we are in abbotsford BC. we have a large cedar hedge (looks like western red)that provides privacy and cuts down hwy 1 noise. But it is getting to tall and the mid section seems to be thinning. It is 30-40' tall I think I thought if I cut 12-15 feet off of it and trimmed the sides it would thicken up but I hear that may not be the case What should I do and when??