Shrubs offer hot color for the desert in summer

Summer in the deserts is both long and intense. Cabin fever sets in around mid-July and one still has almost six weeks to go before there is a significant let up. Gardening slows to a murmur and even the most dedicated are only able to manage a brief window of outdoor work early in the day. Still we all crave color and interest in the garden, even if you are mainly looking at it from behind an air-conditioned window. Luckily for desert gardeners, there are host of excellent shrubs that wait for the heat to put on their best flowering.

Although many common shrubs like hibiscus (Hibiscus rosa-sinensis), lantanas and oleander (Nerium oleander) form the basis of summer flowering, there are many great choices that reward us with their best color in the hottest time of the year.

Flowers galore

While red bird-of-paradise (Caesalpinia pulcherrima) is the poster child of summer flowering in the low deserts of the West, its intense orange to red-orange flowers can overwhelm smaller places or quieter gardens. When the reds are too much, try one of the pure-yellow forms, one of which is sold as Desert Bird. Even more sedate is its close relative, yellow bird-of-paradise (Caesalpinia gilliesii)that has a prolific bloom over most of summer. The buttery-yellow flowers are accented by 6-inch, scarlet stamens. This shrub has a more open form than red bird-of-paradise and is much more hardy growing into USDA Hardiness Zone 8. Grow any of these in enriched garden soil or rocky, native ones. Weekly watering in the hottest summer keeps the plants in good flower, although less frequent watering is necessary in milder areas.

Most of Baja California is extremely arid and the Southern parts of the peninsula offer two outstanding summer flowering shrubs for the area, San Carlos hibiscus (Gossypium harknessii) and red fairyduster (Calliandra californica).

San Carlos hibiscus is a large, rounded shrub that grows 6 feet tall and wide. The leaves are dark green, leathery and cover the light-gray stems almost completely. The deep-green leaves are cooling, making it a good choice for hot, reflective areas like a pool. Throughout summer, the silky-yellow, 3- to 4-inch flowers make a stunning show. This shrub is reliably hardy to the mid-20s.

Red (or Baja) fairyduster has become the darling of the summer-flowering crowd in the region. This openly branched shrub is rarely more than 5 feet tall and has widely spaced, pinnately compound, dusky-green leaves. Clusters of minute flowers have large, showy scarlet-red stamens. While there are hybrids between this species and fairyduster (Calliandra eriophylla), these forms tend to be coral to pink. As might be expected, hummingbirds are wild for it. Temperatures below 20°F will cause severe damage.

Both of these Baja shrubs are astoundingly heat tolerant and are used widely against west- or south-facing walls, or in any situation where reflected heat is intense. Each can be watered as little as every three to four weeks in summer and both do extremely well in native, rocky and alkaline soils.

Desert cotton (Gossypium thurberi) is a close relative of the San Carlos hibiscus but is native to the hills of southern Arizona and south into Mexico. This deciduous, upright shrub has light-green leaves with three prominent lobes. It grows 8-10 feet tall and is hardy to the low 20s. Flowering begins in late summer, once the winds have shifted and the humidity rises. The 1- to 2-inch flowers are white to pale pink and are prolific.

Island-influenced color

While it is hard to think that the Caribbean islands and the deserts of Southern Arizona have much in common, two summer-flowering species come to the region from those islands, skyflower (Duranta erecta) and Barbados cherry (Malpighia glabra).

Skyflower, often known simply as duranta, is a loose-limbed shrub with soft, light-green foliage. Throughout summer, cascades of light-blue to indigo flowers open in sequence along a long, hanging inflorescence. Flowers are followed by round, gold fruit that persists for months and is the source of the common name. Newer selections like ‘Sweet Memory,’ which may be the same as ‘Geisha Girl,’ with its purple corolla lobes rimmed in white do not set or hold as much fruit, which is something of a loss to me. There is a white-flowered form, as well as one with white-variegated foliage that are intermittently available in the region. Skyflower loses leaves in the low 20s, but is root hardy to 20°F.

Barbados cherry was first introduced in the Phoenix area as a dwarf form often sold as ‘Mariquita.’ This form grows only 3 feet tall, but the species is capable of becoming a larger shrub or small tree to 10 feet tall. Barbados cherry has an intricate branching pattern that gives the plant a dense, regular shape. The 1-inch, pinwheel-shaped flowers are pastel pink in summer and are followed by deep-red fruit that is much favored by birds. Hardy to the mid-20s with minor leaf damage, it is reported to be root hardy to 20°F.

Both species are tolerant of highly alkaline soils but need deep soaks two or three times a month in summer in the hottest areas to flower and look their best. Both grow and flower well in either full sun or light shade.

Other choices

I have long been a fan of firebush (Hamelia patens). Although hardy only to the high 20s, it is a stunning choice where winters are mild or it can be well-protected. The 1-inch, tubular, red-orange flowers occur in clusters at the ends of the branches. While flowers form quickly in spring, they don’t open until the mercury has topped 100°F for days on end. The flowers continue to open over the entire summer and are followed by small, round, black fruit that birds adore. As the days cool in fall, the leaves turn burnished copper color before they fall for a brief time over winter.

The Chihuahuan desert regions offer countless excellent plants, but one that I value particularly for its summer color is Mexican bauhinia (Bauhinia divaricata). This small tree or shrub, usually 6 feet tall, has an open, loose branching style. Like all bauhinias, the stems are brittle and will break easily in the wind or with careless pruning, so consider location carefully with this species. This species is hardy, although deciduous, at temperatures to 20°F, and is root hardy to at least 15°F. The 1-inch flowers are white with tinges of pink and appear to lay along the limbs.

Australia is not a place we normally consider for summer-flowering shrubs, but one hybrid is a clear choice. This hybrid between Eremophila polyclada and E. divaricata (often sold as ‘Summertime Blue’) is a delicate-looking shrub with thin, needlelike leaves widely spaced on the branches. The tubular, 1-inch flowers are delicate, sky blue and bloom repeatedly through summer.

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- Mary Irish

April 2008