Definition - What does Root Bound mean?
When a cannabis plant is root bound, also known as pot bound, it means that its roots, instead of expanding outwards as they should have, begin to push the soil up out of the container, and in some cases, force their way out of the container they are in. They may also encircle the inside of the container.
Root-bound plants are likely to occur when a plant has remained in a container that is too small for it to achieve or maintain healthy growth.
Hydrolife explains Root Bound
A root-bound plant, cannabis or otherwise, cannot effectively make use of any water or nutrients given to it. In many cases, the plant is also top-heavy and is prone to falling or blowing over, which puts the buds, leaves, and branches at risk of breaking off.
To avoid a root-bound cannabis plant, make sure that it is transplanted into larger and larger containers as it grows. As a rule of thumb, the diameter of branch spread at a cannabis plant’s widest point should not exceed the diameter of the pot it is in.
Conversely, if a cannabis plant is in a pot that is too big, the media can retain more moisture that the plant can use, resulting in any number of fungal diseases that can kill a cannabis plant.
Another way to prevent marijuana plants from becoming root bound is to use something other than plastic containers to house them. Some growers use air pots or fabric pots to prevent root-bound plants. They prevent this issue by allowing plant roots to 'air prune'. Not only is this thought to prevent root-bound plants, but also encourages more root growth than would be possible in a sealed container.