Gardening in Summer-Dry Climates

Plants adapted to summer-dry climates are displayed in a formal arrangement astride a shallow channel at The Huntington Botanical Gardens in Los Angeles.

Gardening
Where
You Are

A tapestry of heaths and heathers, barberries, and dwarf conifers creates a richly textured Seattle garden.

A sinuous stairway invites exploration up and into a San Diego garden of succulents and palms.

GARDENERS IN SUMMER-DRY climates don’t need charts and maps or statistics on rainfall to tell them that their climate is summer-dry. Our climates may be called hot- or cool-summer mediterranean, oceanic, semi-arid, submediterranean, or west coast maritime, but we are all summer-dry and we have much in common as gardeners.

Not that there are no regional garden styles. Gardens reflect the convergence of the physical and the social—of the physical facts of climate, topography, and natural vegetation and the personal and collective experience of those who live and garden there.

Regional variations are seen all along North America’s summer-dry West Coast. Gardens in Seattle or Vancouver are distinctly of the Pacific Northwest. For the most part, they little resemble the gardens of Los Angeles. Even when we try to replicate a garden from another place, say a California garden in Portland or a Northwest garden in San Francisco, we can hardly avoid expressing something of the local vernacular.

A Seattle garden features rosemary, lavender, sage, and other sun-loving plants common in California gardens.

There are measurable physical differences from north to south as well. Not just in rainfall totals but in the timing and intensity of rain. Not only summer drought but how long it lasts and how hot and dry the soil and air. Coastal Oregon and California’s Central Valley are palpably different kinds of summer-dry, not only in the number of cloudless days but in the brightness of the sun, the quality of the light, and how colors present themselves in the presence or absence of mist or fog.

The similarities among summer-dry climates are every bit as striking as their differences. We plant mostly in fall, less often in spring as most of the world does. In summer, many of our finest perennials go from full-on floriferous to a restful dormancy. We can grow plants from just about anywhere in the world, but if those plants are accustomed to summer rainfall, we will need to water them.

With a warming climate and a growing population, our challenges are becoming increasingly alike as well. From north to south, when winter rain and snow fall short we can expect summertime restrictions on garden watering. North to south, wildfires are ever larger, more destructive, and harder to control. Invasive species and the near-catastrophic loss of natural wildlands are challenges we share with one another and with the world.

Gardeners up and down the Pacific coast also share an upbeat conviction that the way we garden can make a difference. In the face of worldwide habitat loss, species extinction, and unsustainable pressures on natural resources, we have moved decisively to reduce our impact on water supplies, to make gardens that attract and sustain wildlife, to use and reuse local materials, and to work with rather than fight the summer-dry climate.

The summer-dry garden is not necessarily a dry garden. Plants from other summer-dry climates may need occasional or even moderate watering to make it through our particular version of summer-dry, especially if winter rainfall has been less than normal. A plant can be well adapted to summer dryness in its natural setting and still need a little help from the gardener.

Gardening in harmony with the summer-dry climate begins with an understanding of where, exactly, you are. It is also useful to know something about the conditions in which the plants you select grow naturally.

Summers are dry or mostly dry, but how dry and for how long? Winters are wet or usually wet, but do storms come rarely with heavy rain, or are there many winter days with only a light drizzle? Summer heat and winter cold both help to determine what plants we can successfully grow. Do winter temperatures drop below freezing? How far below, how often, and for how long?

THE SUMMER-DRY CLIMATE
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SUMMER-DRY CLIMATES OF THE WORLD TEND to be on or near the west coasts of continents where temperatures are moderated by proximity to the ocean and where semi-permanent atmospheric pressure systems influence the path of approaching storms. In these regions, summers may be cool, warm, or hot; winters are mild; and rainfall is concentrated in winter.

The seasonality of precipitation is the defining characteristic of summer-dry climates, which otherwise vary widely in how rainy days are spread out over the year. Rain falls mostly in winter in both Cape Town, South Africa, and Los Angeles, California, but in a year of normal rainfall, Cape Town receives 2 inches of its 20-inch annual total, or 10 percent, in summer. Los Angeles receives 0.14 inch, or less than 1 percent, of its 15-inch total in summer months. Los Angeles has a more intensely summer-dry climate.

Even within the same summer-dry region of the world, climates differ markedly in the amount of rain that falls in the mostly summer-dry season. In the region surrounding the Mediterranean Sea, the northwest coast tends to receive some summer rain. Some parts of that coast—near Barcelona, for example—may see so much of their annual rainfall in summer that they can hardly be called summer-dry. The southern Mediterranean coast, from Morocco to Tunisia, and the eastern Mediterranean, from southwestern Turkey to Israel, usually are much drier than the north coast, with little or no rain in summer.

