Experts and BHG readers answer.
What to do with root bound begonia?
Thanks for writing. Without seeing the plant and knowing for sure what variety it is, it's tough to give much specific information. But many begonias can easily be cut back. Use the top 4 inches or so of the growth to use as cuttings; pot up in moist potting mix, sand, or perlite. Keep them moist and out of direct sun and they should root pretty readily.
You'll be able to tell if the plant is rootbound by carefully sliding it out of the pot. If you see lots of roots growing in circles around the rootball, then the plant is rootbound. Gently loosen the roots and repot in a container that's 2-4 inches wider.
---Justin, Senior Garden Editor, BHG.com