As plants grown in containers mature, their developing roots eventually will run out of space.
When this happens, the plant becomes "root-bound". The roots will try to escape out any drain holes in the pots. In some cases, they will try to slip out of the soil and over the lip of the pot. And, in nearly every situation, the roots will begin to grow in overlapping circles that follow the inner walls of the container. As roots take over the interior space of the container, little room is left for soil to hold water, which may lead to root death. Allowing root-bound plants to continue to grow in this fashion will not only stunt the plant's growth, but also it can bring about the plant's overall demise.
In many cases, plants grown in decorative containers should be repotted from time-to-time. How often can depend on the type and number of plants in the container. The size of the container, the environment in which it is placed, and the sort of potting medium can also impact the frequency replanting is required. Many Japanese Maples, for instance, can grow slowly and live for many years in a large pot – but watch out that the plant doesn't break through the bottom of the container and begin rooting into the soil below. Succulents like many Sedums and Echeverias can also live for many years in the same pot -- just be careful to install them into a lightweight potting soil that doesn't get soggy. Fast spreading perennials like Phlox, Bee balm & Black-Eyed Susans, on the other hand, may rapidly multiply, filling up a small container within a single growing season. The good news – when you repot these perennials, you'll be able to divide them to make more plants!
One basic planting rule: When moving a plant from a container into the soil – whether a decorative pot or a planting bed – break up some of the roots. *
Unlike the top growth on a plant, roots will not stop growing in circles unless we break them and spread them out. So, if you have purchased a potted plant that's bursting the pot's seams, plan to break up the roots a bit with your hands. Or, if the plant is heavily root-bound and you cannot separate the roots by hand, try using a tool like a Big Grip Multi-Purpose Planting Tool to saw off the bottom few inches of roots from the plant. Then, begin teasing the partially released roots apart, spreading them out into the soil in which you are planting.
Yes, it feels like you are mutilating your plants when you hack, saw and tease roots apart. But, if you leave the roots circling by just popping them out of the pot and into the ground with the root growth still showing the perfect formation of the pot from which the plant was extracted, your plant will not thrive. In a few weeks, months or even years when your plant subsequently dies, you will dig it up only to find the now-dead roots formed in nearly the same molded form as when you put it in the ground.
Older schools of thought recommended that in addition to breaking up roots, top growth should be pruned equally before planting to "balance" the plant. Instead, remove only any dead or broken top growth at planting time. Leave as much top growth as possible on the newly installed plant so it can draw whatever energy it needs from its branches and leaves.
*As the saying goes: "All rules are made to be broken." Some plants like squash-family veggie starts and even some trees have very delicate roots that do not benefit from being broken. If in doubt, ask your plant supplier if you should or should not break up the roots on a specific plant.