What's your garden climate?
There are no green thumbs, only people who give plants what they need to thrive. Most plant needs - good soil, water, fertilizer - are easy to satisfy. You can't, however, change the climate to suit the plant. So it makes sense to choose the plant to suit the climate. With that guiding premise, the present incarnation of Sunset Western Garden Book was first published in 1967 (previous versions go back to the 1930s). It identifies 24 different climate zones and provides an encyclopedia of plants that tells which zones each grows in. We've just completed the book's most extensive revision ever, expanding the encyclopedia to cover some 6,000 garden plants and, with the help of about 70 weather observers around the West, revising the climate zone maps to make them even more useful.
The all-new Sunset Western Garden Book goes on sale in March. Meanwhile, you can preview some of the maps and find your own climate zone in the following pages. Scan the maps to find where you live, then read about the zone that defines your gardening.
What defines a climate zone?
Six important factors define each of the zones.
Latitude. Generally, the farther a spot is from the equator, the longer and colder its winters, and the more daylight in its summers.
Elevation. High gardens get long, cold winters and low night temperatures all year.
Ocean influence. Weather that blows in off the Pacific Ocean tends to be humid and mild, and laden with precipitation in winter.
Continental influence. Our continent originates its own weather, which is colder in winter than in areas of ocean influence, hotter in summer, and more likely to get precipitation any time of year. The farther inland you live, the stronger the effect.
Mountains and hills. The Coast Ranges take some ocean influence out of the air as it passes eastward. The Sierra further weaken ocean influence. East of the Rocky Mountains, continental and Arctic air dominate. In the opposite order, first the Rockies, then the Sierra, and finally the Coast Ranges reduce the westward influence of continental air.
Local terrain. South-facing slopes get more solar heat than flatland; north-facing slopes get less. Slope also affects airflow: warm air rises, cold air sinks. Because hillsides are not quite as cold in winter as the lower ground beneath them, they're called thermal belts. Above thermal belts, elevation makes the air cold. Lowlands are cold-air basins.
Get to know your zone
In Sunset Western Garden Book, climate zones are listed in numerical order, from harsh zone 1 to mild zone 24; here we describe only the zones in the areas covered by this regional edition of Sunset. (Although only part of Idaho appears in the map on page 82, the whole state is mapped in the book.) Temperatures are given in degrees Fahrenheit.
Why don't we use the U.S. Department of Agriculture's 11-zone climate map? Its zones are based on minimum winter temperatures alone; that's useful for determining plant hardiness, but just not sufficient for predicting how plants will perform from region to region. For example, the USDA map puts the Olympic rain forest into a zone with parts of the Sonoran Desert. Try growing Sitka spruce in Tucson.
Sunset Western Garden Book's zone scheme considers winter minimums, too, but it also factors in summer highs, length of growing season, humidity, and rainfall patterns to give a more accurate picture of what will grow where.
As you scan the information that describes your zone, you may find that your garden has different characteristics. If, for example, winter lows in your garden are consistently different from what your zone would lead you to expect, chances are that you're gardening in a microclimate that pushes you into another zone. Local terrain - the climate-modifying wild card - is almost always the cause of such microclimates. Read descriptions of the nearest climate zones, and you'll probably find one that fits your situation better.
Zones 1, 2, and 3 are regions where snow falls and stays on the ground (for a week or all winter) every year. Zone 1 is coldest, its extreme winters caused by latitude, influence of the continental air mass, elevation (the higher you go, the colder it gets), or some combination of the three. In this zone, gardeners plant with a 75- to 150-day growing season in mind, though frosts can occur any night of the year.
Second-coldest Western climate
Gardens in zone 2 get some snow in winter, but because they're usually lower in elevation or modified by a large body Of water (Great Salt Lake, for example), they don't get quite as much cold as zone 1. The difference is crucial: 'McIntosh' and 'Sierra Beauty' apples, for example, grow well in zone 2, but not in zone 1.
In zone 2, windbreaks, trees planted for shelter, and heavy mulches can make it possible to grow plants that would otherwise perish from the effects of wind, cold, and the low winter sun, which can burn the trunks of young thin-barked trees.
During a 20-year period in zone 2, annual low temperatures ranged from -3 [degrees] to -34 [degrees].
Cold country's "banana belts"
This is the mildest of the snowy-winter climates. The zone 3 areas of Idaho and Nevada are the ones that are often called "banana belts." Of course, the only place you can grow the banana satisfactorily outdoors is in tropical climates. But the comparatively mild winter lows of zone 3 allow gardeners to grow such plants as English boxwood and winter jasmine.
