Ground covers for dry climates

Arid climates place special demands on landscape ornamentals. Low humidity and rainfall and high summer temperatures dictate the use of tolerant plants as well as management techniques that cope with the high potential for water loss. You can achieve successful ground-cover plantings in dry-climate regions in two basic ways. The first involves using conventional ornamentals that are tolerant of heat, dry air and periodic moisture stress. However, landscapers often find it necessary to plant these ground covers in amended soils and sustain them with regular irrigation for good performance.

The second approach calls for native species and other regionally adapted plants that can survive with less soil improvement and that require relatively little supplemental irrigation. Both of these approaches are important for dry-climate landscapes, requiring the grounds manager to learn a variety of plant material, design strategies and management techniques. Here, we'll discuss the latter approach.

Native and regionally adapted species often provide special challenges. Plants from arid regions rarely grow as continuous, carpet-like ground covers. Further, many of these species have distinct periods of growth, flowering and dormancy that relate to seasonal moisture availability. Plus, few of them tolerate shade, and even fewer withstand poor drainage or excessive irrigation. Thus, you must choose species appropriate for your situation and then manage them carefully (see section, "Watering," page 91). Many times, this means exercising some benign neglect.

When working with native species, it is best to avoid large, continuous plantings of a single ground-cover species. Instead, develop a more natural-looking design by combining several plant species in natural-looking drifts or groupings. In this manner, you can arrange heights, colors and textures in combination with coarse rock or organic mulch to create pleasing compositions of color, form and movement within the landscape. This design strategy is useful for large perimeter and foreground areas. It also is a flexible approach; if a specific plant dies, it will not compromise the design or appearance of the ground-cover planting. This type of landscape also lends diversity of character and seasonal variation to its aesthetic value and, by design, will look different throughout the year.

Grade the site with topographic relief for added visual character, and then locate plants to take advantage of small microclimates within the contoured landscape. Contouring also can be helpful for capturing rainfall.

Entries, courtyards and raised planters often are places where color and refreshing foliage are the most welcome. By studying these areas and identifying other microclimates in the landscape, it is possible to incorporate a wide variety of ground covers into dry-climate landscapes. This enables you to make important design refinements while keeping maintenance and water use at appropriate levels.

Drip irrigation provides a distinct advantage over sprinklers by supplying moisture from below the plants. This avoids wetting foliage and surface organic matter, something that can promote disease in xeric landscapes. Plus, because of lower application rates, drip irrigation can run for longer periods without runoff. Thus, water penetrates the soil more deeply, which encourages deeper rooting and, ultimately, greater drought resistance. Drip systems also enable you to use the same line to apply different amounts of water to various trees, shrubs and ground covers. By using emitters with varying discharge rates and by adding or subtracting emitters as needed, you can tailor irrigation to meet the needs of individual landscape plants.

Among western natives and xeric plants, the following species have proven to be highly successful in a variety of landscape conditions.

Baccharis Perhaps the best all-around western-native ground covers come from the genus Baccharis, commonly known as coyote brush. These plants are native to coastal, inland and desert regions, where people appreciate them for their rich, evergreen foliage. Baccharis pilularis is native to the coastal bluffs and sand dunes of Central California and is in widespread use in arid regions well beyond its natural habitat. Two popular cultivars, 'Pigeon Point' and 'Twin Peaks', are excellent plants for erosion control on hillsides, along highways and in commercial and residential landscapes. These are robust shrubs, reaching 3 to 4 feet in height and spreading 4 to 6 feet. Establish them from rooted cuttings in the late fall, 24 to 30 inches apart, to provide a mass ground-cover effect. Coyote brush has a wide tolerance of soil conditions and accepts either drip or spray irrigation. It also tolerates shearing and edging. Grounds managers often rejuvenate plants every 4 to 5 years by cutting them back to the ground.

A recent introduction for use in low-desert landscapes is Baccharis 'Centennial'. This is a hybrid between B. pilularis and a desert species, B. sarothroides (desert broom). It has the prostrate habit of the former with the tolerance of dry heat and drought of the latter. It provides greener foliage than most desert plants and succeeds in transition plantings on golf courses, in mediansand along roadsides. Prune in late fall to manage its size and remove branches of upward growth habit. 'Centennial' grows well with bubbler or drip irrigation, which it needs during the hottest parts of summer.

