Most gardeners are aware of zone systems that help us determine the plants we can grow in our gardens. Perhaps the most commonly used zone map in North America is the USDA Plant Hardiness Zone Map, which is based on area temperature readings. It defines 11 zones, with each zone determined by the average lowest temperature in that area. Zones rise in increments of 10 degrees: the higher the zone number, the warmer the area. However, these zones are only guidelines, not hard and fast rules for what you can and cannot grow. Here are some additional factors that affect your choice of plants:
The USDA Plant Hardiness Zone Map divides areas by temperature, but it is not perfect. Each area and garden has mini-zones (called microclimates) that no zone map can show. Microclimates are areas that are different from the surrounding area in topography, exposure to wind and sun, drainage, or other factors. The differences in these microclimates may add or subtract a zone or even two. For example, a south-facing wall may be a zone warmer than the rest of your yard because the heat absorbed by the wall warms the soil in that particular area.
The weather also plays an important role because zones represent the average low temperature for an area. A particularly harsh winter in Zone 5, for instance, might actually be a Zone 4 or Zone 3 winter. Likewise, an especially warm winter might only be as cold as a Zone 6 or Zone 7 winter. This is especially true for gardeners who live on or near a border between two zones.
The kind of plant you grow makes a difference. Trees, shrubs, and woody vines are exposed to whatever winter temperatures your area receives. Herbaceous perennials and biennials, on the other hand, die to the ground each year. This keeps their crowns protected in the soil from some of the harshest weather. The fact that they die back also allows you to mulch them. Several inches of winter mulch can help moderate the effects of cold weather.
Strong, healthy plants withstand harsher conditions than plants struggling to survive. If your plants were stressed by a summer drought, they might be more susceptible to winter damage than if the summer had been moist and mild, for example.
Soil also plays a role in plant hardiness. Many herbaceous plants survive better in sandy soils because these soils drain well. Clay soils, on the other hand, retain water. When the water freezes around plant roots, it can compromise their hardiness.
The USDA Plant Hardiness Zone Map does not take into account summer heat, either. Many plants, such as Himalayan blue poppies (Meconopsis spp.), do not withstand warm temperatures, even though they tolerate quite a bit of cold. The American Horticultural Society has created a Plant Heat-Zone Map that marks areas in the U.S. by the number of annual days above 86°F (the point at which plants suffer heat stress). Heat-zone ratings are gradually appearing in catalogs, books, and plant labels.
Other zone systems
Natural Resources Canada and Agriculture and Agri-Food Canada released a map of Canadian zones that is quite different from the USDA map. Additionally, many gardeners on the West Coast rely on a zone system created by Sunset Publishing that takes into account other factors besides temperature. The important thing to remember is that no zone system is perfect, and should only be used as a guide.