The nature of the vegetation in different climatic zones is directly related to the climatic characteristics. In Mediterranean, prairie or desert climates, the main limitation for plant growth is water scarcity. Wild plants growing in Israel are exposed to the dry season that is typical of these climates. In view of the severe shortage of water, it is appropriate, even in ornamental gardens, to grow plants that are suitable to drought that characterizes the Israeli climate.
Water is needed for life processes, to maintain turgor pressure, to transfer hormones, salts and food dissolved in them. Plants cannot grow without water, but there are plants that are able to retain the amounts of water they receive from the soil, in different ways. A large amount of water is lost, for example in all types of plants, in the process of photosynthesis when the stomata are open to absorb CO2.
Plants use three main strategies, which may be combined with each other, in order to reduce this loss.
Disappearance in the dry season:
This group includes annuals. In annual wild plants, the mechanism for saving water is mortality during the dry season. Annual plants in ornamental gardens are considered water-eaters, as we “insist” on growing them even in summer, and then they require multiple watering. Wildflowers in Israel, on the other hand, which bloom in the spring, do not need watering, as they bloom with the renewal of the rains. The following fall, annuals germinate as the rains begin. In these annual plants, there are germination-directed mechanisms that prevent germination while there are un-sufficient amount of water, thus causing germination dispersed by time. The mechanism allows germination of only some of the seeds each year and preserves the rest for years to come. This allows for high survival rates of the annual plants. In gardening and agriculture, one strives for the germination of all the seeds all the time, which is contrary to the instinct of survival.
Perennials cannot avoid water loss by avoiding growing in the dry season, as the plant is active year round. This group has various mechanisms that reduce the intensity of the effect of water shortage and regulate the utilization of water according to their availability:
A. Xeromorphic structures (xerro = dryness) that allow the amount of water lost to be reduced:
- Small leaves: Characteristic of desert and Mediterranean plants. Each leaf loses less water because stomata close during hours when the radiation is strong. The concentration of stomata is rather high, to allow for maximum CO2 absorption during their short activity during the day. Ornamental plants with small leaves: Myrsine africana, Eriocephalus africanus, Salvia Rosmarinus, summer savory and many others that are common in gardening. Cypress and juniper species have small leaves called scales or needles, which are also suitable for dry conditions.
- Light color of the leaves: Lighter color causes the reflection of the sun’s rays and leads to less warming of the surface. This reduced warming results in reduced water loss and retention of it in the leaves. The pale leaves are bluish, grayish or whitish. Often the light color is caused by hair covering on the epidermis (the outer layer on the surface of the leaf). Looking through a magnifying glass or under a microscope, one can see hairs on the leaves of these plants – stellar, simple or branched. Olive leaves, for example, are coated on their underside with flat stared hairs. In landscaping, gray and bluish foliage is valued in architectural-design for garden diversity. Examples of such plants are: wormwood, bare-fruited elderberry, whitewash safflower, European olive, triple acacia as well as many others.
- Cuticle and wax coating: A cuticle is a thin, transparent, impermeable layer covering the stems and leaves. The cuticle is made of cotine – a hydrophobic substance (impassible to water), sometimes wax is excreted onto the cuticle further preventing the loss of water from the leaf. A variety of plants have fatty or waxy layers covering the surface of the leaf. Examples are spiny snipe, large-flowered magnolia, common oak, broad-leaved buckthorn and others.
- Multilayer epidermis: An epidermis is a layer of cells surrounding the body of the plant. Layering of the epidermis increases the separation of plant leaves from the environment and prevents water evaporation from the cells. Sometimes a multi-layered epidermis is accompanied by a cuticle. Multilayered epidermis and cuticle are found in many of the Mediterranean trees and shrubs, such as: the gum goddess, common oak, broad-leaved buckthorn, stream hawthorn and others.
- Spartoidstems. These are usually found in almost completely leafless plants. The stem assimilates (photosynthesis) and the stomata are sunk into the stems. Water evaporation is low when the stomata are submerged in the stem, because the water that evaporates from the peony remains in the groove in the stem and does not “escape” into the air. Examples: Retama raetam, Spartium junceum and Russelia equisetiformis.
- Upright and narrow leaves(grain and cereal-like): These leaves grow vertically, and their wide surface does not face the sun, it therefore heats up less and loses little water. Perennial cereal herbs have such leaves. But also other monocotyledonous plants: Dietes bicolor, Kniphofia uvaria, iris species and others.
- Succulents: Plants from different groups and families have fleshy leaves and stems. These are usually grown in low-water soils with a high salt level. The purpose of the succulents is mainly to dilute the salts in the water of the leaves and lower their concentration. Examples: cacti species such as sabra and echinocerus as well as succulents from other families: aloe species, Euphorbia species and more.
B. Physiological adjustments:
This phenomena are difficult to spot at first, these are plants with a high capacity for water absorption:
Desert plants usually have a large root volume relative to the plant. This enables the absorption of water from a large surface despite their shortage.
- Some plants growing in warm areas, have particularly efficient photosynthesis process called C4. CO2 absorption is more efficient and allows the stomata to close quickly (thus preventing the loss of water vapor from the plant).
- Plants from the Crassulaceae family, called CAM plants – in which the stomata open at night, absorb CO2 that is stored in special acids until it is used during the day. When there is enough light needed to continue the process of photosynthesis.
The main methods for adapting to drought in the country’s vegetation are: annuals (avoiding growth in the difficult season) or the disappearance of the above-ground parts in summer, such as in geophytes, closing stomata during heated hours and a large and branched root system.
Fleshy vegetation is rare in our wild foliage (Caralluma is almost the only example of this). Unlike the fleshy American cacti (such as the sabra, which is not native).
When planning a water-efficient garden, it is worthwhile to base it on the right choice of plants. Tropical plants have large leaves and lack the structural and physiological mechanisms described above.
Gardens based on the right choice of plants will suit the local landscape and the water-saving order. These gardens will have Mediterranean or desert plants and plants that originate from areas with significant seasonality (areas in Australia, North America, South Africa and others).