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| Rhododendrons respond to cold weather. |
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 using an
antidessicant on these plants in early winter. We try to spray the plants with antidessicant on a dry day when there’ll be
no precipitation and the temperature is above freezing (around 40 – 50 degrees
is good); you shouldn't apply the spray in freezing temperatures.
It's important to 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.
-Ken Almstead, CEO and Arborist
