By the last hot week of July, most plants in central San Joaquin Valley gardens have stopped producing new growth. This is a normal reaction to high temperatures which cause plants and trees to enter a state of semi-dormancy. It’s a survival mechanism; in extremely hot weather, plants’ roots simply cannot draw up enough water quickly enough to replace that lost by transpiration through their leaves so the plants slow or stop growth during the hottest months to conserve moisture and energy.
Drought- and heat-tolerant plants, including succulents, cacti, Mediterranean herbs and California native plants, also become semi-dormant in summer. We increase irrigation in summer for thirsty lawns and landscape plants, but mature drought- and heat-tolerant plants actually need less water in summer than might be expected.
Overwatering of drought- and heat-tolerant plants is the most common cause of death. Their root systems have evolved to survive with very little water and many species, especially cacti and succulents, are also able to store water in the plant itself. Other species including many Mediterranean herbs that are considered invasive such as artemisia, tansy, rosemary and members of the marjoram family have root systems that extend long distances in search of water.
Young drought-tolerant or native plants will need consistently slightly moist soil in the root zones during their first two to three years. Well-established drought-tolerant plants may need irrigation just once a week or when the soil has completely dried. A soil moisture meter (available at many nurseries, garden centers or online) can be a handy tool for determining when to irrigate your drought-tolerant garden.
Over time you’ll learn to recognize signs of drought stress in your xeriscape garden. Cacti will shrivel a little; succulents will begin to lose leaves; rosemary tips and lavender stalks will droop and then dry out. Most drought-tolerant plants can rebound from drought stress with additional irrigation, but they seldom rebound from root rot caused by overwatering.
A good landscape irrigation design will place drought- and heat-tolerant plants in individual irrigation zones separately from lawns and heavy water users like roses. Lavenders and roses may complement one another visually when planted together but the lavender roots will rot if they receive the same amount of water as the roses.
Drought- and heat-tolerant plants require well-draining soil that allows water to flow away from the roots. The soil in their planting zones should be amended with large amounts of compost, humus and sand to improve drainage especially if the soil in the garden is heavy clay, common in the Valley.
Mediterranean herbs thrive in our hot, arid climate but, when planted in containers, the roots of the tiny sprig of thyme or oregano that you bought this spring probably has already completely filled the pot you put it in. Herbs in pots often need to be replaced annually since they so quickly become rootbound. Plan on watering your potted drought-tolerant container herbs every other day when temperatures are high until water runs out the drainage hole. Potted succulents and cacti may need just a tablespoon or two of water every few days to keep them healthy in hot weather.
Elinor Teague: firstname.lastname@example.org