Plant hardiness zones are the gardener’s guide to which plants can be expected to survive in any given area. They are based on average winter low temperatures. Most of Southeast Wisconsin, including Ozaukee County, is in Zone 5b, which means that an average winter will see temperatures plunging to minus 10 to minus 15 degrees. The original hardiness zone map was redrawn in 1990 and was based on weather data from 1974-1986. An interactive map, with slight modifications and based on data from 1976-2005, is available on the USDA website at http://planthardiness.ars.usda.gov/PHZMWeb/. Changes were made because winter temperatures were not getting as cold as they used to and gardeners were able to successfully grow plants that in years past had succumbed to frigid temperatures. Midwest readers are probably saying, “But wait. Didn’t we just have temperatures in the 20 to 25 or more below zero range?” Right you are, which puts SE Wisconsin in Zone 4. I await with trepidation the fate of some of the plants in my yard! Local temperatures are not the same as global climate, however, and the reality is that global temperatures have been steadily rising. I could give a strong argument that the burning of fossil fuels, the destruction of forest land, especially in the tropics, and the subsequent rise in CO2 levels are major contributors, but that is something to be debated in another column. What I want to discuss here is how the average gardener can grow plants that may be on the hardiness fringe, and give a few examples of plants we are likely to be successfully growing in the future, should this warming trend continue.
Plant hardiness is not affected by winter temperatures only. The ability to withstand summer heat, soil type and drainage, and exposure to sunlight are also contributing factors. When I was living in Central Illinois I found it very difficult to grow Firs (Abies.) These plants are generally native to northern areas or higher elevations and would suffer in prolonged summer heat. Wisconsin is much more to their liking. Coral Bells are another plant that like my Cedarburg soil better than what I had in Peoria. Central Illinois is known for its deep, rich, silt loam soils. They may be great for growing corn and beans in the summer, but tend to hold a lot of moisture in the winter, causing roots to be exposed from the alternating freezing and thawing of the soil. My sandy loam Cedarburg, WI soil is much more conducive to growing shallow rooted plants such as Heuchera. Though they tend to dry out quicker, plants are much less susceptible to winter heaving.
Winter sun can be a problem with evergreens in areas with prolonged snow cover. A few years ago we had the worst problem with winter sun damage in recent memory, resulting in brown foliage and areas void of live foliage. Fortunately, most of my plants have recovered from the damage, but signs of the severe injury still linger on some. This problem can be prevented by placing susceptible evergreens on sites that are in sun during the summer but in shade during the winter. The lower angle of the sun during the winter has allowed me to grow a couple of conifers that would otherwise not be possible in our area. The most notable is a dwarf form of Japanese Cedar (Cryptomeria japonica ‘Globosa Nana’) that I purchased in Durham, NC five years ago. The price was right and it was a beautiful plant, so, being a gardener, I thought I’d take a chance with it! I sited it where it receives full sun in the summer, but complete shade in the winter. So far this Zone 6 plant has been growing well with minimal damage.
One of the enjoyments of gardening is to try plants that stretch the hardiness limit. A rule of thumb is that you have to kill a plant three times before you give up on it! Sometimes it takes that many tries to find the right location. There are a few plants that I grew in Central Illinois that I would love to try in my Wisconsin garden. Flowering Dogwood (Cornus florida) and Sassafras (Sassafras albidum) are two trees that come immediately to mind. I was even able to grow Crape Myrtle (Lagerstroemia indica), a Zone 7 small tree, as a shrubby perennial. It would die back to the ground each winter but came back from the roots in the spring. Though I don’t expect the streets of Cedarburg to be lined with palms anytime soon, don’t be afraid to stretch the hardiness zone and try something new. With proper placement, you may just be successful.
Cryptomeria japonica ‘Globosa Nana’
The Cottage Gardener 