Many of you with rhododendrons in your yard can tell the temperature outside by looking at the droop of their leaves. Rhododendrons are highly thermotropic plants: their leaves move in response to the temperature. As the thermometer falls the leaves begin to droop and curl; when the temperature goes well below freezing, the leaves are tightly curled and pointing straight down.
It’s hard to believe that these leaves will unfurl and rise again in response to warmer weather – but shrubs and trees are remarkably resilient. There are times however, when they can use some extra help.
As winter approaches, I recommend applying antidesiccants (also called antitranspirants) to most broadleaf shrubs. This is not a remedy for the curling leaves of your rhododendron – those leaves are supposed to curl and actually help the plant survive the cold. The stomata (the tiny holes on leaves that allow the escape of moisture) are tightly closed, preventing any moisture loss. The problem for broadleaf evergreens is not the cold, but the warmth. When a winter thaw occurs, your rhododendron leaves will straighten out, rise up and start sweating (actually transpiring). Because of the frozen ground, the roots may not be able to replace this water loss. Add some chilly winds, and the moisture loss can be dramatic. This is where leaf damage and loss -- and even plant death – may occur.
The antidesiccant adds an additional oily or waxy coating to the leaves, keeping the moisture inside. Many evergreens in our area are especially vulnerable to winter damage, particularly holly, rhododendron, cherry laurel, skip laurel, mountain laurel, Japanese skimmia, leucothoe, aucuba and boxwood. I recommend that you or your tree care company use an antidessicant on these plants in early winter. Pick a dry day when there’ll be no precipitation and the temperature is above freezing (around 40 – 50 degrees is good) to use an antidesiccant spray: do not use it in freezing temperatures. Coat both the top and undersides of the leaves. Antidesiccants are typically organic and biodegradable; they will wear off the leaves by spring.
Rose canes and hydrangea stems will also benefit from the spray, as will young trees with thin bark. Some evergreens with needles do not need antidesiccants; plants like arborvitae and spruce can actually be harmed by it, or at least lose their blue color.
When a winter thaw occurs, as often happens in our area, another coat of antidesiccant will help the evergreens stay hydrated until spring.