Identify the purpose of your landscape design. Are you designing an entire property landscape that will show off your home to best advantage, or an enclosed, private, back-yard garden? Are you trying to maximize the environment for a favorite tree, shrub or flower bed that you intend to plant? Or are many plants are already in place and you need to design around them?
Landscape architecture includes walls used to create shade, divert wind, and provide privacy; raised garden beds that provide soil drainage and keep rabbits away, walkways and steps to direct traffic and make climbing easier; and rock or masonry channels that divert water and prevent erosion.
Start your garden design with a list of plants to be used, and their needs. Group plants according to their water requirements. This allows for efficient watering by group, and takes less time. Also group plants into those that can handle full sun and those that need part shade, especially western shade.
On paper, position trees and shrubs spaced according to their mature width. Do not be fooled by the appearance of a tree when young. It will outgrow its 3-5 foot width and become, depending on species, a 15-30 foot wide tree. Trees that are 30' wide need to be 30' apart from each other and from houses. Trees with aggressive side roots need to be 30' apart to avoid competition.
In your garden design, create microclimates by careful placement of 4-8' high walls, raised garden beds and earthen mounds around low areas for water collection. Keep the sun's path both winter and summer in mind while doing this. Also plan arrangements of tall shrubs and trees that can provide shade for other plants. The mature size of trees should be proportionate to the total area covered by the walls if you have an enclosed garden. Smaller walled enclosures need smaller trees or large shrubs instead.
If you can, select plant species that are adapted to the climate of the desert southwest - hot, dry, infrequent rain, periodic drought and alkaline soil and water. Just because a garden is in the desert, though, does not mean it has to be cactus and sharp thorns. Many thornless and leafy plants are adapted to desert conditions.
Select young trees to plant that come in 5 gallon containers rather than very large ones. They will require less water initially and grow to the same size as bigger trees. They are also more adaptable and less likely to fail. Avoid large, water-hogging trees such as Western Cottonwood, Sycamore, Aspen, or ornamental fruit trees that will rob water from other plants.
Many plants need part shade in the hottest parts of the year. Walls and the shade of trees or large shrubs can help. Know which trees are inhospitable to plants growing underneath because of chemicals released by their roots.
Most desert plants need well-drained soils and do best in raised garden beds or on top of earthen mounds at least 12-18" above surrounding soil. Raised garden beds formed of rock or masonry block are often used. Avoid treated lumber because the poisons in the lumber can be picked up by any plant used for food, such as mint, basil, tomatoes or edible flowers.
Double masonry walls can be built, a tall outside wall and a lower inside wall, about 18-24" apart. Filling the space between them with soil provides a well-drained, linear space for flowering perennials or garden herbs.
Walkways can be made of tamped earth, gravel, brick, concrete block, or even more creatively, mosaic stone with an infinite variety of patterns. In Tucson, Phoenix, and elsewhere, monsoon rains frequently lead to erosion. Paths on slopes should be lined on the sides with rock, brick or block to reduce rainwater erosion and water flows that turn to ice sheets in winter. Steps may have to be situated at internals along a path to reduce steepness. Steps can be made of concrete, block or treated lumber.
Drip irrigation has the disadvantage of forcing roots close to the surface where they get baked in the hot desert sun. Instead of drip irrigation, use flood irrigation to periodically water plants. Provide raised soil ridges to channel rainwater and dig low areas in the soil (with bare soil bottoms) to collect rainwater so that nearby plants can receive water that slowly soaks into the ground. Have an overflow channel of rock or block so that rainwater can escape without erosion if it exceeds the depth of the pool.
During a drought, run stored rainwater or tap water into these low areas to provide needed water. Pools for a group of plants can be flooded every two to four weeks. Individual trees can be set up with their own basins to collect rainwater and flood their roots to the drip line every one to four weeks. This is especially necessary for fruit trees.
A rainwater collection system that collects rainwater from a roof and stores it in large barrels will help provide water when rains fail.
Use mulch that is suited to each plant. Mulch reduces evaporation from the soil. Penstemons, for example, prefer crushed decomposed granite, which provides nutrients, or small gravel. But avoid using only rock for the landscape to avoid a moonscape look. Bare soil between plants in the desert is fine. However, some plants benefit from organic mulch made of bark or leaves over their roots.
See also Microclimates and Plant Placement.