Selecting Ornamental Trees for New Mexico
Curtis W. Smith, Extension Horticulture Specialist, Department of Extension Plant Sciences
John Mexal, Professor, Department of Agronomy and Horticulture
John White, Extension Horticulture Agent, Doña Ana County Extension Office
Rolston St. Hilaire, Associate Professor, Department of Agronomy and Horticulture
College of Agriculture, Consumer and Environmental Sciences New Mexico State University
Trees provide many benefits to us and our environment. They cool our cities, cleanse the air, recycle oxygen and reduce noise levels. In addition, trees enhance our quality of life by providing habitat for desirable wildlife and creating a restful environment. However, with New Mexico’s varied landscape, where less than one-third of the state covered with native forests, selecting trees that will thrive in this environment is challenging. Trees planted in our cities and around our homes grow under climatic and soil conditions that may not naturally support tree growth. Nevertheless, New Mexicans need trees that tolerate our soils and climate. The purpose of this publication is to provide a description of trees that are adapted to New Mexico. While, many of the trees recommended in this publication are not native to New Mexico, they will adapt and thrive with appropriate care.
Selecting a Tree
An important consideration when selecting a tree should be the planned function of the tree in the landscape. This planned function will determine which tree is chosen and where it is planted. Trees may be chosen for their shade, flowers, seasonal leaf color, fruit (presence or absence), wildlife habitat, size and architectural form. Growth rate also is a consideration, but often not be the primary reason for selecting a tree. Trees that grow rapidly tend to have a short life and create hazards, because they often have weak wood and increased disease and insect problems. Trees should be considered a long-term investment as a well-placed, attractive part of the landscape that can substantially enhance the property value.
When choosing a tree for the landscape, consider the location in which the tree will be planted. Mature tree height and spread should be considered when selecting a site. Distance from structures, roads, walkways, walls and other paved areas are factors that must be considered. For example, if planted under power lines, trees eventually will interfere with power line maintenance and create electrical hazards. Poor site selection eventually could lead to structural damage, necessitating removal of the tree or pruning that could decrease the tree’s form, function and value. Potential problems can be avoided by matching the tree to the site. If you desire a specific tree, be sure to find a site appropriate for that tree. If your concern is a specific site, select a tree appropriate for that site. Many tree problems in New Mexico result from a failure to match trees and sites.
An unseen potential problem is the root system of an actively growing tree. As a tree grows, the root system expands beyond the tree’s drip line. (The drip line is the area of soil beneath the ends of the tree’s branches.) Trees planted close to walkways or other pavement can cause of the pavement to lift up due to root growth creating a hazard to foot traffic. Roots also can clog sewer or septic lines, resulting in costly repairs. Most of the tree’s roots are in the first 18 inches of soil. However, the roots can spread a distance 1.5 or more times the height of the tree from the trunk. Thus, a 40-foot tree can have roots exploring soil 60 feet or more beyond the trunk. As a general rule, most of the large roots, which cause structural damage, will be found under the dripline of the tree at maturity. Smaller, less destructive roots extend much further. However, as these roots absorb water and nutrients, they may cause problems with septic systems.
To adequately care for trees, homeowners must irrigate and fertilize well beyond the drip line. The homeowner should anticipate the future size of the tree (above and below ground) in the landscape.
Another way to avoid problems is to choose trees adapted to the soil and climate. Soil conditions to consider include depth, drainage, caliche layers, texture, pH and salinity. Problems resulting from these factors can often be avoided by knowing your soil conditions and selecting trees adapted to these conditions. Your county extension agent can help determine your soil conditions.
Finally, consider selecting trees adapted to New Mexico’s planting zones (Fig.1). These planting zone guidelines are based on climatic conditions under which the trees must grow. However, these guidelines do not consider microclimates due to changes in elevation over short distances and location relative to structures. Sites near the zone boundaries should expect harsh conditions more often than areas located farther from the zone boundaries. In New Mexico, extreme weather will sometimes damage or kill trees. However, planting within the zones indicated (Table 1) should minimize the risk of tree death due to climatic stress.
Figure 1. Average number of frost-free days and average date of last frost in three growing zones. From Climatological Data, Annual Summary-New Mexico, 1982, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce.
Some tree species may suffer winter injury from severe winter storms. Winter injury can vary from death of succulent tissue, to small branch dieback, to damage to the southwest side of the trunk, to death of the entire tree. However, this should not discourage experimentation in the landscape. A tree’s unique beauty may be enjoyed for many years, even if it is eventually replaced. Additional information can be obtained from your County Extension agent or nursery professional.
The list of trees in Table 1 is not intended to be comprehensive. This list is intended to provide guidance in selecting trees, identifying their positive and negative characteristics, and offering suggestions on where these trees can be planted safely. Do not let this list discourage the use of other interesting tree species that may do well in the care of a conscientious homeowner.
Table 1. Selected tree species for New Mexico. N.B. Eleagnus angustifolia (Russian olive) and Ailanthus altissima (Tree-of-heaven) are not recommended for planting in New Mexico. These two species are prolific seed producers and spread rapidly.
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Printed and electronically distributed January 2005, Las Cruces, NM.