The frequency at which you should water will depend upon soil type and the drought hardiness of the plant. Under the hottest, driest conditions, citrus and other non-succulent plants should be watered every other day on sandy soils and every few days on clay soils. Unless they are cacti or succulents, plants in containers should be irrigated daily. To leach salts, flush containers out at least once a month with repeated applications of water.
Skipping a single irrigation can be deadly for recently planted trees or shrubs. Even plants that are considered drought hardy can be damaged by water stress soon after transplanting. For example, recently transplanted mesquite trees should be irrigated like any other non-succulent plant. This is because these recent transplants do not benefit from the substantial root system that enables mature mesquites to tolerate desert conditions. Mature mesquite trees can extract water from deep within the soil profile; they have a taproot that can be up to 150 ft long.
Plants should be irrigated deeply throughout the year. Frequent, shallow irrigations encourage root development near the surface, where temperatures get higher and the soil dries quickly. Irrigating every night for 10 minutes will maintain a lawn, but this will lead to salt problems and shallow root development. It is better to irrigate for a longer period of time less frequently, so that plants root deeply and the salts are leached out of the root zone.
The early morning is the ideal time to water. Do not allow water to stand around trees and shrubs while temperatures are high. High temperatures while the roots are in water can result in a condition called scald, which can kill plants. Citrus trees are particularly susceptible to scald.
Mulch placed around shrubs, trees and vegetables, around plants in pots and in flowerbeds can reduce water loss from the soil, moderate soil temperatures. In some situations, they can also control weeds and reduce erosion. "Mulch" is a general term given to a layer of organic material, rock or plastic sheeting that is spread on the ground around plants.
Many types of organic mulches are available. Organic mulches available in home garden stores include peat and bark chips. Medium to coarse grade bark is best for use as mulch; fine is better if you are going to use it as a soil amendment. Agricultural products or byproducts, such as hay or wheat straw can be used as mulch. Alfalfa hay is a good general-purpose mulch that contributes a considerable amount of nitrogen to the soil as it decays.
You can make your own organic mulch by composting a variety of organic materials, which includes leaves, manure, kitchen scraps, lawn clippings and other yard wastes. At the time you use it around plants it should be decomposed to the point that you can no longer tell what it is made of.
Mulches insulate the soil from extreme heat and help to maintain soil moisture. A 3-inch deep organic mulch will reduce temperatures in the upper eight inches of soil by 10 degrees. A 2-inch thick layer of organic mulch can reduce evaporation by as much as 70 percent.
A soil covering can greatly reduce weed problems. Two inches of wood chips or gravel will substantially reduce most weed problems, although tough weeds like Bermuda grass and nutgrass will grow through even four inches of these mulches. Black plastic covered with a couple inches of bark, gravel or other material will provide extra protection against weeds. Just by shading the soil, you prevent some weed seeds from germinating.
Mulch is a valuable tool in meeting the gardening challenges that come with summer in the low desert. If you already have organic mulch around your plants, now would be a good time to check the depth. If it isn't at least 2 inches deep, you may want to consider adding more.