“My gardening friends are advising me to use an anti-desiccant spray on my boxwood and evergreen rhododendrons to help protect them this winter. Is this worth doing?”

— Alvaro Hernandez, Highland Park

During the cold winter months, evergreens continue to lose water vapor through their leaves (or needles, which are modified leaves). The leaves must replace the water by pulling it up from the roots.

Winter injury due to the leaves drying out can occur when water is transpired or lost from the leaves more quickly than the roots can supply it to the leaves. This can be caused by long dry periods of cold weather and thawing, along with winter winds.

Evergreens exposed to more sun can also be more susceptible to winter burn. Discolored or “burned” foliage will start to appear on the plants in spring as the weather begins warming up. Signs of winter burn include golden or brown needles, as well as foliage that has developed an overall yellowish or off-green color. Leaves may appear bleached.

The damage commonly occurs on the south or southwest side of the plant or on the windward side of a plant that is exposed. The symptoms tend to show up quickly in spring when days are sunny and warm.

Anti-desiccant sprays work by adding a protective waxy coating to the leaves of broadleaf evergreens, such as your boxwood and rhododendrons, to help slow down water loss through the leaves. Generally, they will help retain moisture in the leaves for up to four months in the winter, though they tend to vary in terms of their performance. I do not use them to treat my broadleaf evergreens at home, as I feel they are unnecessary in most garden situations.

The Chicago Botanic Garden does not use anti-desiccant sprays as part of the routine maintenance program but focuses on siting broadleaf evergreens properly. They generally benefit here when placed in a site that has some protection from winter sun and wind.

The Garden does have boxwood growing in full sun and exposed locations that will occasionally show some winter burn. Any evergreens planted in the last three years will benefit from supplemental water, if late fall and early winter conditions are warm and dry. They will be more likely to suffer winter damage if they are under drought stress as they go into winter.

I do not feel you need to use an anti-desiccant spray on your plants — especially if you have not had any winter burn on them in past years. If you decide to try using one, then it is important to follow the label instructions. They are best applied to dry foliage, with temperatures around 40 to 50 degrees Fahrenheit and no rainfall in the immediate forecast, because the spray needs time to dry.

The plants should be completely dormant before applying the spray, so your timing will most likely be in late November or early December. Be sure to apply the spray to both sides of the leaves for proper protection. Do not spray waxy blue conifers such as blue spruce, as they have a natural coating that you do not want to damage.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.