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39 Principle 3 Appropriate Plant Selection Amy Croft and Heidi Kratsch It is a misconception that water-efficient landscapes must be designed using only plants native to the region. Some plants native to the Intermountain West are particularly adapted to the higher-elevation, dry, and often harsh conditions of the region. But many plants native to other parts of the country and the world with similarly harsh climates perform just as well. The suggested plant list at the end of this chapter includes both native and non-native plant species and cultivars that have demonstrated outstanding performance in managed landscapes in our climate. It is important to understand the characteristics that have made these plants drought-adapted and, therefore, unique in the ways they perform and the treatments they require under managed conditions. For this reason, the first part of this chapter is devoted to an explanation of how plants have adapted to arid environments. Water-Efficient Plant Adaptations Arid and semiarid climates are characterized by the scarce and variable nature of their water resources. Therefore, plant life in such climates is conditioned by available moisture. Plants native to arid and semiarid climates have adapted with characteristics that enable them to withstand drought situations. Adaptations include various leaf formations and angles to the sun, leaf colors, and different types of root systems. Some plants utilize more than one of these traits. All of these characteristics help a plant tap into or preserve the limited water necessary for its survival. 40 Water-Efficient Landscaping in the Intermountain West Most plants adapted to dry climates have extensive root systems that allow the plants to take advantage of all available water resources. Some plants have deep taproots that are able to draw from permanent ground water (Figure 3-1a). Some have sinuous roots that are able to penetrate into rocky cracks and crevices or follow narrow seams of soil to gain access to water (Figure 3-1b). Some have fleshy roots that can store water during periods of drought (Figure 3-1c). Others have a shallow but widespread root system to take advantage of light summer rains. Some plants exhibit a combination of root systems. For example, yucca has a deep taproot but also has a shallow root system, thus enabling it to grow with no supplemental irrigation. Many plants native to arid and semiarid landscapes have root systems that are much more extensive than their shoots. An extensive root system ensures plant survival during periods of drought, and such plants are ideal for a water-efficient garden. Because drought-tolerant plants rely so much on their root systems for survival, it is critical that their root systems are healthy. Healthy roots take in water and soil nutrients essential to plant health. Thus, a successful water-efficient landscape will be one that accommodates the root systems of xeric plants. This means that the soil bed must be prepared in such a way as to welcome their growth habit. Most xeric plants prefer well-drained soil. Excess moisture around a plant adapted for drought conditions can be detrimental. If the soil does not drain well, it should be amended to create more pore space, or special attention should be given to the amount of water applied to a xeric plant growing in such soil (see “Principle 2: Soil Preparation” for more information). Figure 3-1. a. Tap root. b. Sinuous roots. c. Fleshy roots. The most common cause of plant death in a water-efficient landscape is overwatering. illustration: Adam Richards Appropriate Plant Selection   41 The most common cause of plant death in a water-efficient landscape is overwatering. Therefore, plants should be watered frequently only until root systems are established. Plants are not fully established until vigorous twig or top growth returns. For herbaceous plants, this may take several months. For trees and shrubs, this may take one to two years. Trees dug from the field take longer because most of their roots are left behind. Once plants are established, water should be applied only when necessary. In general, low-water-use plants should be watered thoroughly, then left unwatered until the soil is dry or until they show water stress. Irrigation needs will vary within each hydrozone , so this could be as often as once a week or as little as once a month (see “Principle 6: Efficient Irrigation” for specific watering needs based on hydrozone). Lawns and annual plants often require more frequent irrigation than trees and shrubs because of their...

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