Until this 2006-07 winter, it seemed like our winters were getting
milder. After all, there was less snow and cold. One recent year it stayed
mild until Christmas. Our minimum
temperature around Pittsburgh rarely sank below 10° Fahrenheit.
This trend created a false sense of security when it came to the
hardiness of outdoor plants. Landscapers, nurserymen and homeowners
began planting more trees and shrubs that were previously considered
"marginally hardy" in our area.
Hardiness is determined by the minimum low temperature experienced
in a geographical area. These zones are well illustrated on maps
like the one shown below. Pittsburgh has always been a solid Zone 6,
meaning the minimum winter temperature can easily drop to -10° F. During
particularly cold winters, we can become Zone 5 with temps
reaching down to -20° F.
(Click to enlarge)
|| 0°F to
||-10°F to -20°F
The local weatherman offered a harsh reminder of
that reality last night when he mentioned the -19° F we experienced
during the winter of 1994. Our current forecast calls for a low
of -3° F the next two days. This sort of low temperature is real Zone 6
stuff, and any plants that are only hardy in Zone 7 or Zone 8 may be
in big trouble.
What can happen to marginally hardy plants? If the cold doesn't
them, it may just freeze them back to the ground, leaving the roots alive. Back in the 80's, when our winters were
more consistently Zone 6 while venturing into Zone 5, there was one plant that
"froze to the ground"... Convex Leaf Japanese Holly.
Japanese Holly (Ilex crenata)
One of our clients had several of these Ilex planted along their
front foundation. No sooner than those plants got to be 4-feet tall, we
would have a Zone 6 winter and the plants would freeze to the
ground. The following spring we would cut them back to low stumps
and they would begin growing back. During the grow back period there
were large gaps in the front landscaping, but at least they weren't
The good news right now is that we do have some snow cover. Snow insulates
plants from severe cold, like a blanket. Nurserymen much prefer
having some snow cover, instead of an "open winter" where there is
severe cold without the protection of snow.
Snow "blanket" around shrubs
(Hold the salt please!)
This spring, if you find that some of your marginally hardy plants
got whacked, you may want to replace them with nursery stock that is
hardy in Zone 6. If you really want to play it safe, chose plants
hardy in Zone 5.
Stay warm (and think Spring!)
Getting your Ash kicked
your trees in Spring