To renovate old Buxus hedge, when is the best time to cut back to 15-30 cm shoots? It appears that new shoots are pushing through now. My American Horticulture Society Pruning and Training book suggests late spring, but that may not apply to Seattle.
I consulted a local organization, Plant Amnesty, and their information sheet on Buxus says that April is the best time to prune. It is important not to prune when it is either too cold (leaves will turn grey) or too hot (same result).
Peter McHoy’s book, A Practical Guide to Pruning, also says to do your renovation pruning in mid- to late spring, and further suggests that drastically reducing the height of the hedge should be done in stages, over two to three years.