What kind of plants can be used?
First we interviewed the following local experts, Ms. Betsy Lagunas, (23) Mr. Jorge Gomez, (20) Ms. Anahi Padilla, (32) Mr. Wynn Anderson (2) and Mr. Oscar Mesta (28) who provided various lists of plants and their recommendations for what grows best in El Paso.
These local experts provided information regarding root systems, placement of plants, definition of low-water use and stressed the importance of an establishment period before leaving the plants to survive on their own. Establishing means allowing the plant’s roots to grow to its mature rate. For trees, this can take between 2 to 5 years during which time they need supplemental water in addition to the rainfall. Low water use usually refers to the established plant’s ability to survive only on rainfall.
Research began by investigating the drought resistant characteristics of these plants. An excellent source was Desert Plant Survival (11). Investigation turned to the soil, plant, water relationships found in the books Water Relations of Plants and Soil (22), The Plant in Relation to Water (26), and Plant Environment and Efficient Water Use (31).
Learning about the survival techniques of these plants, it became apparent that in order for our plants to survive in a passive capture system, more information on root systems was needed. Mr. Oscar Mesta provided invaluable information about the myths of root systems and their depth requirements. He provided the article The Structure and Function of Trees (15) that lead to other beneficial information such as “most root systems thrive if the soil moisture reservoir provides for storage of 18 inches of water. (Pierre, pg. 4) (31) and maximum radial horizontal spread of shrub root systems usually occurred at depths above 1 m (3 feet). (Gibbons & Lenz) (17). Figures 1 and 2 illustrate root systems from several different plant types.
Figure 1. Native guayule about 30 years old in Ector stony loam in Brewster County, Texas. (Pierre)
Figure 2. Roots of the sod-forming tobosa (PLMU, Pleuraphis mutica) and three perennial forbs, silverleaf nightshade (SOEL, Solanum elaeagnifolium), wrinkled globemallow (SPHA, Sphaeralcea hastulata), and desert zinnia (ZIAC, Zinnia acerosa) in a fine-loamy, Typic Calciargid, Dona Ana at Site 2. Plant tops are drawn at same scale as roots. The deeply penetrating roots of wrinkled globemallow indicate that it is better able to survive droughts than the other species shown. (Gibbons & Lenz)
With this information, the list of plants was trimmed. We looked for plants that were native to El Paso, drought resistant and only low water use. Then we checked the City of El Paso’s list of approved plants (1) to ensure compliance with city ordinances. Finally, we found the annual rainfall requirements most of which was acquired through the US Department of Agriculture’s database (43).
We converted the annual rainfall figures to daily rates. For the shrubs on our list, we compared our results to the published research, “Estimation Of Relative Water Use Among Ornamental Landscape Species”, conducted by M. Carmen Garcia-Navarro, Richard Y. Evans (16). This article provided a graph (Figure 3) of the daily water consumption for three types of shrubs. Since we selected spiraea shrubs for our plant list, we used their results, made the unit conversions and found that our results correlated with the study.
Figure 3. Regression of average daily water use of lysimeter plants on average daily water use of plants grown in 3.8 l containers (P < 0.001). Well-watered and stressed plants are represented by filled and open circles, respectively. Values associated with a species are followed by the species initial (A: Arctostaphylos, L: Ligustrum, S: Spiraea, V: Viburnum). Courtesy of “Estimation Of Relative Water Use Among Ornamental Landscape Species”
To maximize survival, only low-water use plants were considered.
In order to minimize the capture area ratio, the plant’s leaf and root area were considered.
The minimum daily average water consumption for deciduous and evergreen plants was 0.0262 and 0.0297, respectively, and these rates could be considered ET rates.
An establishment period of two years would be incorporated in the plans.
In the landscaping design, we would follow the landscaping requirements and “water-tight” plants for the El Paso area provided by the El Paso Water Utilities Conservation Department. (40-42)
For any grass areas, we would follow the Texas Water Development Board’s Landscape Water Demand model. (36)
The minimum depth storage of the soil profile would be 18 inches.
Well-drained, aerated soils were needed since in arid regions, drainage increases water-use efficiency because of its role in salt control and excess soil water removal will permit plant roots to proliferate the entire soil profile rather than being restricted to the upper few inches, which may later become too dry to supply the needs of the plants. (Pierre, p.219).(31)