But the tag said it was hardy? 

You are frustrated or maybe even angry.  You spent a ton of money on your landscape and the plants that you bought were supposed to be hardy for your area.  The tag that came with the plant indicated that it would tolerate a minimum temperature of 0 to 10 degrees F.  It didn’t get any colder than that in your yard so why then did your plant suffer so much damage and possible death?  The answer isn’t simple but I think I can help explain.

In 1927 the Arnold Arboretum published Hardiness Zone maps for the continental US.  These were intended to help guide gardeners in choosing plants for their area.  In 1960, the US Department of Agriculture got into the act, publishing its first map, based on the data from 450 weather stations around the country.  In 1990, the US Department of Agriculture, in cooperation with the US National Arboretum, updated the USDA hardiness map, using data from between 4,800 to 14,500 weather stations.

If you Google US hardiness zones you will come up with a map where you can enter your zip code and find out what your town is rated at.  Marysville and Lake Stevens for example are rated a zone 8, which means normally we have minimum winter temperatures of 10-20 degrees.  Arlington is a 7-8 which means you could experience 0-10 degree minimums.

What the winter hardiness map doesn't tell us is how many times the temperatures dropped that low, and how long these low temperatures lasted. There are a number of plants that can survive 5 degrees F for a couple of hours, but could not survive these temperatures for a longer period, or more than once during a winter. Cold temperatures for one night is not the same as cold temperatures for a period of weeks, which is exactly what we experienced in December.  

Another factor not taken into account by maps is winter acclimation. A plant growing in our gardens in midsummer can be easily killed by temperatures in the 20 degree F range. The same plant, if properly acclimated, can withstand temperatures of -20 degrees F. We see the exact same thing in some late spring frosts. After a certain number of hours at a specified temperature, each type of plant will switch from a dormant winter mode to a growing spring mode. It is at this point that winter hardiness is lost. If a late frost occurs while the plant is still in its dormant mode, there is little, if any, damage. If the late frost occurs after the plants have switched to active growing mode, even a mature tree may be killed.

Think back to the fall of 2008 and remember how beautiful it was.  There were lots of sunny and warm days, not the kind of weather that helps a plant get ready for winter.  As a result of this Indian Summer, our plants were not as hardy as they might have been if the fall had been a more typical crappy cold and wet season.

Something else that affects winter hardiness is a lack of summer heat. A warm summer increases sugar production, which allows plants to survive more stress in the winter. In areas without summer heat, a particular plant may only be hardy to 20 degrees F, while in an area with hot summers, the same plant may easily be hardy to 0 degrees F. Again, remember back to last year and how cold the spring and summer was and that will help explain why the damage was so severe. 

What about the snow you say?  In most cases, snow is a gardener’s friend.  Except for the damage it can cause on limbs due to the weight, snow on the ground will help insulate the soil and actually protect our plants.  Plus, whenever it is snowing it is almost never as cold as when it is clear and the stars are out.  Don’t worry about snow killing your plants.

Too much TLC can lead to reduced winter hardiness as well.  People who fertilize heavily or subscribe to companies that apply fertilizer on a monthly basis are setting their plants up for a disaster such as we recently experienced.  I had a customer tell me she had the best looking yard on the block last fall and now has the worst.  Hopefully she will learn from her mistakes.  You have to let plants slow down and take a break.

Finally, late pruning will cause some plants to produce new growth which can be easily killed since they have not had a chance to become acclimated to the cold temperatures. 

So to summarize, the hardiness of a plant can be affected by a warm fall, a cool summer, extended cold snap, excessive fertilizing, or late pruning.  Three out of five of these conditions happened for sure last year.  And that is why so many plants now look like hell.