A rootbound plant means that the roots have completely taken up the pot, often circling and creating a dense web of roots. This can form a compacted, hard ball that comes out of the pot with effort, retaining the shape of the pot. If often means the plant isn't healthy because the roots have replaced the potting soil. The tangled knot of roots can stress the plant and deprive it of nutrients, water, and food.
Solutions for a Rootbound Plant
There are two primary solutions for a root bound plant. First, you can repot your plant, putting it in a larger pot so the roots have room to expand. This is a good solution if you want your plant to keep growing and have a larger pot to use. But, if you have a pot you are particularly fond of, or you don't want your plant to get larger, it is more desirable to root prune your plant. Root pruning a rootbound 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 rootbound 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. If your plant is living in a terrarium, is a bonsai, or a containerized tree you can root prune to maintain the small size.
How to Root Prune
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. 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. 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.
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 harming your plant, but plants' roots can take a lot of abuse.
When you are done, take a stick, pronged cultivator, or a fork and loosen the 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 and not continue to grow in circles and strangle the plant.
Repotting the Plant
Get ready to repot the plant 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, where the plant stem meets the roots, is at soil level.
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 move around the sides of the pot, making sure you have filled all the holes. Water generously when the repotting is 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.