Definition - What does Root Bound mean?
Any plant’s root system that is confined in a container for any length of time can become root bound. When a plant is grown naturally in the ground, it spreads its roots out throughout the soil. However, in a pot or container, the roots start to twist and rotate inside the pot until the plant becomes root bound.
In other words, the plant’s entire root system becomes bound by the container that holds it. Most root-bound plants are simply plants that have grown too large for their container or pot.
On occasion, plants grown outside near foundations, walkways, footers, or pipes can also become root bound if the plant’s roots cannot spread freely through the soil but are instead held prisoner by some barrier that inhibits their natural growth.
MaximumYield explains Root Bound
Root-bound plants often wilt quickly. It is not uncommon for a root-bound plant to start to suffer from yellowing or dead foliage. It may also exhibit stunted growth. Often, the sheer force of the plant’s root system pushing outwards as it grows might deform the pot as it attempts to spread its roots out of the container's confines.
Clay, ceramic, or glass pots frequently break under the pressure from a root-bound plant. A severely root-bound plant will form a mass of roots and contain very little soil when removed from the container. A root-bound plant will need to be transplanted outdoors in the garden, replanted in a larger container, or have some of its root mass pruned away. Some root-bound plants can also be divided and planted in more than one container.
To avoid root-bound plants in a container garden, some growers opt for fabric containers, or Air Pots, both of which allow a plant's root zone to breathe. As roots are exposed to air, they are air-pruned, rather than risk being bound by the container's impermeable wall. Another way to avoid root-bound plants is to transplant often.