I have planted emerald green arborvitae evergreens in our above ground planters two years ago. last spring i replaced about half of them due to winter kill this last fall i covered them with burlap and watered good before freeze up and lost almost all of them this winter.The planters are 16" wide by 68" long and 16" deep. there are four plants per planter and six planters. i am wondering what can be done to provide screening and what to change to help them survive