When you grow a plant in a container, be it a tomato for transplanting, a dwarf tree, or anything in between, their roots will continue to grow even when the top has reached its full size. Over time, they will begin to grow in circles to follow the container walls and become root bound. If left unpruned long enough, the roots will eventually eat up most of the soil in the pot and the plant could die of stress or starvation.
If you will be transplanting your plant to a larger container or outdoors, it is best to do so before it becomes root bound. However, if your plant is in its permanent home already or if you’re not able to transplant in time, you will need to prune your plant’s roots to keep it healthy.
Let your container do the pruning
You can easily eliminate the risk of root binding and make less work for yourself by planting in pots that naturally keep the roots pruned. This is possible through two mechanisms: air pruning and light pruning. Root contact with light and air triggers a reaction in the root tips to stop growing; this is the same reason why plants roots don’t grow above the surface of the ground in your garden.
Air pruning is the method used by Smart Pots and other fabric pots. When the roots grow to the edge of the soil, they come into contact with the air that passed through fabric walls. Further, the material that the containers are made of allows the root tips to grow into the fabric, but not sideways through the fabric, thus pruning the roots that grow into the container walls.
Light pruning occurs when using a white container such as white grow bags. The white color does not block light transmission like black or black-lined pots do. Instead, the light is able to transmit through the plastic to the edge of the soil inside the container where it prunes any roots that extend that far.
The air or light that passes through these containers’ walls does not affect root growth within the soil itself. It only affects the roots that grow as far as the edge of the container, pruning them at that point and preventing them from circling and becoming bound. If your plants will live in a container of this type for many years, it will still need to be fertilized and given fresh potting soil periodically.
Root pruning by hand
If your plant’s roots are getting too big for its container but you cannot move it to a larger pot, you’ll need to prune it by hand. Some plants are more sensitive to having their roots disturbed, so it’s a good idea to look up specific instructions before root pruning houseplants and other ornamentals.
The first step of root pruning is knowing when to do it. If your plant is root bound, it is time to prune! However, if you want to prevent your plant from becoming root bound and stressed from an overly small pot, you should do some root pruning maintenance every two to three years. For veggie starts and other transplants, you only need to prune the roots if they are circling at the time of transplanting.
To remove the plant from its pot, tip it on its side (or even upside down) if you can and gently pull it out by gripping the plant as close to soil level as possible. Be extra careful not to damage it! If it won’t budge, use a hori hori to loosen the tension around the inside edge of the container. Never use force to get a plant out of its pot.
Next comes the scary part… But don’t worry, your plant will actually thank you for this! Use a hori hori or pruning shears to cut away the outer part of the soil and root area. If your plant has a taproot, corm, or bulb, avoid cutting that part, but do prune the tiny feeder roots. Then use a single prong cultivator, such as a Cobra Head, and tease apart the roots. If your plant is especially root bound, you may want to trim the feeder roots at the interior of the root ball as well. You can prune up to two-thirds of your plant’s roots, although the actual amount you prune may not be that much. These steps will encourage your plant’s roots to grow in a new healthy direction instead of continuing their previous circling pattern.
Repot your plant at the same depth as previously. You will need to add new soil (not the old dirt with trimmed roots in it!) to replace what you cut away, which will also provide fresh nutrients for your plant. Water your plant right away, and give it a feeding of liquid kelp or Thrive Alive. Your plant may show signs of shock for a few days; just keep it properly watered and out of direct sunlight during the recovery period. The new roots that it will grow are better able to absorb nutrients than old roots, and with the fresh soil and new space, your plant will be better than ever.