Multiple Trunk Desert-Adopted Trees
It’s far too common for landscape professionals, including wholesale tree growers, to ignore the natural growth characteristics of desert-adapted trees when pruning and shaping in the nursery or in the landscape. Trees native to mixed conifer and hardwood forests have a generally upright growth habit, dominated by a strong central leader. In their native forests, competition for survival is for sunlight. The ability to grow straight and tall gives these trees a significant competitive advantage over trees with less upright or slower growth habits. On flood plains or in riparian areas, increased soil moisture affords trees the luxury of not having to conserve water resulting in faster, more vigorous growth and often more columnar form (e.g., Cottonwoods). With the availability of water and nutrients not limiting factors, competition for space and sunlight become more intense. These situations result in taller, more upright trees growing closer together. Resistance to wind remains high since individual trees function as windbreaks for each other.
In their native habitats, desert trees species branch at or just above ground level. While they may begin their lives as single leader seedlings, vegetative buds at the leaf nodes and along the trunk are soon stimulated and multiple branching results. This is particularly true of arid (xeric) sites where trees such as Velvet Mesquite (Prosopis velutina) and Catclaw Acacia (Acacia greggii) often develop dense hemispheres of branches covering the ground around the tree bases.
In the southwestern deserts, critical competition is for water. In these habitats, trees are typically highly dispersed and tend to grow about as wide as they are tall, producing branches that often extend to the ground. This low branching helps to conserve moisture within the tree’s dripline by shading the ground from the direct rays of the sun. It also diverts winds around and over the leaf canopy, instead of through it and reducing water loss due to evapotranspiration. The shade produced also inhibits the germination of other seeds close to the base of the tree which serves to reduce competition for available moisture and nutrients. This unique growth habit cannot be ignored when developing pruning practices for desert region trees in landscape applications. The goals of pruning desert trees must be to promote tree vigor and health while enhancing and complimenting the natural, genetically controlled, forms of these native species.
For pruning information please visit www.aridzonetrees.com FAQ “Pruning Desert Trees”.
When we design and construct residential and commercial landscapes, neither water nor space are limiting factors, so desert trees can and do grow aggressively. This growth can cause problems in areas where people and vehicles need to pass, such as along sidewalks and paths or in parking lots. Pruning is used to control and direct the growth of woody plants, particularly trees. Unfortunately, in many instances, tree structure and natural stability are destroyed in the process. For example, many multiple trunk Thornless Mesquites are pruned to a single, crooked trunk in the mistaken belief that the tree’s width can be reduced or controlled. Most properly pruned and shaped, multiple trunk trees take up no more horizontal space than do single trunk trees. The trunks (3, 5, or 7, in most cases) can be selectively pruned to establish the lowest main laterals or scaffold branches at pedestrian friendly heights. Over time, branches can be "laced out" or thinned to improve sight lines and pedestrian access. With the thinning of the lower branches, wind is still diverted around and over the tree allowing them to pass more easily through and around the tree.
The introduction of cloned or vegetatively propagated varieties of desert species has made the task of pruning and shaping trees, for increased pedestrian access, easier. Trees being considered for ‘Variety AZT’ cloning must first be evaluated for upright structure, proper branch angles, response to pruning and other physical and horticultural qualities before being considered as potential “Mother” stock. With cloned trees, genetic uniformity, consistent and predictable growth habits, and growth rates make the process of fostering desirable tree forms simpler. Such trees respond to pruning in a reliable and predictable way making long term maintenance, to retain and encourage this form, easier and ultimately more successful.
Growers of desert tree species typically produce both single (a) leader and true multiple trunk trees (trees which produce multiple vertical branches at or near the soil level) as well as "low-breaking multiple trunk" (trees which branch from 6” to 24"). A higher percentage of singles are produced by most growers than exist in nature. This tendency can be attributed to: 1) Convenience--it is often easiest for a nursery worker to select the largest stem and tightly stake it up, thus avoiding having to make critical pruning decisions required to produce attractive multiple trunk trees; 2) Convention--a client's preconceived notion of what a tree "should" look like often incorporates a “straight-trunk,” rounded leaf canopy (e.g. "lollipop" effect); and 3) Confusion--the incorrect assumption that a single leader tree takes up significantly less space than a multiple and that multiple trunk trees interfere with pedestrian and vehicle traffic. As a result, “single trunks” are often specified for use in parking lots, street plantings and pedestrian paths, when the selection of cloned multiple trunks with vertical growth are equally appropriate.
There are situations in the landscape where even narrow multiple trunk trees would not be appropriate (e.g., a three to four-foot-wide planting strip between a sidewalk and a wall). In such instances it would make sense to select desert tree species with a vertical rather than a spreading habit. Choices include Palo Blanco (Acacia willardiana), Shoestring Acacia (A. stenophylla), and Mulga (A. aneura). Such trees would function better than species with crowns that would be crowded by the wall or interfere with pedestrian access.
The use of multiple trunk desert trees in the landscape should be encouraged in all facets of the landscape industry, from design to construction to retail sales. The natural grace and beauty that multiple trunk, desert-adapted trees bring to the urban landscape are important factors in the health, aesthetics and durability of constructed desert landscapes. Tree form or structure also plays a role in wind throw. Retaining this attractive, more natural, multiple trunk form provides easy pedestrian access, offers ample shade, essential structural support and lowers the risk of wind throw without compromising the other uses of the landscape. Wind throw is not a problem unique to desert-adapted tree species like Thornless Mesquites. Proper planting, pruning, tree form and placement, irrigation and fertilization are all keys to vigorous long-lived landscape trees.
(a) The definition “Standard” found in Hortis Third (Macmillan Publishing Company, New York) more accurately describes small statute, patio-type trees (with 3 to 6' of clear straight trunk) like tree roses, tree Fuchsias and Azaleas than it does the vast majority of desert adapted tree species. AZT uses single leader trunk to better create a distinction image between single trunk, low breaking multi trunk, and true multiple trunk specimens.