There are two primary solutions for a root bound plant. First you can re-pot your plant, putting it in a larger pot so the roots have room to expand. This is a good solutiong if you want your plant to keep growing and have or can get a larger pot. But, if you have a pot you are particularly fond of, or you don't want your plant to get larger, in some cases it is more desirable to root prune your plant.
Root pruning a root bound plant sounds scary, but it's easy (if your plant isn't too big) and can even save the life of your plant.
It takes some nerve to root prune a root bound plant, but it truly is a kindness for a plant that has outgrown its pot. It is also a way to keep a plant small, for a terrarium, a bonsai or a containerized tree that you don’t want to move to a bigger pot.
First, take your plant out of its pot and examine the roots. When doing this, particularly with a delicate plant, don’t just pull the plant out of its pot. If the plant isn’t too big, tip the pot over and tap the rim. Also, if the pot is flexible, try to squash the sides of the pot. Put your hand at the base of the plant, or slide your fingers through the foliage and slide the plant out, by tipping the pot sideways. You may also have to run a long knife around the perimeter of the pot to separate the roots and soil from the inside of the pot.
Take a look to see if the roots are they taking up most of the pot or if they creeping out the drainage hole. If either of these are true, that means the plant is root bound and the plant either needs to be moved to a larger pot or the roots need pruning.
To prune the roots, start with a pair of scissors, pruning shears or sharp knife. Cut around and under the plant’s root ball removing both roots and soil. You can be pretty aggressive, cutting away both large and small roots. This may feel a bit barbaric and like you are committing planticide, but plants' roots can take a lot of abuse.
When you are done, take a stick, pronged cultivator or fork and loosen soil and roots around the surface of the root ball, teasing out tangles and spreading the roots. This encourages the roots to expand into the soil around the ball, not grow in circles and strangle the plant.
Get ready to re-pot by adding potting mix to the bottom of your container. Add enough soil so that your now smaller root ball will sit on the soil and the top of the plant is about an inch below the rim of your pot. You always want to make sure that the crown of the plant
Then place your plant in the pot and add soil around the newly trimmed root ball, making sure soil gets into all the cracks and crannies that now exist between the root ball and sides of the container. You may need a stick or trowel to stick down the sides, to make sure you have filled all the holes.
Water generously when done and add additional soil if needed. Make sure to keep your plant well hydrated for a few weeks so it can recover and thrive.