(Editor's Note: This article was originally published on July 30, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
I am familiar with the mediterranean climate of southern California, so this discussion will be from that perspective. I have found the dry season to usually be from June to November, though often the rain stops in April or once in a while starts in October. This amounts to at least five months with virtually no rain. The hottest weather is from July until the middle of October. Temperatures vary from barely 80F right at the coast to 90F or more inland. Southern California appears to have the longest dry season of the world's mediterranean climate zones. It might sound like this would be a very plant-poor region, but hundreds of native and introduced plants are found in my local Santa Monica Mountains and similar variety exists up and down the coast and inland to the high mountains and deserts.
You might think that this would be a land rich in cacti, but it isn't. Though cactus deserts do not get much rain, they do get traces of rain throughout the year or have winter and summer "rainy" seasons. They don't typically have the 5 month dry season in the hottest part of year, year after year. There are some cacti in the mediterranean climate zone of California, but they are limited to coastal areas where dew and fog drip account for additional precipitation in the summer and where summer temperatures are less hot.
Many Mediterranean climate plants use the same strategies to survive the summer as cold-climate plants use to survive the winter.
Grow in favorable weather, set seed, and die This is what annuals do. They sprout from last year's seeds, grow, bloom, produce seeds, and die, usually all by the middle of summer. Southern California has a wealth of annual flowers and grasses. These species are very responsive to the quantity and timing of rainfall and under the right conditions, wildflower displays can be spectacular. California grasslands are full of annuals and that is why they turn brown in summer.
"Sleep" through the hard times This is what geophytes ("bulbs") and some perennials do. They grow during the rainy season and are dormant in the summer and fall. It is almost like these plants are eager to go dormant as the foliage is often dead by the time the plant blooms, like the scarlet larkspur in the photo at the left.
Even if the rainy season doesn't amount to much, the geophytes often have enough reserves to grow and bloom. Bulbs are usually small, but the wild cucumber can produce a tuber up to 100 lbs in weight. Perennials such as Indian paintbrush and shooting stars go dormant much like annuals, except that the perennials remain alive in their bases and roots.
Deciduousness Plants lose a lot of water through their leaves. Some shrubs like purple sage will shed some leaves in response to dry weather. This is called being drought deciduous. The picture at the right shows a purple sage plant that has lost at least half of its leaves in an effort to survive the summer.
However, there are many native plants that do not die or go dormant during the dry season. Most of these plants are trees and large shrubs. Some survive the dry season by avoiding the dry season in a sense, by growing in relatively more mesic microclimates. They grow in canyons where their deep roots can tap underground water. Some grow on the north sides of hills, where the soil dries out more slowly. Others grow along year-round streams. Notice how the coast live oaks in the photo at the top upper right are all growing in canyons and the valley bottom. Along the central California coast, summers are rainless but often foggy. The fog condenses on leaves and drips to the ground and can account for several inches of precipitation a year. This allows redwoods to grow in sheltered canyons.
Other plants practice water conservation. Many mediterranean shrubs such as toyon, islay, and laurel sumac have leathery, waxy, or fuzzy leaves. This helps to reduce water loss due to evaporation and thus reduces the plants' water demands and allows them to live in more arid sites. This also results in many plants having and olive, bluish, or greyish color. Compare the color of the foliage in my photos to the color of the foliage of plants from a rainier climate.
Growing mediterranean climate California native plants at home requires a change in gardening mindset. We usually think that plants need supplemental water in hot, dry weather. However, many of these California natives will die under the watering regimen given to conventional gardening plants. Many California native plants have not been domesticated to any degree and they will not survive domestic pampering. They should be grouped with other drought-tolerant plants and given very minimal summer watering. If necessary, add small gravel to the soil to aid drainage. Also, when growing California natives, one has to be accepting of the fact that your native garden will look at its worst in the summer and fall. The benefit of this is that a minimal amount of gardening work is required during the hottest days of summer.
If you would like to try native California plants but have summer rainfall, I recommend trying annuals or growing small perennials in pots that can be moved out of the rain. Probably the most readily-available mediterranean climate California native is the California poppy.
When I am out hiking in the summer, I have to marvel at the plants. They can't go sit in the shade, go home and turn on the air conditioner, or even get a glass of water. Intellectually, I know how plants survive the mediterranean summer, but on another level, I wonder how any survive at all.
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Photos from top to bottom: Grassland in early September; Scarlet larkspur in mid July; Purple sage in late June; Grassland in late May; Chaparral in July; Chaparral in June; Grassland and Chaparral in July. All photos taken in the Santa Monica Mountains.
 Dallman, Peter R., Plant Life in the World's Mediterranean Climates, University of California Press, 1998