Q: How do I prevent my arborvitae from dying every winter?
About five years ago, we planted a row of arborvitae in a raised bed. We usually lose two of them each winter, but last winter we lost seven. Is there anything we can do to prevent them from dying?
—Michael Frederick, Clinton Township, Mich.
Roger Cook replies: Arborvitae are usually just about bulletproof, but they're not immortal. In wet areas, I've seen them get root rot, but this isn't usually an issue in raised beds, where underwatering is the more common problem. Once a bed freezes solid, the roots can't take up any moisture, which means winter winds can dry them out to the point of killing them.
So try this: Set up drip-irrigation lines or a soaker hose and give the trees a good long drink once or twice a week from October until the ground freezes. Cover the soil in the beds with a 2- or 3-inch-thick layer of pine bark mulch, but keep it 6 to 8 inches away from the trunks. This will help hold in soil moisture and prevent the roots from drying out.
Finally, spray the trees with an antitranspirant, such as Wilt-Pruf. This clear, polymer coating reduces the amount of moisture a plant loses during the winter, particularly when it's windy. Make sure the product is labeled for use on arborvitae and that it's applied at the right time. Generally, you want to spray these shrubs early in winter and again in January, whenever daytime temperatures rise above freezing.
If you don't want to bother with antitranspirant, wrap each shrub with burlap before winter sets in. Or protect the entire group with a windscreen made of burlap stretched between wooden stakes.