Pruning Advice - Late Winter, Early Spring

Early Spring: Resist the Urge
to Prune Evergreens!

After a long winter, the much
awaited spring thaw is a time
of relief and celebration for all
of us. Though late winter and early
spring are primarily a time of rebirth,
for some evergreens, this time of year
can be deadly.

According to Bill Cullina, Propagator at The New England Wild Flower
Society, and renowned native plant author, evergreens, and especially
“broad-leaved” evergreens such as rhododendrons, mountain laurel,
hollies, and some magnolias and viburnums may be showing damage after
the tough winter we had. In addition to damage from the weight of snow
loading, what you might be seeing is winter desiccation, also known as
winter burn.

“If damage has occurred, resist the urge to prune off the dead branches right
away, as they may resprout either from the tips or a bit lower down on the
stem,”advises Cullina. “By late May, it will be obvious that certain branches
or the whole plant are not coming back, and you can prune down to just
above the live growth at this time. Fertilize the injured individuals lightly,
and try to keep them watered during summer droughts."

Winter burn is not an inevitable result during cold, snowy winters. If you
site your evergreens properly and protect vulnerable ones with burlap or
anti-desiccants, this unsightly and debilitating affliction can be prevented
entirely. Whenever the relative humidity falls below 100%, leaves lose
water. Most plants can control this somewhat by closing off the small pores
(stomata) that permit gas exchange in and out of the leaf. Rhododendrons
and their relatives, however, have no way to close these pores, and this is
the reason they are particularly hard hit during some winters.

Remember that, even in late February, the sun is strong enough during
midday to give you sunburn, and as it shines on leaves it heats them up just
as it does your skin. At this time of year, the air is typically very dry, so
water can quickly evaporate out of stomata into the parched atmosphere.
Normally, a tree or shrub will draw water from the earth to replace what is
being lost through leaves, but if the ground is frozen, replenishment is
impossible. With no water coming from the roots, the leaves and even twigs
and larger branches simply dry up. Often desiccation injury that happens in
February doesn’t appear until later, when temperatures warm and the dead
stems completely brown. One sure sign of winter burn is damage that is
limited to those areas above the snow line. Snow may cause problems for
leaves higher up, but it shields the lowest ones quite effectively. Newly
planted shrubs with under-developed root systems are particularly
susceptible.

Needled evergreens, including yews, pines, white cedar, and hemlock may
exhibit signs of winter burn also, but damage is subtler. Needles that are
winter-burned typically die back partway from the tip, and only in severe
cases will the entire needle brown and fall.

Salt injury is easily confused with winter burn, but it occurs either very near
salt water or along roads and highways where automobile spray lofts road
salt onto the foliage. The best way to tell salt damage from winter burn is by
the location of the affected tree or shrub. If it is near a busy road, chances are
it is the former.

The best way to protect your evergreens, and especially the broad-leaf
evergreens, from desiccation injury is to site them in a spot that is shaded
from the noonday sun during winter. Remember that the sun is much lower
in the sky at this time of year, and often your neighbor’s house or tree line
will do the job. You can wrap or drape existing plants with burlap in
December and remove it once the ground has thawed in late March.

Anti-desiccant sprays (available at most garden centers in the fall) can also
be quite effective if applied in autumn. They simply plug up the pores,
lessening water loss from the leaves. Be sure to spray both the upper and
lower surfaces of the leaves or needles.

For more horticulture advice visit the website of the New England Wild Flower
Society at www.newfs.org, sign up for membership, or visit Garden in the Woods at
180 Hemenway Road, Framingham MA 01701. Bill Cullina is the author of Native
Trees Shrubs and Vines, and The New England Wild Flower Society Guide to
Growing and Propagating Wildflowers, both available online at the web site. All
purchases support the conservation of our native flora. Phone 508-877-7630.