In any summer-dry climate, annual rainfall and precipitation patterns can vary dramatically from one year to another. Abnormally dry or wet winters may repeat for several to many years. A series of dry years may be followed by an exceptionally wet year or by a year in which the rainy season is unusually long or short. Variability is one of the few constant features of the summer-dry climate.

Spiky agaves, aloes, and a succulent euphorbia flourish in San Diego, where average annual rainfall is 10 to 12 inches.

SUMMER-DRY CLIMATES OF THE PACIFIC COAST
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THE WEST COAST OF NORTH AMERICA RECEIVES most of its rain in winter, with the summer season mostly to almost completely dry. From Vancouver to San Diego, there typically are weeks or months in the warm season when no rain falls.

There is great variation, of course, from north to south in rainfall amounts and in timing of the wet and dry seasons. To the north, the length and intensity of the warm-season drought decreases, with more rain falling in spring, summer, or early fall. Seattle receives an annual average of 38 inches of rain, with about 12 percent falling in the summer months and rain spread out over about 150 days. To the south, the dry period lengthens and increases in intensity. San Diego receives an average of 10 to 12 inches of rain, with less than 2 percent falling in summer and only 43 days with any measurable rain at all.

Temperatures also trend higher from north to south. The annual average high in Los Angeles is 15 to 20 degrees above that in Vancouver, and the average low is at least 10 degrees higher. Averages mask the extremes, and extremes matter. Summertime highs in Los Angeles often edge into the 100s for days at a time. In Vancouver, the hottest summer days usually are in the mid-80s. Lows at or below freezing in Vancouver occur an average of 14–40 nights a year. Los Angeles rarely sees even a light frost.

Differences in local climates or microclimates can be even more remarkable. Summer temperatures in Burbank, California, may be 10 degrees hotter than temperatures 9 miles south in downtown Los Angeles. Downtown Los Angeles may be 10 degrees hotter than Santa Monica, less than 15 miles away. Temperatures farther inland can be 20 to 30 degrees hotter than those right along the coast.

Neighborhoods on the west side of San Francisco often are blanketed with thick summer fog, while those just a few blocks east are basking under sunny skies. When dense fog spreads across the bay to envelop Oakland, a short trip through the tunnel under the Oakland hills usually delivers bright sunshine in a little over half a mile.

Similar variations are found to the north. Fifty miles north-west of Seattle, the city of Sequim receives less than half as much rain. Olympia, almost 50 miles to the southwest, receives a foot more rain and has a frost-free growing season more than 80 days shorter.

Perennials in the Waterwise Garden of Bellevue Botanical Garden near Seattle thrive in the mild, moist climate of Puget Sound.

Summer fog brings significant moisture and cool temperatures to the San Francisco Botanical Garden.

ORIGINS OF THE PACIFIC COAST CLIMATE
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THE CIRCUMSTANCES THAT PRODUCE THIS distinctive arrangement of climates are complex and interrelated. Summer-dry climates, as with climates worldwide, are largely determined by such globally significant factors as distance from the equator, prevailing winds, the direction and temperature of ocean currents, and the shifting location and strength of atmospheric pressure systems that govern the path of approaching storms. Weather along the Pacific coast is determined as well by the meandering jet stream, by fluctuating ocean temperatures associated with El Niño and La Niña events, and by the size, timing, and moisture content of atmospheric “rivers” that make landfall along the coast. Regional and local variations in climate are shaped by distance from the ocean and topography.

Proximity to the ocean plays a central role. Oceans gain and lose heat more slowly than adjacent land. As a result, air moving over the water is warmer in winter and cooler in summer than temperatures on land. This has a moderating effect on climates along the Pacific coast, an effect that diminishes as you move inland. Proximity to the ocean also brings fairly reliable daily breezes and sometimes strong winter winds.

Air temperatures are further modulated by the temperature and direction of ocean currents. From about the border between British Columbia and Washington, the cold California Current flows south along the coast and the warmer Alaska Current flows north. In summer, as the California Current flows south, winds from the northwest propel the surface water south and then west, causing even colder water to rise from below to replace it. This creates a band of especially cold water along the coast, where moisture condenses and fog forms as warmer winds from the ocean pass over it. As the land heats up later in the day, the fog usually dissipates, but it often stays long enough for fog drip from trees to soak the ground.

Precipitation patterns on the west coast of North America are largely determined by the location and strength of two seasonally shifting atmospheric pressure systems over the northern Pacific Ocean. In winter a low-pressure trough over the Gulf of Alaska, the Aleutian Low, moves south, sending storms toward much of the Pacific coast. In summer the low-pressure trough weakens and moves north and a high-pressure ridge, the North Pacific High, takes its place. The high-pressure ridge sends storms northward around it, blocking storms from more southerly parts of the coast. Sometimes this ridge stalls off the coast for weeks or months at a time, keeping most of the Pacific coast dry.

