Karl Foord, UMN Extension Educator
If a particular temperature recording station has an average minimum low temperature of between - 30F to - 25F then the station would be in 4a. If the range was - 25 F to - 20 F then this station would be part of the 4b hardiness zone. The same logic determining the temperature ranges for Zone 2b [-45 to -40 F], Zone 3a [-40 to -35 F], and Zone 3b [-35 to - 30 F].
The average part of this calculation gave me concern because if it is an average then there must be numbers less than and greater than the average. The question then becomes how wide is the distribution around the average?
Minneapolis is in the 4a plant hardiness zone with a low minimum temperature range of -30 F to - 25 F. Over the last one hundred and eleven years low minimum temperatures have exceeded this range 10% of the time, but only by four degrees at the most1. Temperature data for the last 10 years (2000-2009) places Minneapolis in hardiness zone 4b. If we are willing to accept a 10% chance of a low minimum beyond the -20 F to -15 F range, then Minneapolis would be placed in zone 5a. The question is how many of your 5a plants would have died given the low temperature of -24 on January 30, 2004. The relatively milder recent winters gave rise to a revised plant hardiness map that can be viewed here. This map was subsequently rejected by the USDA.
I think it is critical to point out that the low minimum temperature is only one of a number of factors that come to play on a plant's winter hardiness.
Other factors include snow cover, temperature patterns favoring the development of dormancy, moisture conditions, and microclimate effects to name just a few.
Snow functions as an insulator protecting the root system of overwintering plants. Nine inches of snow can lead to a 42 F differential between a - 14 F air temperature and a 28 F soil surface temperature.
Reliable snow cover increases the temperature that the plant experiences and puts the region in a higher plant hardiness zone. This is why locations like the Upper Peninsula of Michigan are functionally zone 5 due to consistent deep snow falls, whereas without this snow the UP would likely be zone 3a.
Temperature Patterns Favoring the Development of Dormancy
The plant's ability to withstand cold temperatures is a function of the metabolic status of the plant. If the plant experiences gradually decreasing temperatures and is allowed to achieve full dormancy then it has achieved its optimum genetically programmed degree of winter hardiness. However in winters that are warm in the beginning of December followed by a significant temperature drop to average temperatures at that time of year, the plants are not metabolically prepared and will be damaged.
Cold dry winds tend to desiccate plants especially evergreen plants with exposed leaves. Plants entering winter under draughty conditions are further stressed leading to weak plants and mortality.
Local conditions can modify the climate experienced by the plant. Protected locations reduce the stress caused by cold dry desiccating winds. Highly exposed locations can increase plant stress. Slope affects airflow as cold air sinks into lowland areas called frost pockets. South facing hillsides capture more heat which can be advantageous for plants like grapes, but disadvantageous if the heat stimulates early flowering subjecting the plant to the risk of late spring frosts. There is an art to finding the right plant for the right location. I am a big fan of Japanese maples. Their microclimate over the winter is my garage. I for one am not willing to accept the risks of the one in ten chance of a true 4a or even 3b winter that we might experience in the Minneapolis area zone 4a. Especially consider microclimates if you live in 4a or 4b and have a lake home or cabin in 3b or 3a.
Data from the Minnesota Climatology Working Group for Minneapolis