What's your garden climate?

Sunset, Feb, 1995

Sunset's exclusive climate zones, newly revised and updated, can help you choose the right plants to grow in your garden

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.

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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.

ZONE 1

Coldest-winter West

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.

ZONE 2

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.