Bougainvillea Using vining plants as ground covers is a widespread practice. Some of the best vining plants for warm and dry climates are the bougainvilleas. They are popular and widely known for their colorful flower bracts. Most species and cultivars are vigorous plants that need only periodic deep watering once established. They thrive in sunny, frost-free environments from the coast to low deserts. Landscapers often place bougainvilleas at the tops of slopes and banks to enable their long woody stems to cascade downward. Their flowering begins with the heat of spring and summer and extends through the last warmth of fall.

The most popular types of bougainvillea for ground-cover uses are horticultural varieties that range from mounding shrubs to large vines. Bougainvillea 'Rosenka' is a robust, mounding shrub with fluorescent pink-orange bracts that spreads 15 to 20 feet. B. 'Jamaica White' has large white bracts with a pink tinge; B. 'San Diego Red' is a vigorous vine growing 15 to 25 feet with bright red bracts. Authorities recommend drip or bubbler irrigation. Avoid late season fertilizing and watering-which leads to frost-sensitive growth-and prune overgrown plants and stems in spring after the risk of frost is past. Another tip: A bit of moisture stress during summer helps stimulate flower production.

Gazania Few ground covers enjoy as much popularity in warm-climate regions as gazanias. These perennials are valuable for their fast growth, colorful daisy-like flowers and tolerance of many soil and moisture conditions. A variety of flower colors is available, with the best floral display occurring from mid-spring through the summer and intermittently during other months. Because both clumping and trailing species are available, gazanias are useful for small or large spaces. They perform best with full sun and good drainage, and once established, they require only moderate amounts of supplemental water. Like many other fast-growing perennials, gazanias can be short-lived. Plants also can experience seasonal dieback. Thus, you should divide clumping varieties every 3 to 5 years to maintain their vigor. Trailing types can regrow to cover bare spots and thus help maintain more uniform coverage.

Among the many choices of gazania to use for ground covers, Gazania rigens leucolaena, trailing gazania, is perhaps the most durable and drought tolerant. This selection has yellow flowers, soft gray-green foliage, grows 4 to 6 inches high and spreads many feet. Plant trailing gazania with rooted cuttings 24 to 30 inches apart, and water with overhead spray irrigation. Once established, gazanias typically require water every 2 weeks during summer. Recent introductions flower more intensely and are extremely vigorous.

Lantana Some of the best ground covers for dry climates come from the subtropical and tropical climates of Central and South America. Such is the case with two adaptable and colorful species of lantana. Lantanas are fast growing plants that produce abundant, vibrantly colored flowers from late spring through fall and intermittently all year. They are strongly scented and benefit from pruning in late fall to manage their size and help regenerate their foliage. Once established, lantanas are heat- and drought- tolerant. Under prolonged moisture stress, the leaves wilt and drop, but new ones quickly regrow with watering. Lantana is sensitive to frost and sometimes will die in cold weather. However, in cool-climate regions, landscapers use lantana as an annual for its summer-long flowering.

Lantana montevidensis, trailing lantana, is a low, spreading shrub with arching stems that reach 12 to 18 inches high and spread 5 to 10 feet. This species is notable for its intense purple and white flowers and its usefulness in covering large banks, slopes, parkways and garden perimeters. Landscapers seldom use it as a single-species ground cover. Rather, they most often plant it around larger shrubs and trees or mix it with vining ground covers such as honeysuckle or ivy. Plant rooted lantana cuttings 15 to 24 inches apart and water by overhead irrigation. Established plantings need watering every 2 to 3 weeks during the summer, and virtually no supplemental water is necessary during the winter. Lantana can go many years without heavy renovation, but mowing or string trimming every 3 to 4 years helps control woody-stem buildup.

Lantana camara, sometimes called yellow sage, is a large, spreading shrub that is remarkably tolerant of dry heat, sun, drought and various soils. While not a true ground cover itself, it has been successfully hybridized with L. montevidensis to produce many colorful ground-cover varieties. These are typically low, mounding and spreading plants that you can group together in mass plantings or spot with other plants around rocks, on banks and in raised planters. Some popular cultivars from this cross include: L. 'Confetti', growing 2 to 3 feet high and 6 to 8 feet across with yellow, pink and purple flowers; L. 'Dwarf Orange', a mounding form 2 to 3 feet high and 3 to 4 feet wide, with bright orange flowers; and L. 'Spreading Sunset', a low, spreading variety, 15 to 18 inches high by 4 to 6 feet across that produces shocking orange-red flowers. These cultivars grow well with drip irrigation and tolerate cutting to control size and shape in late fall.

