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Index : Miscellaneous Gardening Topics
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- 47. Plant Hardiness Zones - Top
- The first step in determining what garden and landscape plants grow in the Tucson area is to look to the plant hardiness zone maps. We have two to choose from. One is the U. S. Department of Agriculture Hardiness Zone map, used by much of the country. The other is the the Sunset Western Hardiness Zone map, which includes the area west of the Rocky Mountains.
The USDA hardiness zone map is based on winter minimum temperatures and little else. This is fine for relatively flat areas of the country, where topography has little effect on climate. It also works well where average annual rainfall tends to be rather uniform. But in the western region of the country elevations and rainfall amounts fluctuate radically, even within the same localized area. This has a major effect on temperature ranges, length of growing seasons, and plant adaptability. For this reason, Western climate zones were developed by Sunset Publishing in cooperation with the University of California.
Using the Sunset Western climate zone map, Tucson is in zone 12, described as the intermediate desert. The elevation range in this zone begins where the low desert ends, at 1500 feet, and extends up to 3300 feet, where the high desert begins. This zone is considered semi-arid, with annual rainfall supporting lush desert growth. Growing seasons are long, averaging 8 months between frosts, and 9 months between killing freezes (28° F).
The Sunset Western Garden Book lists over 6,000 species of garden and landscape plants and their hardiness zones. There is also a CD version of the book, in which a plant search can be conducted by hardiness zone.
Most plant reference books do not list Western hardiness zones. However, by cross-referencing with the Sunset Western Garden Book, the hardiness zone for any given plant can be determined.
Mail order garden catalogs almost exclusively use USDA hardiness zone designations. Here in Tucson we are in zone 9, which distinguished by average minimum temperatures from 20° to 30° F. A common mistake in ordering plants is to select from colder hardiness zones, 8 or lower. These plants would be able to withstand our cold, but not our summer heat and drought. Plants in colder hardiness zones also often have cold dormancy requirements. For instance, certain fruiting plants, such as raspberries, must have a minimum number of chilling hours during the winter. If they do not receive this cold, they will not produce fruit and the plants will eventually die.
Select fruit varieties adapted to our climate. Many varieties of apples are advertised to grow in zone 9, but only two, Anna and Dorsett Golden will adapt to the Tucson area. Other deciduous fruit trees such as peach, apricot, plum and pear have a limited number of varieties which will grow in our zone 9 conditions.
Some plants may have they cold dormancy requirements met in zone 9, but still may be unsuited to the desert. An example is the red maple tree (Acer rubrum). It is advertised for zones 3-9, and will grow in eastern regions of zone 9 where rainfall and humidity is high. It will not grow successfully in our zone 9 desert!
Some plants listed for zone 9 may not be adapted to our soil conditions. Mountain laurel and gardenia are listed for zone 9, but both require an acid soil. Our very alkaline soil causes these plants to turn yellow from iron and other micro nutrient deficiencies. Both need rich, moist, organic soil and lots of moisture and humidity. Again, a bad choice for Tucson.
In determining a plants adaptability to our climate, rely on local reference sources. Books written by authors and about plants of our region can be used. Visiting at local garden centers and nurseries is always a help. Garden clubs and plant societies can suggest varieties in certain plant groups, such as roses, herbs, cacti, etc. Recommended plant varieties can also be seen at the Tucson Botanical Garden, Tohona Chul Park, and the Sonoran Desert Museum.
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Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on July 26, 1998
- Updated: July 26, 1998
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