Today, painters covered up two of my boxwoods with clear plastic tarps to protect them from paint chips/paint. I noticed some moisture forming on the underside of the tarp in the afternoon. When the painters removed the tarp, the tops of the boxwoods had turned brown and were shriveled. What happened and is there permanent damage?
The Gardener’s Answer
Hello, Jessica in Maryland: It sounds like the new growth on your boxwoods was burned by the plastic. Not to get too technical but when clear plastic comes into direct contact with plant foliage, it can burn as a result of increased temperatures. Even on a cloudy day sun rays reach the ground, and more than 90 percent of these rays pass through clear plastic. As the heat is absorbed, it increases air temperatures; that is why a properly made greenhouse is so effective. Clear plastic acts as a transparent medium allowing short wavelengths of light to be absorbed, and the longer wavelengths that are absorbed by the plants do not allow the heat to be released, thus increasing temperature/humidity. In your case the plastic was touching the plants and this is why they were burned. Fortunately, it will not be detrimental to your evergreens. You should prune out the burned foliage and discard. It is still early enough in the season that they will put on new growth. Ideally the painters should use a black plastic that will reflect sunlight and not allow the temperatures to rise so drastically, protecting the plants from potential burn.