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17


The West’s Climate Zones


IN THIS BOOK, CLIMATE ZONES ARE LISTED FOR EVERY PLANT. To identify the Sunset climate zone where you live, check the map at left, then turn to the page(s) listed there for your state to locate your specifi c climate zone. Find a description of your zone on pages 18–21; if you live close to a zone line, also read about the neighboring zone, which might be a better fi t for your microclimate. The following factors make each zone unique.


Temperature Every plant has a mini- mum temperature that it can survive, so zone designations start there. Summer highs are also important, especially in the desert, where gardening mostly shuts down in summer.


Latitude Generally, the farther an area is from the equator, the longer and colder its winters. High latitudes also increase the number of daylight hours in summer and the length of night in winter.


Elevation Gardens high above sea level get longer and colder winters, often with intense sunlight, and lower night temperatures all year.


Ocean infl uence Weather usually moves from the ocean onto the conti- nent, bringing mild, moist air with it, especially in the cool season.


Continental air infl uence The North American continent generates its own weather. Compared with coastal cli- mates, it is colder in winter and hotter in summer, and is more likely to get precipi- tation any time of year. The farther inland you live, the stronger this continental infl uence. Incessant wind also becomes a major factor in open interior climates.


Mountains, hills, and valleys These determine whether areas beyond them will be infl uenced most by marine air or by continental air. The Coast Ranges take some marine infl uence out of the air that passes eastward over them. The Sierra-Cascades and Southern


California’s interior mountains further weaken marine infl uence. East of the Rocky Mountains, continental and arctic air dominate. In the opposite direction, fi rst the Rockies, then the interior ranges, and fi nally the Coast Ranges reduce the westward infl uence of continental air.


Microclimates Local terrain can sharply modify the climate within any zone. South-facing slopes get more solar heat than fl at land and north-facing slopes. Slope also affects airfl ow: warm air rises, cold air sinks. Because hill- sides are never as cold in winter as the hilltops above them or the ground below them, they’re called thermal belts. Low- land areas into which cold air fl ows are called cold-air basins. Microclimates exist within every garden (see page 22).


Growing season The average num- ber of days between the last frost in spring and the fi rst frost in fall is the growing season. Growing seasons, how- ever, don’t tell the whole story. For exam- ple, Zone 5 has a much longer average growing season than Zone 3a, but Zone 3a’s extra heat makes it a much more favorable climate for heat-loving fruits and vegetables.


Soil Although soil is not a climate con- dition, it is closely related to climate. Abundant rain usually drives soil to the acid side of the spectrum, while scant rain drives it to the alkaline side. Further, soil’s ability to drain when it’s raining and to hold moisture when it’s not is critical to plant health.


WHY DON’T WE USE OTHER CLIMATE ZONES?


The U.S. Department of Agriculture (USDA) calculates climate zones based on winter minimum temper- atures. It provides a useful index for plant hardiness—the ability to withstand frost and low tempera- tures—but it has some drawbacks. This focus on cold-tolerance alone places the Olympic rain forest and parts of the dry Sonoran Desert in the same zone; and maritime Kodiak, Alaska, with continental Albuquerque, New Mexico. Similarly, the American Horticul- tural Society (AHS) heat zone sys- tem is based on the average number of days above 86°F (30°C) —the temperature at which cellular protein is damaged in many plants. This system can help gardeners choose plants that won’t cook in hot-summer climates, but it ignores other factors, like winter cold, growing season, and humidity. The Sunset system bases each


of its climate zones on a broad range of factors, such as cold, heat, humidity, wind, proximity to the Pacifi c Ocean, snow cover, and length of the growing season. In the end, it gives Western gardeners a more accurate picture of which plants will thrive where they live. Our thanks to the many experts


who continue to help us refi ne the Sunset climate zones.


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