When buying new plants, most customers try to choose one that looks reasonably healthy. However, it is easy to buy a plant with healthy-looking foliage, only to discover that it is extremely root-bound when it comes out of the pot. Plants become root-bound when they have been grown in a pot that is too small for too long. They will have many woody roots circling the bottom and even the sides of the container.
If the problem is not too severe, the roots can be gently pulled apart before planting, spreading them around the planting hole when placing the plant in. However, if the plant is very root-bound, it may be impossible to pull the many circling roots apart. If the plant is placed in the hole with too many circling roots, it will have a very difficult time growing any roots out into the surrounding soil, severely limiting its water uptake. I planted a cranberry bush (Viburnum trilobum) a couple of years ago that was very root-bound, but, being in a hurry to get it into the ground, I neglected to adequately separate the roots. Although we watered it quite often, it constantly looked drought stressed and slightly wilted. The next spring, we dug it up and sure enough, it had only grown one tiny root outside of the original, intact circle the shape of the nursery pot. If we had left it long enough, it may have grown more roots and survived, but I doubt it ever would have been very healthy.
The best way to remedy this situation is to rearrange the plant's roots before putting it into the ground. To do this, fill a large bucket with water, then submerge the plant's roots into the water. Swish it around a bit and massage the roots; the idea is to wash off most of the soil so that you can see the structure of the roots. Once the roots are free of soil, they can be examined and, hopefully, untangled. This sounds pretty straightforward, but let me assure you that if you have a large plant, it can be a job for two people! Although the plant will almost certainly recover from a few broken or pruned roots, it is best to be as gentle as possible. Be sure the planting hole is ready before you strip the soil from the plant's roots, but if it is not ready immediately, the plant can wait a short time in the bucket of water (not in the blazing sun, however). When planting, make a small mound in the middle of the planting hole and spread the plant's roots around the mound, pointing down into the soil. Fill the hole with soil, being especially careful to avoid leaving air pockets around roots that could dry them out. Pat the soil down firmly and water the plant thoroughly.
Ensuring that a plant has healthy roots gives it the best possible start in your garden. If we had planted our poor cranberry bush this way the first time, it would have grown much better from the start. After we finally dug it up, washed and spread out its roots, and replanted it, it has been much healthier and happier.