruning is to slide the root ball out of the pot and examine it. Thick roots pressed right to the edge of a root ball, or circling its outside, indicate that the time to operate has come.
To root prune, take a sharp knife, grit your teeth, and slice a one-half to 2-inch layer of soil from all around and underneath the root ball. The larger the root ball, the more soil and roots can be removed.
Then go over the whole root ball again, this time with a pronged hand cultivator or a stick, loosening soil and roots on the outside. Also prune back any lanky or damaged roots.
With roots pruned, the plant is ready to return to its pot. Put a stone or screen over any drainage holes, then shovel in enough soil so that the base of the plant's stem sits one-half to 1 inch below the rim. Gradually fill the space between the root ball and the pot with potting soil, tamping it in place with your finger tips or a blunt stick.
Watering is important during the couple of weeks or more that a plant is recovering from root pruning. Right after pruning, give the plant a thorough soaking. When new growth begins, make sure all the soil stays moist — the old soil because even with its reduced size it must still support the plant, and the new soil to encourage new root growth there.
(Cactii and other succulents are an exception: Let their soil thoroughly dry between waterings, even after root pruning.)
Finally, prune the stems so the reduced root system has fewer leaves to support. Besides, the whole purpose of root pruning has been to keep the plant from growing larger.
Rest assured that plants tolerate all this pruning. Just look at bonsai trees, whose heights can still be measured in inches after hundreds of years. They are kept that way with regular root and shoot pruning.
For more about all types of pruning, see my "The Pruning Book" (Taunton Press, 2010).