Plants purchased in containers at a garden center may look fine in the store, but all too often they are root-bound—with dense roots that are tightly packed into the container. Many people imagine that a plant with lots of dense roots is a better choice than a plant with sparse roots, but this is actually not the case. When planted in the garden, a root-bound plant often just keeps developing its roots in a tight circular fashion and never begins to send those roots out into the surrounding soil. The plant can eventually choke itself. A much better choice is a plant with a loose root ball, with plenty of loose, bare soil around the roots.
Identifying a Root-Bound Plant
Before you buy a plant, turn the plant over and examine the bottom of the container. if you see roots poking through the drainage holes, chances are good that the specimen is root-bound (the condition is sometimes known as being pot-bound). Severely root-bound plants may even be hard to remove from the pot, since the roots can be firmly entwined through the drainage holes.
It is also perfectly fine to inspect the root ball by sliding the entire plant out of its container. Knowledgeable gardeners do this all the time when shopping for plants, both with potted herbaceous plants and even small shrubs and trees. Garden center personnel have no problem with this, provided you don't damage the plant while you inspect it. Simply grab the plant by its main stem between your thumb and forefinger, and lift it up while tugging downward on the pot. You only need to extract a few inches of the root ball to know whether the plant is root-bound. If you see a dense mass of white encircling roots around the edge of the soil, this is not a plant you want to buy, if possible. This kind of root ball may well easily slide out of the container in a mass of white roots formed into a hard ball. A plant becomes root-bound for a number of reasons, none of them good ones. The specimen may have been neglected—the extreme development of roots may be a response to not getting enough nutrients or water as the plant was growing.
The ideal specimen will reveal a few white roots exposed around the perimeter of the root ball, with plenty of dark soil also visible. The root ball may begin to crumble slightly as you extract the root ball from the container. This is an ideal specimen to buy.
If You Must Choose a Root-Bound Plant
Ideally, there are enough choices of a given type of plant type that you can inspect several specimens until you find one in which the root structure is ideal. But this is not always possible; sometimes you are forced to choose a root-bound plant because there are no others available. Fortunately, there are ways to improve the chances for a root-bound plant to succeed in the garden, though it may seem a little brutal to the uninitiated.
When you are forced to plant a root-bound plant, make efforts to untangle the roots with your fingers before planting it. If you can tease the root-ball into a loose bundle of hair-like roots bristling out from the plant, these roots will rather easily find their way into surrounding soil once you plant the specimen into your garden.
If the roots resist untangling by hand, you can even cut slits into the root ball with a knife or sharp garden trowel. Most plants are quite tough, and by severing the roots you make it easier for the plant to send new roots out into the surrounding garden soil. Some gardeners slice the root balls as a matter of course whenever planting a specimen, no matter if it is a mature shrub in a gallon container or tiny seedlings in a bedding plant six-pack. Make a series of vertical slits along the sides of the root ball, then slice a deep X in the bottom surface of the root ball before planting. With small six-pack plants, it's usually enough just to slightly tear at the root ball with your fingers before you plant each one.
If you plant a root-bound specimen as is, the plant typically languishes as the roots simply continue to spiral around the plant, which may eventually choke the plant to death. Plenty of expensive shrubs and trees meet this unhappy end, a fate that can be avoided if you simply make multiple cuts into the root ball before you plant it.