For the third year, I have removed a boxwood standard with its root ball from a decorative urn and planted it for the winter in a protected bed. To this point, I have had no problems. Each fall, I simply pop the plant out of the urn and plant it in the ground. I am concerned that the plant will become either root bound or the soil of the ball needs replacement. Next spring, should I carefully remove some of the soil or root prune before replanting?
This is such a great way to protect and prolong the life of your boxwood, which would otherwise not have much chance of survival in its container in a hard winter here in Toronto. When you remove the boxwood from its container in the fall, you will be able to see whether the roots are taking up much of the space in the container. If so, the plant is root bound, and root pruning would be beneficial in the spring when you are about to put it back into its container. Once the boxwood is in the ground and winter has taken hold, its roots will not continue to grow.
If your container soil is several years old, it may have become depleted of nutrients. Repotting your boxwood in fresh container soil, or top-dressing your large container may help your boxwood thrive. As for removing all the soil around the root ball, this isn’t necessary, but it may be helpful, before you repot your boxwood in the spring, to gently remove enough so that you can do a thorough inspection of the roots for breakage and damage, which you would prune away.
Here is an informative video on the basics of root pruning that you may find useful: