The landscaper installing plants in the American Southwest encounters four distinct problems:
1. The soil quality is generally poor.
2. Irrigation is necessary throughout the year.
3. Higher altitudes can produce extremely hot daytime temperatures and very cool nights.
4. High winds dry out plants quickly and often damage them physically.
Arid soils generally fall into three categories: pure sand or gypsum, adobe, and caliche. Sand lacks both nutrient content and humus. Adobe is a heavy, clay-like soil that holds moisture better than sand but needs humus to lighten it and improve its aeration. Caliche soils are highly alkaline due to excessive lime content. They have a calcareous hardpan deposit near the surface that blocks drainage, making plant growth impossible. The hardpan layer may lie right at the surface or from several inches to several feet below ground level. The deposits may occur as a granular accumulation or as an impermeable, concrete-like layer. Generally, these are the characteristics of arid soils:
• lack humus
• require frequent irrigation
• are nutritionally poor; nutrients are continually leached out by the irrigation water
• are highly alkaline (pHs of 7.5 to 8.5 and higher)
• are low in phosphate; phosphate may be rendered unavailable by the high pH
• lack iron or contain it in a form unavailable to plants
• have a high soluble salt content resulting from alkaline irrigation waters and from manures and fertilizers that do not leach thoroughly
When installing plants in the Southwest, the landscaper must add organic matter to the soils to improve their structure. Organic matter improves the water retention capability of light sandy soils and breaks up heavy adobe soils. The only way to improve the drainage of caliche soil is to break through it and remove the impermeable layer. The excavated soil can be replaced with a conditioned mix that will support healthy plant growth.
To catch and retain the water so vital, yet so limited, in arid regions, the planting beds should be recessed several inches below ground level to create a catch basin (Figure 10-15). This method traps and holds applied water, preventing loss through runoff. In addition, organic mulches should be applied to a depth of four inches or more to slow moisture loss and create a cooler growth environment for the roots. Trunk wraps and whitewash paint are also applied to the trunks of trees to prevent water loss through their thin bark due to sun scald.
Cactus plants are sufficiently different from other plants to warrant special mention. They can be transplanted successfully by following these steps.
1. Before transplanting, mark the north side of the cactus. Orient this side of the plant to the north in its new location. The plant will have developed a thicker layer of protective tissue on its south side to withstand the more intense sunlight.
figure т-15. A recessed planting bed creates a catch basin for moisture. (Delmar/Cengage Learning)
2. By trenching around the cactus, lift as much as possible of the root system.
3. Brush soil from the roots and dust them with powdered sulfur.
4. Place the cactus in a shaded area where air circulates freely and allow the roots to heal for a week before replanting.
5. Plant the cactus in dry, well-drained soil. Stake the plant if necessary.
6. Water the plant in three or four weeks, after new growth starts. Thereafter, apply water at monthly intervals.
Whenever possible, native or naturalized plants should be selected for Southwestern landscapes. They have a better chance of surviving the transplant and they keep maintenance costs down. In situations in which the soil is especially unsuitable for planting, there may be little choice but to install the plants above ground in planters.