This pitiful object used to be a "Karen" azalea. I had planted a couple of them in a raised bed with good soil and excellent drainage. One of the bushes seemed to be constantly wilting, its leaves drooping and losing color. I watered and watered, the azalea would perk up briefly, then wilt again. The contrast between it and the healthy one became more and more clear. It started to drop leaves. By the end of the year, it was clear that it was dying, and I decided to pull it up. I took hold of the stem, gave a tug, and out it came, with a rootball exactly the size and shape of the original pot I had bought it in. The shrub had been rootbound in the pot. It never had a chance.
Unfortunately, commercial growers produce shrubs in plastic pots in a way that makes it very likely they will be rootbound when we buy them. The roots of a plant or shrub want to expand. They grow constantly. When the growing root reaches the wall of the plastic pot, it has nowhere else to go, so it turns inward, seeking but never finding room to stretch out. This process creates a rootball. A rootball is not in itself such a bad thing. The roots hold the soil together and make it easier to transplant the shrub with less exposure of the roots to drying air and the risk of damage to them. The problem comes when the roots are so firmly compacted into the ball that they can't escape, even after the shrub is planted in the soil. They will never be able to expand, and the fine feeder roots, trapped inside the rootball, may not be able to take up enough water and nutrients to sustain the plant, no matter how much we water. Sometimes the soil of a compacted rootball even becomes what is called hydrophobic, refusing to absorb water. This is certain doom for the plant.
Good planting practices are crucial for root development. This is why it is so important, when planting a new shrub, to dig the hole twice the size of the rootball, to mix the excavated soil with loose organic matter so that the roots can easily penetrate it and expand. This is of particular importance for those of us who live where there are clay soils. If you have ever looked at the slick wall of clay at the side of a newly-dug hole in such soil, you will see that it is just like making a clay pot in the ground. Newly-planted roots wouldn't have a chance to penetrate it, and poor drainage from such a hole will doubly doom the plant.
But with a rootbound shrub, even normal good planting practices may not be enough. You may have to free the roots. The first step is to remove the plant from the pot and soak it in a bucket or tub of water for about an hour, until the rootball has clearly absorbed the water. It should be noticably heavier. Don't prolong this process, however, as it can drown the plant. Then, loosen the roots. With a fibrous-rooted shrub like an azalea, you can take a sharp knife and make several slashes, about an inch deep, down through the rootball. This will encourage new root growth outward. With thicker roots, gently tease them out of the rootball and untangle them. Prune out any broken or dead roots. Then place the plant into the prepared hole with the roots spread out and fill it in the usual manner. Since some of the soil from the rootball will probably have washed out during the soaking process, use the muddy soaking water to water the plant. This will minimize the shock from the change in soil from the pot to your planting bed.
A little extra care at planting time may be the difference between a healthy, thriving shrub and pulling it out dead by the end of the season, still imprisoned by its own roots.