When it’s hot enough to fry an egg on the pavement and bitumen is melting in the street, most garden plants are suffering. They are hanging out for a long wet drink. Most, but not all. There is a group of plants that thrive when it’s hot and dry. These are known as xerophytes and form the basis for a drought-proof style of gardening known as xeriscaping (literally dry landscaping).
Xerophytes are plants from dry climates that have reduced their water needs to survive when times are dry. They get by on the smallest amounts of rainfall by reducing transpiration (water loss in plants akin to perspiring in humans) from their leaves. When water loss through transpiration is greater than the amount of water a plant can ‘drink’ from the soil, the plant wilts and can die.
Water loss beating techniques employed by plants include being able to close their stomates (minute holes in the leaves that lose water) during the day, hanging their leaves vertically so they are shaded from the sun, tough leathery leaves, or covering their leaf surface with fine hairs so the leaves are protected from heat and drying. Hairy leaves look grey or silver rather than the usual leaf green. Some xerophytes even dispense with leaves all together so another part of the plant such as the stem acts as a leaf. Many Australian wattles for example have modified leaves known as phyllodes which give them a water-wise edge to survival.
Common xerophytic plants include succulents, cactus and plants native to Mediterranean and other dry climates. Many native Australian plants including saltbush, lomandra, kangaroo paw, paper daisies and many gum trees have xerophytic adaptations to help them survive dry times.
Plants that have reduced water loss are clearly identified in garden centres often carrying a ‘waterwise’ symbol on their plant labels.
Smart garden methods
Even plants that normally need regular water can get by on less when water is scarce, with water smart gardening techniques. These aim to lower the temperature of the soil around the plant and conserve water loss from the soil. Mulches – both organic such as chopped bark and inorganic such as gravels – protect soil keeping it cooler and moister so the plants growing there can still access whatever soil moisture was available.
To see how effective mulches are, go out on a hot day and put your hand on bare soil, then go to an area that’s also in the sun but covered with mulch, scrape the mulch aside and feel how much cooler and moister the soil feels.
Garden soils can also be helped to retain moisture if they contain added organic matter such as compost. This is why gardeners are urged to dig organic matter into soil before making new plantings. The organic matter isn’t simply adding nutrients, it is also helping the soil hold onto water for longer. There are also synthetic polymers, known colloquially as water crystals, which can be added to soil and potting mixes. They absorb and hold water when the soil is moist and release it as the soil dries.
Sometimes soils stay dry and fail to absorb added water. These soils are termed ‘water repellent’. They can be encouraged to soak up water by applying a soil wetting agent via a hose or watering can.
The downside of a garden given over solely to water-wise or xerophytic plants is that many can’t cope when the long dry turns to a long wet. They may develop leaf or root rots, fail to thrive and may even die.
To avoid the downside, group dry-loving plants together in a well-drained garden bed that drains well when conditions are wet and cover the soil surface with a layer of inorganic mulch such as gravel. Drainage can often be improved by creating a raised garden bed or rockery. Water-wise plants are also smart planting choices for containers.