I have 2 very ancient wysterias that flower profusely each year and completely cover the front of an old building which needs to be renovated. The trunks of the wysterias are between 3 and 4 inches thick. I need to severely cut them back in order to carryout the renovation ( I need to get as close to ground as possible). How hard can I cut them back without losing them and what is the best time of year? Any advise would be grately appreciated.