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Cecily Gill COLUMN

At the gardens


Native trees grow well in Tucson basin

The term “native” in native trees can be pretty confusing. Native to where? Arizona? Southern Arizona? Mountains, washes or valleys? The whole point of knowing whether a tree is native to where you live is so you’ll know if it will adapt to the conditions affecting your yard: mineral soils, hot summers, cold winters, windy springs and long dry seasons.

The Tucson basin, where most of us live, lies within the Santa Catalina, Rincon, Santa Rita and Tucson Mountains. It extends to about 4,500 feet in elevation, about the point where it becomes too cold for Sonoran Desert plants to thrive, and where the beginnings of mountain plants take over. Trees native to our valley are often associated with washes, but not always. They provide welcome shelter for birds, butterflies, lizards and serenading cicadas.

Some desert trees are shrubby in character but can be pruned into small patio specimens in just a few years. This describes our valley acacias pretty well. They tend to be prickly customers with the bonus of sweetly scented flowers. Whitethorn (Acacia constricta) reaches 10 to 18 feet tall, with an open, airy appearance. The catclaw acacia (Acacia greggii), is about the same size, but with leaves clinging tightly along the branches, giving a sculptured look. I favor these open-branched acacias for supplying enough light to plant cacti beneath. The prickles give the plants the nickname “wait-a-bit” as they catch your shirt, but both will enhance any naturalistic garden.

The velvet mesquite (Prosopis velutina) is graceful and broad and needs some room. In eight years you can put a picnic table under it. Size and shape are not always predictable because of inherent variability, but most reach 15 – 30 feet high and wide. Branches can grow low and long and eventually assume massive proportions, especially if a water source is tapped (like a flower bed). Velvet mesquites favor a valley wash or riverside locale. Young mesquites are menacingly spiny, which protects them from getting chomped by animals. Mature mesquites become less thorny with age.

Desert ironwoods are spectacular trees. They grow commonly west of Tucson in the lower deserts, where winters are warmer. But there is a finger of amenable habitat on Tucson’s Northwest Side, giving that part of the valley a different character. Ironwood (Olneya tesota) requires a warmer location, and is not found in colder places near washes. In colder valley locations, frosts at 20 degrees and below will cause damage to branches. The slow growth rate of ironwoods may fool you into thinking they will be small, but make sure you have enough room. Mature trees range from 15 to 25 feet tall. The lovely lavender blossoms appear in early summer.

Blue paloverde (Parkinsonia florida, formerly Cercidium floridum) prefers the moister microhabitats of desert washes. It tolerates the colder, lower valley areas as well as higher locales like Green Valley. Robed in yellow blossoms in spring, it spreads a rounded and wide blue-green crown to 25 feet high and 30 feet wide.

The foothill paloverde (Parkinsonia microphylla) is nearly lime green in comparison to the bluer bark of its cousin. The trunk has knobby knees and black scars, a tough tree used to scrabbling a foothold in the rocky foothills of the Tucson basin. This tree is a slow-starter and may take many years to reach its maximum height of 10 to 20 feet. The spring flowers are bicolored, white and soft yellow.

These trees all belong to the same botanical family, the legumes, a characteristic group in the Sonoran Desert. They are well-adapted to extremes, losing leaves during cold spells and often during dry periods. They also tend to grow slowly at first and applying more water won’t really speed them up. It will just make them gangly looking. Nor can they tolerate total neglect. Water thoroughly twice each month the first summer, then less in succeeding years. Look for these trees at nurseries that have a good selection of native or desert plants. Try buying them in 15-gallon containers. A large size will give a head start.

Learn more about desert trees by visiting the Tucson Botanical Gardens, Tohono Chul Park, the Desert Museum and the UA Arboretum; and by reading up in “Native Plants for Southwestern Landscapes” by Judy Mielke and “Gardening in the Desert” by Mary Irish.

Cecily Gill is curator of the horticulture at Tucson Botanical Gardens, 2150 N. Alvernon Way. She has gardened there since 1986.

PHOTO CREDIT: Photos/Tucson Botanical Gardens

CUTLINE: (Above) Flowers of the blue paloverde, (upper right) catclaw, and ironwood in full bloom.

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