Myoporum One of the lowest-growing ground covers for dry landscapes is Myoporum parvifolium. It reaches just 6 to 10 inches high but has a 12- to 15-foot spread. This plant comes from the hot, dry interiors of southern Australia, where it grows in full sun and average soils. It has small, medium-green leaves and produces clusters of tiny but noticeable white flowers in early spring. In recent years it has become a favorite plant for slopes and roadside plantings in low-desert regions, where it produces remarkable growth with drip irrigation. In desert landscapes, this plant can be short-lived, lasting some 5 to 7 years. However, many designers and grounds managers believe its fast growth and handsome foliage offset this drawback.

Myoporum parvifolium strongly prefers good drainage and full sun. Plant it from rooted cuttings at 5 to 7 feet apart, and they will grow together in 1 to 2 growing seasons. Trimming along walks and removing overcrowded plants are common maintenance practices. This species does not require rich soil and, once established, it needs water no more than every 3 to 4 weeks.

Rosemary An old favorite, prostrate rosemary, Rosmarinus officinalis 'Prostratus', comes from the Mediterranean region. This woody evergreen has deep-green needle-like foliage and a spreading, drooping growth habit. It reaches 12 to 18 inches high with a 2- to 3-foot spread. The crushed leaves emit a spicy scent, and chefs sometimes use them as a culinary seasoning. The clusters of pale-blue to lavender flowers in winter and spring have a sweet fragrance.

Due to its attractive appearance and wide tolerance of coastal, foothill and desert conditions, rosemary is widely used throughout the West. It prefers full sun and is one of the few dry-climate plants that tolerates fine-textured soils and occasional poor drainage. For full coverage on slopes, along parkways and in commercial- and residential-scale projects, install container-grown plants 24 to 30 inches apart and irrigate by overhead spray. Because of its moderate size and well-behaved growth habit, you can let prostrate rosemary grow with natural form, or you can trim or mow it as needed in late fall to early winter.

Several other low-growing cultivars of rosemary have come into the trade in recent years. 'Collinwood Ingram' is a vigorous, spreading shrub with upward branching and intense purple flowers. It grows 2 to 3 feet high, spreads 4 to 6 feet across and works well on banks and in mixed plantings with mulch. 'Lockwood de Forest' has trailing stems similar to 'Prostratus', but its flowers are deeper blue.

Verbena Perennial plants often possess the valuable combination of fast growth and intense bloom, enabling them to provide soil stabilization and accent color until slower-growing shrubs develop size and character. Verbenas are among the most colorful choices that meet these needs. They have intense pink to lavender flowers from early spring to summer and provide at least 3 to 5 years of service before you will need to replace them. They are best in small drifts that create islands of color. Or use them individually on banks and around large rocks and tree trunks. Most species prefer full sun, grow well in warm coastal, inland and desert habitats and have a wide-spreading habit that enables the use of efficient drip-irrigation systems (as opposed to spray-irrigating the entire planting). Trim them back hard in fall or early winter to achieve the best habit, character and flower production.

Two species that are well adapted to warm and low-desert regions of the West include vervain, Verbena rigida, and moss verbena, Verbena tenuisecta. Vervain is a coarse-textured species with flattened stems and intense purple flowers that grows to 12 inches high and 3 to 4 feet in diameter. It is most useful as an accent plant in medians, along the edges of naturalized plantings of trees and shrubs or around boulders in dry streams. Moss verbena is a spreading perennial that works well in mass groupings that cover large areas. These groups quickly grow together and reach 12 to 15 inches in height. This species is effective when you arrange it to flow in soft, organic shapes throughout the landscape. Seed from both species can germinate and produce new plants.

Watering People have a strong tendency to apply too much water to landscapes in dry regions. This often is to satisfy the water needs of the many exotic species we use. However, it also is a natural human reaction to try to help plants withstand the onslaught of sun and high temperatures by being generous with water. The unfortunate outcome is excessive growth and an increase in pests and diseases-for most types of plants, but western natives in particular. Because these species tend to tolerate drought well, schedule your irrigation so that the soil remains somewhat dry most of the time.

While you may have to devote more careful thought to water management in dry-climate landscapes, that doesn't necessarily equate to more work. With proper consideration to species selection, design and cultivation, you can have beauty and dependability, even in the hottest climates.

Bob Perry is professor of landscape architecture at California State Polytechnic University-Pomona.

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