Much planting here is based on winter lows of around 10 [degrees], though in an occasional winter, Arctic air forces temperatures much lower. Winter minimums limit the choice of broad-leafed evergreens, because drying winds dehydrate plants growing in frozen soil. Wind protection, mulch, shade, and careful late-autumn watering will help with many borderline evergreens.
During a 20-year period, minimum temperatures in zone J ranged from 13 [degrees] to -24 [degrees]. While growing seasons can be shorter here than in zone 2, winter minimums are always higher in zone 3.
Southwest's high desert
Zone 10 consists mostly of the 3,300- to 4,500-foot elevations in parts of Arizona and New Mexico. It also includes parts of southern Utah and southern Nevada. It has a definite winter season; from 75 to more than 100 nights each year have temperatures below 32 [degrees]. In the representative towns of Douglas, Arizona, and Albuquerque, average winter minimums range from 31 [degrees] to 24 [degrees]. Lows of 25 [degrees] to 22 [degrees] often come in April. The lowest temperature recorded is - 17 [degrees].
The cold winter season calls for spring planting and a spring-through-summer growing season. (In neighboring zones 12 and 13, most planting is done in fall.)
What distinguishes this climate from zone 11 are more rainfall and less wind. Annual rainfall averages 12 inches, with half that amount falling in July and August. In the eastern parts of zone 10 (Pecos River drainage), the summer provides more precipitation than does the winter.
Nevada's medium to high desert
This zone shares some similarities with its extremely different neighbors - the cold-winter zones 1, 2, and 3, and the subtropical low-desert zone 13. Like zones 1, 2, and 3, zone 11 has cold winters, and like zone 13, it has hot summers. Overall, zone 11 is characterized by wide swings in temperature. Hot summer days are followed by cool nights; freezing nights are often followed by daytime temperatures of 60 [degrees]. On average, 110 summer days go above 90 [degrees], with the highest recorded temperatures hovering between 111 [degrees] and 117 [degrees]. About 85 nights have temperatures below 32 [degrees], with maximum lows between 11 [degrees] and 0 [degrees].
Zone 11 includes Las Vegas, where gardeners lose many trees and shrubs every year, blaming the climate when the problem is usually bad soil or inadequate water (dry wind and bright sunlight combine to dry out evergreen plants). If you've been careful about watering and your correctly zoned plants still don't grow well, talk with your county extension agent about plants suited to your native soil.
Arizona's intermediate desert
The crucial difference between zone 12, the intermediate desert, and zone 13, the low desert, is the amount of winter cold. Even though zone 12 averages only 5 more freezing nights than zone 13 (20 in Tucson compared to 15 in Phoenix), it has harder frosts spread over a longer winter cold season. Put another way, zone 12 averages about eight months between freezes, nine months between killing frosts (28 [degrees] or lower). Zone 13, on the other hand, averages more than 11 months between killing frosts, if it gets them at all. Extreme low temperatures of 6 [degrees] have been recorded in zone 12. The mean maximums in July and August are 5 [degrees] to 6 [degrees] cooler than the highs of zone 13.
Many subtropical plants that do well in zone 13 aren't reliably hardy here, but with protection they succeed against the extreme winters.
Although winter temperatures are lower than in zone 13, the total hours of cold are not enough to provide sufficient winter chilling for some deciduous fruits.
From March to May, winds to 40 miles per hour can damage young, tender growth. Windbreaks can help. Here, as in zone 13 and the eastern parts of zone 10, summer rains are usually more dependable than winter rains. And as in zone 13, the best planting time for cool-season crops (cabbage family members, salad greens, root vegetables) starts in September or October.
Arizona's low desert
Ranging from below sea level in California's Imperial Valley to 1,100 feet around Phoenix, zone 13 is subtropical low desert. Average summer maximum temperatures range from 106 [degrees] to 108 [degrees]. Winters are short and mild. Frosts, anticipated between December 1 and February 15, are brief. Although the average minimum winter temperature is 37 [degrees], with just 15 nights below freezing, lows of 19 [degrees] to 13 [degrees] have been recorded.
The gardening year begins in September and October for most vegetable crops and annual flowers, although crops like corn and melons are planted in late winter. Fall-planted crops grow slowly in winter, pick up speed in mid-February, then race as temperatures rise in March and April. Spring winds and summer storms are factors in gardening: the rains help with watering, and dense clouds shield plants from the hot sun.