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Disclaimer: The information provided here was gathered from research literature published by the University of Arizona, other professional Landscape and  Horticultural organizations and our experience at Arid Zone Trees. Always consult local landscape experts for recommendation for your specific area.

 

Thornless Hybrid Mesquite

Few trees have been more instrumental in popularizing desert-adapted trees in landscape designs than Thornless or Chilean Mesquites. By providing abundant shade, lush green leaf canopy and graceful fissured brown trunks, Thornless Mesquites are another of the trees that dispelled the myth that desert landscapes were hot, barren, spiny and uninviting.

While its horticultural characteristics and taxonomy remain disputed, the Thornless Hybrid Mesquite is among the most popular and widely used trees in the desert landscape palette. As the common name indicates, trees are thornless, making them ideal for landscape applications from re-vegetation of disturbed desert sites to streetscapes, park plantings, commercial and residential landscapes. The form and texture of Thornless Hybrids blend almost seamlessly with surrounding native Sonoran desert trees and shrubs. They are easily incorporated into urban landscapes and are increasingly being used in more formal or traditional landscape designs. Other uses include: as theme trees along streets and commercial projects, as screens, wind breaks or barrier plantings, as transition trees, in re-vegetation projects or any landscape applications where ample shade is desired. This mesquite is well suited for planting in turf and non-turf areas.

At maturity, Mesquites can be up to 30 feet tall and as wide with symmetrical, dome-shaped, spreading canopies. The foliage is lacy and fern-like with compound leaves consisting of anywhere from 20 to 40 leaflets. Cold hardy to 10 to 15 degrees F, they can be planted throughout most regions of the desert southwest. Trees are semi-deciduous, losing only a portion of their leaves in warmer winters in the Phoenix, Arizona and Palm Desert, California areas. Las Vegas and Tucson, Arizona will have a little more leaf shed due to the lower winter temperature typical of those communities. Leaves remaining through the winter are shed rapidly in spring just prior to bud break.

In late spring, Mesquites produce yellow-green, unremarkable, catkin-like flowers. By early summer curved and sometimes curled tan to brown seed pods mature and are shed. With supplemental irrigation and fertilization, Thornless Hybrids grow at a rapid rate. Mature, established Mesquite trees can be naturalized to survive on annual rainfall in desert soils with good water holding capacity. Established trees are most lush and exhibit best growth and appearance with supplemental irrigation during the hot, dry summer months. Trees in well maintained lawn areas will exhibit the greatest growth benefiting from the water and fertilizer applied to the turf.

Thornless Hybrid Mesquites will tolerate an array of soil types and growing conditions from harsh, full sun to shady bosque or grove-like plantings. The first 2 to 3 years following transplanting are critical in establishing a well distributed root system. In typical landscape settings where water and nitrogen are abundant, Mesquites can produce large dense leaf canopies from rather limited root systems. These trees are often easily damaged or completely uprooted by the high winds associated with the summer rainy season.

Proper staking, seasonal pruning and regular deep irrigation during the spring and summer help Thornless Mesquites resist the damaging effects of high winds. Wind damage is NOT an unavoidable horticultural shortcoming of Thornless Hybrids. Making trees "seek out" water and nutrients, by appropriate arrangement of irrigation emitters and application of nitrogen, fosters the development of a more dispersed root system and reduces the risk of wind throw.

During tree establishment, pruning to remove about 20% of the canopy during the growing season will help promote root development that is proportional to the shoot growth of young trees. In areas with severe summer storms (monsoons), it is important to complete this pruning prior to the beginning of the storm season. Additional pruning, 3 to 4 weeks later, will lessen the risk of wind-throw and branch damage. Removing more than 20% of the canopy can inhibit rooting and encourage undesired re-growth made up of dense flushes of branches and leaves. Selective pruning will foster the development of a symmetrical canopy with well-spaced branches. Stake trees only when absolutely necessary and then only briefly. A low-breaking, upright tree occupies no more room than a standard-trunked specimen yet retains the natural wind resistance of trees found growing native in desert settings.

Shade is a welcome addition to all deserts landscapes but can inhibit the growth and flowering of some under-story plantings. The shade produced by Thornless Hybrid Mesquites can range from filtered to quite dense. Carefully consider the ultimate shade that can be produced by these trees and the impact of that shade on the growth and flowering of under-story plants and turf.

Since their introduction in the 1950's, horticultural professionals have become accustomed to referring to Thornless Mesquites as Chilean Mesquite or Prosopis chilensis. The actual botanical name of this specie has long been a subject of discussion. Within the landscape industry, Prosopis chilensis, Chilean Mesquite and Thornless Mesquite have evolved to become synonymous trade terms with only limited botanical relevance. To avoid confusion it is advisable to specify trees on plans as Prosopis chilensis, "Thornless" or simply Thornless Hybrid Mesquite and avoid using a botanical name altogether.

The increased acceptance of arid landscape trees, mesquites included, has fostered a demand for greater uniformity in appearance and growth characteristics. Efforts to increase uniformity of Thornless Mesquites have tended to emphasize vegetative/clonal propagation (rooting a small portion of the plant). Vegetative propagation methods produce uniform trees by limiting the genetic diversity within the propagation stock.

Genetic limitation has advantages and disadvantages. Genetic diversity protects plant populations from devastating attacks by injurious insects and diseases. Criteria for selecting parent trees for clonal propagation tends to focus on the phenotype or external features of trees. Upright branching, dense foliage, trunk structure or rapid growth are examples of phenotypes growers might use in selecting parent stock. Appearance alone gives little indication of rooting habit, cold hardiness, disease and insect resistance or adaptability to varying soil or growing conditions.

Seed and clonal Propagation programs must diligently and regularly evaluate the horticultural qualities of the trees currently in production, while continuing to look for new and better selections to incorporate. The popularity and varied uses of Thornless Mesquites requires that growers offer both uniform, standard and multiple trunked trees and more informal/native-looking specimens. Producing an assortment of shapes and forms of Thornless Mesquites will require different parent selections for clonal propagation.

Given the attractive physical and horticultural qualities of the Thornless Mesquites it is little wonder they remain popular and widely used trees. Efforts by nursery professionals to improve the quality of trees coming to the market insure that Thornless Mesquites will continue to meet the ever expanding uses of this tree in desert landscape designs.