Both of those protections, antidesiccant spray and burlap barrier, will help your boxwood shrubs survive the winter with more limited damage, though the physical burlap barrier is considered better. Both methods are designed to reduce the amount of moisture that is lost from the leaves through transpiration. Transpiration is a natural plant process but damaging if excessive at a time of year when the ground is frozen and that moisture cannot be replaced.

The antidesiccant spray will coat individual leaves with a filmy layer so that less transpiration takes place in cold, windy weather. It mostly covers the outer leaf tops, leaving the leaf undersides unprotected and is considered to limit transpiration but not stop it. The greatest impact of cold on a broadleaf evergreen is in mid to late winter, so a spray that has been applied to be in place then works best. The coating does wear off over the winter and may not be as effective if applied too soon. Follow directions carefully; coating leaves too thickly may interfere with healthy gas exchange. 

The burlap barrier breaks the wind from its strongest direction so protects all parts of the plant. It can also shield the plant from roadway de-icing sprays and strong winter sun "waking" the plant before temperatures plunge again at night. (That can also lead to winter damage and dieback.) If your boxwood is near a sidewalk or road, or in a sunny position rather than light shade in winter, this has a real impact. You do not need to neatly wrap each boxwood in a tidy garment, just create a fence of burlap on stakes that limits wind, salt spray and sun from their strongest direction. If you do wrap plants more closely in burlap, remove it in late winter so that a damp covering doesn't encourage fungal growth as temperatures rise.

Take measures to see that boxwoods are watered until the ground freezes and during any warm breaks in the winter weather.

Mulch lightly and check that mulch has not drifted up to the leafy base of the shrub where, along with the dense leaf structure, it can make a cozy nest for rodents that will strip the bark from the shrub and may kill it.

Courtesy of NYBG Plant Information Service