By Laura Lukes, UC Master Gardener of Butte County, September 7, 2018
The beauty of evolution is its reliance on trial and error, or adaptation. What works, works very well, and allows life in many forms to exist in some of Earth's harshest environments. The climatic conditions of the planet's seven Mediterranean Zones include between five and seven months of zero precipitation, and many days in a row with high temperatures of over 100 degrees Fahrenheit. These are challenging circumstances for any living thing. Yet a wide variety of plants have evolved clever strategies to survive these long, dry, hot periods.
Generally speaking, there are three major strategies employed by plants to survive annual drought conditions: desiccation tolerance; drought avoidance; and drought tolerance. These strategies evolved through millions of years of adaptation, and are endlessly fascinating in their ingenuity. (Please note that the survival tactics described below, the result of complex chemical and molecular biological processes, are simplified for this article.)
Desiccation Tolerance: To desiccate something is to thoroughly dry it. Tolerance of desiccation gives a plant the remarkable ability to survive almost total dehydration. This strategy is employed by mosses and ferns. Briefly, plants in this category have developed the ability to enter into, and recover from, anhydrobiosis, the cessation of metabolic activity as a result of low intracellular water content. Next time you are hiking in Upper Park or the foothills during the dry months, you can see this phenomenon for yourself. Find a patch of rust colored, crunchy dry moss on a rock, and gently pour a small amount of water on it. In seconds, what appeared to be completely dead vegetation will turn green and supple.
Another form of drought avoidance is early leaf drop. A good example of this is the buckeye (Aesculus), which occupies a unique ecological niche by being one of the first shrubs to leaf out and flower in early spring, and also one of the first to lose its leaves, well before the onslaught of summer heat and drought. Leaves demand precious nutrients and energy, and without them the buckeye can conserve these resources. During years of drought, and during sustained periods of high temperatures, our valley oaks and blue oaks lighten their metabolic load by dropping some leaves earlier than usual.
Drought Tolerance: Lastly, there is this catch-all phrase. Plants in this category are just better at functioning during annual drought conditions, due to a number of creative adaptations. Such plants are also called xerophytes; literally “dry plants.” They remain green all year round, but manage to save or store water, often through structural (usually leaf) morphology. Common structural adaptations for water conservation are:
- Thick, leathery leaves with waxy cuticles, which perform dual functions of cutting down on water loss and reflecting heat away from the plant. Our native Ceanothus (California Lilac) is a prime example of this.
- Small, thin leaves, which effectively reduce the surface area from which water loss can occur. The tiny yet highly fragrant leaves of Santolina typify this adaptation.
- Sunken stomata pits, which trap moist air and reduce water loss rates. Pine needles employ this strategy (as well as being small and thin).
- Hairy leaves, like those found on Cyprus ironwort (Sideritis cypria) or Lamb's Ears (Stachys byzantina), which shade the stomata and reduce contact from hot air, protecting plants from extremes of light and temperature.
Redundancy is a hallmark strategy for species survival (think two kidneys in human beings); and most plants employ more than one method of beating the hot dry summers of the Mediterranean climate. Now that you know what to look for, see how many of these ingenious biological adaptations you can spot.
By Jeff Oster, UC Butte County Master Gardener, October 25, 2017.
Western Redbud, California Buckeye, Toyon, and California Flannelbush are all plants in this category that attract pollinators, provide interest over several seasons of the year, and can be grown in a number of different gardening environments. They are especially effective planted in groups for repetition of form, and can create informal hedges or provide a backdrop for smaller plants.
Western Redbud (Cercis occidentalis) is native on dry slopes of the coast ranges and Sierra Nevada foothills to 4500 feet and eastward to Utah (usually situated next to a spring or seasonal creek). It is a deciduous, multi-trunked shrub or small tree with a long life span, growing two to three feet per year and reaching to six to twenty feet over time. Magenta flowers appear in February to April before the leaves, and last for two to three weeks. The leaves are kidney-shaped and bluish-green; they can provide a lovely garden backdrop for summer-blooming plants, and turn yellow in fall. Reddish-purple seed pods hang from the branches in winter. The bushes are impressive in groups.
Western Redbud likes full sun, and will grow faster if given some summer water. It will tolerate clay soils and soils with lime or acid (pH levels 5.5 to 8). It can provide good stabilization along a ridge or bank. The roots perform the useful role of fixing nitrogen. A good winter chill is necessary for good flower set. The flowers and young pods are edible; the plants will self-seed.
Western Redbud attracts hummingbirds, goldfinches, butterflies, and bees (including leaf-cutter bees) to its flowers. It is deer resistant, and resistant to Oak Root Fungus. While generally pest-free, Western Redbud is susceptible to caterpillars and scales, and to crown and root rot.
California Buckeye will grow in conditions ranging from partial shade to full sun, and it is drought tolerant but needs regular water during the first few years. It will tolerate clay and serpentine soils (pH 4 to 8) and seasonal flooding, and is useful as a slope stabilizer. In the garden it will drop leaves early (going summer dormant) if it gets dry; otherwise leaves will drop in the fall.
The nectar of the California Buckeye attracts hummingbirds, native bees and butterflies.
California Buckeye is poisonous to livestock, Asian honey bees, and fish. Its seeds are toxic unless processed. It is deer-resistant.
While Toyon likes full sun, it can tolerate full shade, and will grow in a variety of soils (pH 5 to 8), and can help stabilize a ridge or bank. It is drought tolerant after a few years, but will tolerate some water if drainage is good.
Toyon attracts a wide range of pollinator insects including native bees, pollinator flies, and butterflies. Winter fruit attracts cedar waxwings, towhees, finches, flickers, grosbeaks, bluebirds, robins, thrushes, mockingbirds, quail, tanagers, warblers and sparrows. Often the ripe berries ferment before they are eaten, so birds may get a little tipsy. Mammals, including coyotes and bears, also eat and disperse the berries.
Toyon is susceptible to fire blight and root rot (if over-watered). Over time, it will become deer resistant.
California Flannelbush likes sun to part shade. It is drought tolerant and needs no summer water but must have good drainage, preferring well-drained (sandy) soils (pH 6 to 8) near natural drainage channels (placement along the top of a riverbank is ideal.
California Flannelbush attracts bees and butterflies, has few pest problems, and is deer resistant.
The following two charts refer to plants referenced in the three part series on native plants that thrive in Chico and Lindo Channel.