Being a desert sited project, ambient air temperatures can be expected to reach in excess of 100 degrees F. regularly during the summer months, and may dip to the mid 20s or lower in the winter. Wind tunnels created by traffic serve to increase the wind chill effect in the winter, and increase evapo-transpiration in the summer. Undisturbed soil is nonexistent. Typically two feet of blended soil is brought in following final grading and top dressed over the site (this practice is referred to as soil plaiting). In areas that had previously been residential it is quite common to find topsoil depths in excess of two feet. Where rock outcrops or caliche reefs occur, the two-foot depth of churned topsoil is all the root zone that exists. For engineering purposes, that two feet of topsoil is compacted to 90% compaction. Into this challenging environment, we plant trees, shrubs, groundcovers, vines and accent plants and do what we can to help to thrive.

Tree Selection:
Trees provide the foundation of the landscape palette. They are often the most expensive part of the plant palette, and should be expected to live the longest (with the exception of a few accent plants like saguaro and maybe creosote) and carry the landscape when shorter lived groundcovers and shrubs wane. For these reasons, the trees we select must be of the highest quality and best adapted to the severe environmental and physical conditions outlined above.

The temperature extremes mentioned above are the primary reason for making good species selections. I have found that the best-adapted trees are the Sonoran desert natives, followed by trees native to the other deserts of North America and the world, semi-arid regions and then sub-tropics. In view of the soil conditions described above, a "native plant" may be considered foreign to a specific roadside location. This is not to suggest that the natural origins of tree species are not important. All the other adaptations that made a particular desert adapted species successful in the first place still give it the home court advantage along our urban highway system.

The key to successfully growing trees under these conditions (regardless of the species) is selecting quality, vigorous trees at the nursery. As consumers we frequently embrace the "bigger is better" philosophy. This presumption couldn't be farther from the truth where landscape trees are concerned. Nursery standards exist to help us and provide useful guidelines for "typical" height, width and caliper for trees of a given species and container size. Very large trees, particularly those growing in plastic containers are prone to root binding (roots that are circling, kinked and girdling). Rootbound plants that should always be avoided.

Whenever possible, plant roots should be examined prior to purchase. Examining roots of plants in plastic containers (1, 5 and 15 gallon) is relatively easy and only mildly disruptive to the root system. Boxed trees pose a different challenge. Above ground removal of the box will allow for examination of the root but can make successful transplanting difficult or, in some cases, impossible. Root examination is possible, but sometimes at the expense of the tree. When establishing budgets and contract criteria you may want to take into account the loss of some trees as a result of these inspections. When inspecting trees, make careful observation and take copious note. Record relationships like the tree height to trunk diameter as it relates to root development per species. Your observations and data from selecting tree for one project will likely prove valuable for future projects. Regular visits to local nurseries will keep you abreast of current production practices and the status of inventories.

Other things to look for when making a decision are trunk taper (a gradual decrease in the diameter of the trunk with a corresponding increase in height) and a well structured canopy devoid of co-dominant leaders with included bark (the latter occurs when the branch angle is sufficiently narrow to cause the bark to grow inward instead of being pushed up and out into a well developed furrow). Be on the lookout for proper pruning practices. If you can hang your hat on the stub of a limb that's been removed, then it's too long and a little more needs to be removed, if the exposed area left behind is long and oval instead of nearly round, then too much has been taken off (if your expertise is such that you can identify a branch bark ridge (bbr) and collar, then you already know what I am describing). The bbr and collar should always remain in tact. Leaf canopies should be reasonable symmetrical and full with good foliage color.

Hopefully its obvious that trees with broken or diseased trunks, branches or roots should be avoided when selecting nursery stock. This includes trees that have been boxed from the field and have trunks that are skinned up from nailing boards to the trunk. Carefully examine tree trunks that have been wrapped with cardboard, old carpet, or burlap. This isn't necessarily a bad practice (after all, it's intended to protect the tree from the boards that are nailed into it). But if they sit too long, they may harbor insects and pathogens, and depending on watering practices, tend to keep the trunk wood wet. Always examine the tree under the wrap and remove the wrap at planting time.

Planting:
With species and individual trees having been selected, it is time to plant. Planting guidelines developed by Tipton et.al. for the University of Arizona and available at the Extension office, have proven useful and on the mark in my experience. No need to add compost to the backfill mix, and no fertilizer until after establishment. It may be worth noting that not everyone in the industry, regardless of the number of papers published on the subject or the quality of the individual doing the reporting, has embraced this concept.

After establishment use fertilizers sparingly (especially when dealing with natives). Again here, keeping detailed notes will allow you to develop fertilizer schedules and application rates based on your historical experience (not just your memory). Remember three things about fertilizers: 1. They're an added cost. 2. They're salts. and 3. They're often the start of a cycle of fertilizing-watering-pruning-fertilizing-watering-pruning. Use compost as an organic mulch on the surface after planting to reduce surface water evaporation.

Now, if a good species and good individual of the species has been selected, and the proper planting techniques used, then growing and maintaining the plant will be easier. In fact, much of the rest of the work is making sure the plant has adequate water, checking and changing hose buffers (if the tree has been staked), and pruning.

Pruning is one of the more glamorous landscape tasks. And, while it is more glamorous than picking up trash, for instance, I think you will agree that neither the contorted shapes one has to assume, the stiff neck from looking upward, the sweat, nor sawdust filled eyes and nostrils are truly the definition of glamour. So here it is, pruning is not glamorous, it does little for the plant except reduce its size and ability to photosynthesize. A given plant should not be pruned on a weekly basis. Watering and keeping the irrigation system in good condition together with litter/trash pickup, should be the backbone of a good maintenance program. Plants in the landscape should be allowed to attain their own size and shape with little influence from loppers and saws.

Pruning may be necessary to remove dead, dying, or diseased limbs, criss-crossing limbs, protrudes, and (when trees are planted in the wrong place) to raise the crown. Youth and species selection may contribute to the need to prune more frequently. Young plants have a tremendous potential for regeneration. Likewise, some species, because of their ability to produce suckers, re-sprout from pruning cuts, and just plain fast growth rate, may be considered weedy. Such species when planted near walks or driveways will likely require twice per year pruning. A few, such as certain mesquites and Palo Verde have required monthly pruning during the growing season in order to keep them from encroaching onto sidewalks. As these trees approach mature size however, growth will slow, and the frequency with which we prune will decrease. In the evolution of pruning, trees that were once pruned monthly have matured to an annual pruning and then on to every other year pruning. In the near future I anticipate (and look forward to) the pruning cycle spreading to every three years. The process described has occurred over the past nine years on plants that are intended to shade sidewalks in the future and so were planted in places that otherwise interfered with pedestrian use.

For 10 years the Phoenix freeway landscapes have served as both classroom and laboratory where we have learned and experimented. The learning process in ongoing in this very harsh and challenging environment. We do know that successful landscape designs should incorporate desert-adapted tree species; and that the careful selection of horticulturally sound trees is critical to the long-term vigor and durability of the landscape.