Once storms reach the coast, topography ensures that their effects are not uniformly felt. Mountains exert a particularly large effect on precipitation, especially if oriented perpendicular to prevailing winds. When winds reach the mountains, air rises and cools, clouds form, and water vapor condenses and precipitates. As the cooled air descends to lower elevations on the other side, the air warms and the wind is drier. The higher the mountains, the greater the effect, but air masses can be blocked or redirected by fairly low mountains, and cold air can pool even in small depressions.

Elevation and latitude have similar effects on temperature, which decreases fairly consistently from south to north and from lower to higher elevations. Moving north 300 miles is, on average, equivalent to an increase of 1,000 feet in elevation. Precipitation usually increases with elevation, but the rate of increase depends on factors that affect the amount of moisture in the air, such as distance from the ocean or other large bodies of water.

Aspect also affects climate. In the northern hemisphere, south-facing slopes usually are warmer and drier than north-facing slopes. South-facing slopes receive direct sunlight and both temperature and evaporation usually are higher. East-facing slopes receive morning sun while west-facing slopes receive hotter afternoon sun. Vegetation that thrives on south-facing slopes at higher elevations or in northern latitudes may grow on north-facing slopes at lower elevations or in more southerly latitudes. Vegetation differs with aspect even at similar latitudes and elevations.

The Ruth Bancroft Garden and Nursery in Walnut Creek, California, protects and displays many plants from arid and semi-arid parts of the world.

TOPOGRAPHY AND CLIMATE
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BECAUSE OF THE NORTH–SOUTH ORIENTATION of the mountains and their effects on temperature and precipitation, local climates on the Pacific coast of North America tend to be more similar north to south than east to west as is common elsewhere in temperate parts of the world. Changes in climate are fairly gradual moving north to south but can be extreme moving west to east.

From Alaska to Baja California, the north–south trending mountain ranges interrupt, deflect, or otherwise channel the prevailing westerly winds. From the Vancouver Island Ranges and the Olympic Mountains south through the Coast Ranges of Washington, Oregon, and California, the effect is the same: cooler summers, warmer winters, higher humidity, and more precipitation along the coast and on west-facing slopes than even a few miles inland.

East of the coastal mountains is a series of usually north–south trending valleys. The Fraser Valley in British Columbia, the Willamette Valley in Oregon, and the Central Valley in California are the major valleys, but there are more. Most of the valleys are in the rain shadow of the coastal mountains, especially close to the east-facing slopes. Climates in the valleys are warmer in summer, colder in winter, and lower in precipitation than along the coast. Temperatures also vary more widely from day to night.

Farther inland, on the eastern side of the valleys, the Cascade Range and the Sierra Nevada form a second north–south chain from British Columbia to just north of the Los Angeles Basin. These mountains are higher, with peaks in the Cascades rising to well over 10,000 feet and more than 100 peaks over 13,000 feet in the Sierra Nevada. Beyond these ranges are the harsher, more continental climates of the Great Basin and the Rocky Mountains, where winters are cold, summers are hot, and what precipitation there is often falls as snow.

The coastal and inland mountain ranges, although continuous, are not without gaps through which weather patterns interact. From north to south, several major rivers cut through the mountains to reach the ocean, creating passages for ocean breezes to flow inland in summer and cold northerly winds to bring winter storm systems to the coastal edge. Such passages are formed by the Fraser River in British Columbia, the Columbia River in Washington and Oregon, and the Sacramento River in California. Dozens of smaller rivers, streams, and mountain passes also funnel winds and weather in both directions.

The mountains contribute to the temperature inversions so common in west coast cities that are surrounded by mountains. Cold air sometimes flows down from mountain peaks, pushing under warmer air rising from the valley or the coastal plain. The warm air above creates a cap that prevents the mixing of cold and warm air and traps dust and smog beneath it. Los Angeles is famous for its temperature inversions, but inversions also occur from Seattle to Olympia in Puget Sound, from Portland to Eugene in the Willamette Valley, and from Redding to Bakersfield in California’s Central Valley.

Another weather-related phenomenon brought about in part by topography is inherent in parts of the Pacific Coast, especially to the south. Hot, dry, easterly winds come off the inland valleys and deserts in fall, bringing sudden increases in temperature and drops in humidity to the coast. Called Diablos in north-central California and Santa Anas in Los Angeles, these unusually strong, downslope winds fan wildfires into flaming infernos.

A similar phenomenon, called the Brookings effect, occasionally occurs in southwestern Oregon, where northeasterly winds pour down the Chetco River toward the coast at Brookings, lowering humidity and raising temperatures dramatically.

Oaks and California buckeyes stand out prominently against the golden grasses of summer on Mount Burdell, a 1600-acre open space preserve 30 miles north of San Francisco.

Lush green grasses and budding oaks are highlights of spring in coastal mountains near Gilroy, California, 30 miles south of San Jose.

PLANTS FOR SUMMER-DRY CLIMATES
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WHERE A PLANT GROWS NATURALLY IS NOT THE