The term "root-bound" means that the roots of a plant have completely taken up space within the pot that contains it, often circling and creating a dense web of roots. This can form a compacted, hard ball that will slide out of the pot in a mass, retaining the shape of the pot, or the roots may be escaping through drainage holes in the bottom of the container. Such a plant is often unhealthy because the tangled knot can stress the plant and deprive it of nutrients, air, and water.
There are two primary solutions for a root-bound plant. First, you can repot your plant, loosening the roots and 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 when you have a larger pot available. But, if the plant is in a favorite pot that you want to keep using, or if you don't want your plant to get larger, it's a better solution to root-prune your plant.
Root-pruning a root-bound plant sounds intimidating, but it's an easy procedure if your plant isn't too big, and it may even save the life of your plant. It takes some nerve to root-prune a potted plant, but it truly is a kindness when a plant has outgrown its pot. If your plant is living in a terrarium, is a bonsai, or a containerized tree, you can root-prune it in order to maintain its small size.
When to Root-Prune a Potted Plant
A potted plant should be root-pruned whenever the plant's roots grow to the extent that they entirely fill the pot and begin to circle around the inside surface, or if the roots visibly escape through drainage holes.
How often this needs to be done will depend on the type of plant and how rapidly it grows, but it is usually easy to tell when root-pruning is necessary because the plant's root ball will slide out of the pot in a solid mass when you attempt to lift it by the stem, or when you turn the pot upside down.
In severe cases, it may be nearly impossible to remove the plant from a tapered container (where the mouth is smaller than the width of the middle of the pot) if the plant is root-bound.
Equipment / Tools
- Hand cultivator
- Pruning shears or sharp knife
- Long knife (if needed)
- Stick, pronged cultivator, or fork (if needed)
- Small bag of potting soil
- New flower pot (optional)
Examine the Root Ball
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 slightly compress the sides of the pot to loosen it.
Put your hand at the base of the plant and slide or gently jiggle the rootball 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.
Trim the Roots
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, as though you are harming your plant, but a plant's roots can take a lot of abuse, and the plant may thank you for it.
For extremely root-bound plants, you can cut away the bottom quarter of the old roots to help regenerate healthy growth.
Loosen the Rootball
Using your fingers, gently tease apart the rootball. If it's seriously entangled, 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 rather than continuing to grow in circles and strangle the plant.
Prepare the Pot
Add potting mix to the bottom of the container. Make sure there is enough soil so that the now-smaller root ball will sit on the soil at about 1 inch below the rim of the pot. Make sure that the crown of the plant—where the plant stem meets the roots—is at soil level.
Repot the Plant
Finally, place your plant back into the pot and add soil around the newly trimmed rootball. Make sure soil gets into all the cracks and crannies between the rootball and sides of the container. You may need a stick or trowel to move around the sides of the pot and make sure you have filled all the voids.
Water the Plant
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.
Tenenbaum, Frances, et al. Taylor's Master Guide to Gardening. Houghton Mifflin, 2001.