Many communities have developed landscape design standards that require the planting of shade trees in new parking lots as part on an effort to mitigate the urban heat island effect. In addition to generating much needed shade, parking lot landscapes contribute to the quality of life in the community, and offer another landscape setting to create a sense of place in the desert. Added trees can help communities meet new EPA guidelines for dust and particulate abatement and meet the Clean Air Standards Act by removing pollutants from the air.

For parking lot trees to thrive and provide the desired shade, landscape professionals must understand how harsh, both physically and chemically, this environment can be. The ability of asphalt to accumulate heat from the sun and reflect and re-radiate it is well known. In the desert southwest, parking lot trees must endure this heat well into the night during a significant portion of the spring, summer and fall months. The size of the planting areas, sometimes called cut-outs, must be large enough to accommodate oxygen exchange, and adequate distribution of irrigation emitters to promote a well dispersed root system. They should allow sufficient space for trees to be properly staked, if needed, following transplantation and be large enough to support the tree as it matures.

Water penetration is critical to root distribution. Prior to paving, parking lots are graded and compacted to support the weight of motor vehicles and resist settling and cracking. Highly compacted soils can act to inhibit tree growth in two ways: 1) roots do not grow into dry or oxygen deprived soils, any factors that limits water and oxygen penetration and distribution (like high compaction) limit the growth and exploration of roots and 2) less aggressively rooting trees may be unable the penetrate compacted soils even when moisture is present. Poor root distribution or a weak root ball makes trees more subject to wind throw and reduces the capacity of the root system to extract moisture from the soil. Planting practices should include excavating a broad planting hole, fracturing as much surrounding soil as possible and back filling with a porous (non-compacted) soil mix.

Chemical barriers can also create problems. Some paving specifications call for the application of herbicides to the soil sub-grades prior to paving. The purpose of these herbicides is to prevent plants from growing up through the asphalt, creating cracks and breaks. Landscape architects should review the construction plans with the project architect or engineer to insure that, if applied, herbicides are labeled for use around trees and shrubs. If tree planting cut-outs are to be added to an existed parking lot, it is prudent to review the original construction specifications for information about any previous herbicide applications.

It is a common misconception that tall single-trunked (standard-trunked) trees must be used in parking lots in order to accommodate pedestrians and cars. As in all other landscape applications, selecting low- branching, V-shaped, upright growing, multiple-trunked desert trees offer the best resistance to wind throw and require the least maintenance. Installing specimens with this form, coupled with proper maintenance insures that, for the long term, trees will have sound horticultural structure and be pedestrian and vehicle-friendly. If cut outs are too narrow or too small to accommodate an upright multiple trunked specimens, they are likely too small to allow the long term survival of most desert adapted trees.