Zone 6 winter - You hardy enough?

Ignoring your plant hardiness zone will cost you!

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.

Source: USDA
(Click to enlarge)

ZONE Minimum temperature
Zone 6    0F to -10F
Zone 5 -10F to -20F

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 whack 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 always "froze to the ground"... Convex Leaf Japanese Holly.

A cold Pittsburgh winter will freeze it to the ground, especially in an exposed location
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 entirely lost.
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 acts as an insulator to winter cold and wind
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

Check your trees in Spring


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