Last year I had a lot of browning on one of my evergreen yew plants. It is next to the house near the dryer vent. The vent's hot air caused my plant to dry out quickly, with some areas dying completely. This year I am prepared to protect this plant and others like it. Here is how I'll do that.
Narrow-leaved evergreens such as yews, arborvitae, and hemlock are especially susceptible to winter browning, also called desiccation. Broad-leaved evergreens such as rhododendrons , azalea, and boxwoods are also prone to winter desiccation injury.
Winter desiccation is a result of water loss in the plant due to the winter sun, or in my case from the hot dryer vent. As the sun heats up the plant, it's needles (or leaves) open its pores and release water to cool itself. Basically, the plant sweats. If the ground is frozen or too dry, the plant cannot take up water to replace that lost through the needles, and thus the needles turn brown.
To prevent winter browning, be sure landscape plants go into the winter with plenty of soil moistures. Irrigate as needed to a depth of 18" for most shrubs. During dry winters, supplemental irrigation during winter months may be required. Irrigate during warm spells when the ground is not frozen. Water early in the day and do not allow water to collect and stand near the trunk where it may freeze and damage the plant. Mulching will also help to insulate the soil and conserve soil moisture.
It is best to place susceptible evergreens in locations where they receive partial shade and protection from desiccating winter winds. If this isn't possible, consider building protective screens to provide shade and windbreaks. They can be constructed with burlap or other materials. Temporary fences, such as snow fencing, can also be useful.
For high-value plants or in difficult areas, an anti-desiccant can be sprayed on the foliage to reduce moisture loss. This is what I use to protect the yews by my dryer vent. Also called antitranspirants, these products coat the leaf and reduce the amount of water that escapes. To be effective, these products need to be applied according to label direction two to three times during winter.
Rhododendrons also have mechanisms to help protect their leaves from winter injury. You may have observed rhododendron leaves rolling, curling, and drooping on cold winter days. This action is normal and reduces the amount of tissues exposed to direct sunlight and desiccating winds.
More information on this and other plant issues can be found on our University of Illinois Extension Focus on Plant Problems website at http://extension.illinois.edu/focus.