Selecting Plants: Will That Plant Grow Here? The Reality of Zone Maps.
No matter how hard we try, most landscape designers and home gardeners cannot stop thinking about the plants that might be incorporated into our projects. Over the years, to keep my dreaming in check and avoid costly mistakes, I've worked out a list of eight general criteria for selecting plants. The first four deal with the physical environment, while the last four are concerned with design essentials. Here, I have listed them in what I believe to be their relative rank of importance:
- Climatic adaptability or hardiness
- Soil requirements
- Insect and disease susceptibility
- Sun and shade requirements
- Size and form
- Growth and development rate
- Texture
- Color and all season interest
The first question anyone making a garden and choosing plants must answer is: "Will that plant grow here?"
The concern is whether my climate will support growth of this plant. Let's begin with the broad concept of macro, or big, climates and then in a future article we can identify conditions that lead to micro, or small, climates.
Scientists have collected temperature data and incorporated them into maps known as Plant Hardiness Zone Maps and Climate Zone Maps. These identify various climatic limits, including temperature, that control plant growth and help garden designers solve different plant selection problems.
The USDA Plant Hardiness Zone Map of 1990 is probably the simplest plant hardiness zone map, making it a quick but valuable reference and probably the one most frequently used. Its zones and temperatures, from zone 6 through zone 10, have also been used to formulate various Plant Zone Maps for Europe.
The USDA map is based on the concept that plants die in winter when temperatures are too cold for them and is developed from ranges of average minimum temperatures. These range from below -50°F to 40°F and above, and are divided into eleven different zones, each of which shows an area of winter hardiness for native plants as well as for those used in agriculture and horticulture. Zone 11 represents areas that are essentially frost free. Zones 2 through 10 are further divided into subzones: "a" with the lower end of the range and "b" with the higher end.
Most plants sold today are given ratings showing in which zone they will probably grow best. The map, however, does not take into account the differences between air and soil temperatures, or between day and night temperatures, or differences in duration and amounts of rainfall, in humidity, or in daylight, or in effects of slight changes in natural topography or stress caused by artificial environments. Consequently, these USDA ratings are of general use but are not guarantees.
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