Last week I discussed weather in Wisconsin. We have had quite the roller coaster of weather since then! I promised to write about plant hardiness this week. To have the most success when choosing a shrub, perennial, or tree, the plant must tolerate the year-round weather conditions it is exposed to, such as lowest and highest temperatures or rainfall amounts.
Plant hardiness is the one factor that gardeners can use to guide plant choice and suitability. It is determined by temperature extremes, both low and high, along with many other environmental factors. As I mentioned last week, Sheboygan County USDA Plant Hardiness Zone is listed as 5a (http://www.garden.org/zipzone/). This simply means that a plant listed as being hardy to zone 5a can withstand consistent winter temperatures between -20 and -15 degrees F. Any plant listed as a zone 5a or below (4b, 4a, 3b, 3a, etc.) will survive well here in Sheboygan.
Plant hardiness is genetically predetermined. Some plants are hardier than others and some varieties are hardier than others. A species of plant growing in a southern location may not be as hardy as one growing in a northern location because of genetics. Examples can be found all over the plant world. A few that come to my mind are Rhododendrons, Azaleas, Hydrangeas, and American Redbud trees.
Other things to take into consideration are the plant parts and how they respond to temperature extremes. Roots, stems, leaf buds, and flower buds usually all are hardy to different temperatures. Many perennials die to the ground in winter. Their roots survive the winter which allows them to show their beauty the following spring! To give you an idea how this may affect your plant, late spring freezes may be the culprit for forsythia having leaves but no flowers. Those late frost can even impact some fruit trees not bearing fruit. The flower buds, being less hardy than the leaf buds, are killed by cold.
Hardiness is truly a representation of global latitude. It shows how location affects our perception of plant hardiness. A particular, plant such as impatiens, coleus, or bananas may be perennial in a southern climate only to become an annual if moved to a colder climate like Wisconsin. Many of these plants are what we use as houseplants! Besides being called annuals or houseplants, another term that I have used for these types of plants is temperennial. So, when looking at catalogs, remember that a grower in Florida may sell perennial coleus, which would be true in his or her climate, but not true for us.
So far, I have focused on hardiness zones. To confuse the issue more, another newer concept is the hardiness zones for heat–in other
words, how high a temperature can a particular plant endure. The American Horticultural Society (AHS) developed the Heat Zone Map between 1997 and 1999 (http://www.ahs.org/publications/heat_zone_finder.htm). It is based on the number of “heat days” experienced in a given area. A heat day is defined as a day in which the temperature climbs to over 86 degrees F. Birch, maples, lilacs, and other perennials will not thrive in hot climates.
Hardiness zones are simply a tool for us gardeners to use. They are simply a guideline to help you as a gardener decide what plants will grow better under the various weather conditions we have here. Just know, what’s growing well in your garden now should continue to grow well. The usefulness of hardiness zones have always and will continue to depend on how well plants are tested and labeled. One thing for sure, the newer Plant Hardiness Zone maps will make it more difficult for people like me, who like to push the plant boundaries, to come up with excuses for high plant mortality.