Plants that can survive the arid, hot conditions of the desert have adapted across millennia to live in a harsh, but incredibly fragile, ecosystem. The majestic saguaro cactus of Arizona is a testament to Mother Nature's ingenuity.
Bringing other plants into the desert environment requires attention to soil, sun, and water. Many non-native plants simply can not be compensated for the extreme heat.
Desert soils are often relatively poor because conditions are not conducive to developing thick organic layers of nutrient rich soils. Gardening is challenging and requires particular attention to developing a rich soil matrix to support plant life.
The Desert Southwest in particular has a unique problem not common elsewhere: caliche. Caliche is a layer of calcium carbonate (i.e., lime) that forms a solid stratum a few inches to a few feet below the soil's surface. If you have caliche, you may already be aware of the need to drill through that layer if you want to plant something taller than an opuntia. Trees, in particular, often have deep tap roots that necessitate drilling. Sometimes, you can break through a layer of caliche with a pickax if it isn't too thick, however, it isn't uncommon for several strata of caliche to exist. Failure to ensure good drainage guarantees stunted plants in a best case scenario and more likely will result in death.
Removing caliche is the best guarantee of gardening success, especially where trees and larger shrubs are concerned, however that can be impractical in many cases. An alternative suggested by the Arizona Cooperative Extension is to drill "chimneys" through the caliche that allow improved drainage. Nevertheless, the larger the plant, the larger the area taken up by the mature root ball so caliche removal and replacement with an enriched organic soil may be necessary if you want to grow trees or larger shrubs.
Soils may also have an extremely high pH because of their alkalinity. Testing is essential so amendments can be added to mitigate the soil's inherent character and improve mineral absorption. In particular, iron deficiency can be a problem. If your plants have yellow leaves, but green veins, it's probably an iron uptake problem. Check with your local nursery or county extension for remedies appropriate to your situation.
One of the primary requirements for plants, and a substantial challenge in the desert, is water. Some plants, like roses, can do well as long as they have some protection from the hottest sun and ample amounts of water. Raised beds with their rapid drainage don't work as well in the desert as bermed holes that have raised edges to contain and direct water toward the plant's root system. In addition, supplementing desert soils with water retainers such as Zeba Quench™ that, along with good mulch, may reduce the amount of water wasted and improve plant health. It doesn't hurt that Quench is corn-starch based and biodegradable unlike similar sodium-based products that may degrade more slowly and affect soil health.
A number of non-native trees that have done well in the desert include olive, cottonwood, and mulberry. Unfortunately, all three have been outlawed in the Phoenix area because of the high levels of pollen, which have caused major allergy issues for many people. Small trees like palms and mesquite both do well. Natives are usually scrubby and small, often with needle-like leaves, tough, and adapted to a relentless climate.
Finding trees for a desert garden can be challenging. The best plants take the least water and are indigenous to the area. However, as long as the plant is reasonably drought tolerant, there is no reason not to experiment with additional plant materials. The following are drought-tolerant, relatively low maintenance, and some even produce something useful:
Pomegranate (Punica granatum)—deciduous, 12-15 ft. tall. Edible fruit. Useful as a hedge and for shade. Any soil type and light pruning to desired form.
Bottle tree (Brachychiton populneus)—evergreen tree that eventually can grow up to 45 feet tall. Fast growing with glossy green leaves. The bottle tree has interesting woody, brown pods. It requires regular water, especially in summer, but is low maintenance.
Fig (Ficus carica)—Big leaved trees that love the heat and produce sweet fruits. Figs can be espaliered to cool exterior south-facing walls. They grow fast to about 20 feet tall or more. Prune to shape and clean up during fruiting are regular chores.
Citrus (Citrus) like lemons, oranges, and limes do well especially around Phoenix with its expansive heat island. They need water, but can take some drought, and a good feeding schedule. Once upon a time the entire Valley was citrus from one end to another and the April breezes carried the scent for miles. If you are located in a higher altitude region, plant in pots and winter indoors.
Desert Ironwood (Olneya tesota)—This is a true desert scrub tree and essential to the desert ecosystem which is threatened by sprawl. It grows to about 20 feet in height and is extremely drought tolerant. With enough water, it's an evergreen and can be used as a shade tree. A member of the pea family, the tree blooms in April and produces seedpods with brown beans that can be ground into flour.
Sweet Acacia (acacia smallii)—The semi-deciduous sweet acacia reaches 15-30 feet in height, grows quickly, and provides an attractive small tree that provides filtered shade. In spring, it has ball-shaped, yellow flowers. Maintenance includes regular pruning of the trunk to maintain shape. Well-adapted to desert life.
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