Answer: Since your mother lives in a historic home and boxwoods are certainly part of that heritage, you are right to be concerned about any form of severe pruning being a risk. I have observed shrubs suffering extreme winter storm breakage regenerate, albeit very slowly (at a rate of about 6 inches a year), however the damage did not consist of an all over shortening, but rather no more than about half the plant at a time. The plants also looked terrible for some years following the incident.
Some authorities suggest that it is fine to give boxwood a hard cutting in order to force it to grow from old wood. For instance, Michael A. Dirr in his Manual of Woody Landscape Plants mentions some old overgrown specimens with trunks in the 4 to 6 inch range which had been cut back to 1 and 2 feet from the ground and were filling in again nicely. (They were located in the Spring Grove Cemetary in Cincinnati, Ohio.) However, I would be extremely cautious about taking this drastic approach, especially if these plants are as large as you say and if there is any possibility that they are in less than fine health to begin with.
If you do decide to go ahead with any form of severe pruning, I would suggest that you first root a good number of cuttings from the original shrubs so as to preserve the original plant stock from the grounds. That way, should anything untoward occur, you will have genetically identical plants to use as replacements.
I would do the pruning in late winter and place the plants on a careful monitoring schedule. I would give the plants an organic mulch consisting of good rich compost and be prepared to ensure they receive adequate watering for at least the entire year following the pruning. I would also be prepared to risk the loss of some or all of the shrubs.
Finally, I have two other suggestions. The first is that it is quite likely some of the other historic properties in that area of the state have faced a similar dilemma. It would be most useful to know what they have done and with what results, since the varieties of old box, the microclimate and the soils specific to your area will have the most affect on the outcome. Perhaps your historical society could help you contact other property owners who have similarly aged boxwoods.
The latter is that you might contact the American Boxwood Society at ph# 540-939-4646 to get their advice.
Good luck with the boxwoods!
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