Buddleia is surprisingly hardy..maybe a lot more hardy than most folks think. Try cutting back flat to the ground in the late fall, put a few bricks around it (Make sure you allow for root spread) and fill with polystyrene/straw/mulch etc. a polystyrene lid with a brick to hold it in place...uncover in the spring when the night lows are above 28f. Once spring has sprung this plant can grow from nothing to 8 or 10ft in a season in ideal conditions. It'll flower a few weeks later than those that are not cut fully back ....but flower it will!