Pruning Box Plants
Am I too late to prume two large box balls in pots please?
Yes, I'm afraid you are. Pruning Box plants now will leave them very susceptible to frost damage.
Box should have it's last clipping by the end of August to allow the exposed shoots time to harden off before the winter.
The earliest it would be safe to prune, depending on where you are, is in May. But make sure you leave it until all risk of frosts has passed.
Traditionally Box is trimmed on Derby Day which is on June 1st next year.
I hope that helps.