Benefit of xeric landscape plants is lower water use

This happens to most of our native mesquite, palo verde and acacia. They are in tune with our desert environment and, in particular, its spotty rainfall. Plants like these are xeric in their water use. They use less water than alternative trees and shrubs and are controlled with applications of water.

Xeric plants in the nursery trade come from desert regions of the United States and some parts of the world. You won’t find them labeled in the nursery as “xeric.”

Trees and shrubs that use more water than xeric plants and whose growth can’t be controlled with water are considered mesic in their water use. Oftentimes they are sold as “desert plants,” but they aren’t xeric in their water use.

Mesic trees and shrubs include vitex, oleander, African sumac, bottletree and Chinese pistache. Trees and shrubs that grow well in lawns are dead giveaways as mesic in their water use. Don’t confuse drought tolerance with low-water use. Plants like oleander and vitex may be considered drought-tolerant, but they are definitely not xeric in their water use.

The two biggest benefits of xeric plants in landscapes are their lower water use and control of their growth using water. Xeric plants should have their own water supply or irrigation station.

If saving water and controlling their growth is a primary reason for planting them, then put them all together on the same irrigation valve. This gives you, not them, control of their growth.

This is known in the irrigation trade as hydro zoning or the creation of hydro zones. Smaller xeric plants like desert marigold or autumn sage can be on the same irrigation valve, but the amount of water they receive is less (fewer drip emitters). That results in less water being applied to the soil and available to the plant.

Q: I am converting my lawn area to a desert landscape. It has a 20-year-old African sumac tree growing in the lawn that shades the front of the house. The rest of the plants can be removed, but I want to keep this tree healthy because it shades the south side of my home.

A: The roots of your large African sumac tree favored the lawn in its search for easy-to-get water and fertilizer. When the lawn is removed, most of its water and fertilizer is also gone, and the roots of the tree will be found close to the surface of the soil.

The shape of any tree resembles a wineglass; the roots spread out at the base of the glass. The big difference to the wineglass analogy is the length of tree roots growing in a lawn; undisturbed tree roots will spread twice the height of the tree. A 30-foot tree could have a 60-foot root spread in a lawn.

Tree roots growing in a lawn are shallow. Medium-sized trees like African sumac should have most of their roots growing to a depth of 24 inches. That requires watering periodically to at least this depth. Because of a lawn’s requirement for frequent irrigation, most of the tree roots will be found no deeper than 8 to 10 inches and stretching as far away as twice its height.

Your job after removing the grass is to recognize where the tree roots are growing and replace the water it gets from the lawn. Six or eight drip emitters won’t give the tree enough water, and it will not distribute the water to a large enough area to satisfy its needs. Established trees that grew in a lawn need water distributed to an area similar in size and location to the lawn.

Use drip tubing for irrigating this tree rather than individual drip emitters. Drip tubing has emitters embedded into the walls of the tubing. The most common tubing available has drip emitters embedded every 12 inches along its walls that deliver nearly a gallon of water each hour.

A spiral of this tubing surrounding the tree trunk and looping about 250 feet in length should deliver enough water when replacing the lawn. One hour of irrigation should deliver about 200 gallons to the tree each time is watered. After irrigation, push a 4-foot length of rebar into the soil in random locations to make sure water wets the soil to a depth of 18 to 24 inches.

Q: My soil looks OK, but in some spots it stays wet for a couple of days after I water. I can water plants in some areas OK, but in other locations plants have died because the roots have stayed too wet. The irrigation water doesn’t seem to drain very well in some areas while in other areas it seems to be fine.

A: Most likely there is something in the soil preventing the water from draining. It is somewhere below the planting depth. This could be a layer of clay soil — called a soil lens — preventing water from draining. Frequently these are found in lower parts of the valley heading toward the Las Vegas wash.

This clay layer could be 2 feet below the soil surface or much deeper. If it’s shallow, then drilling or auguring 3- to 4-inch diameter holes in the soil can “sump” or French drain the water away from plant roots. That allows for soil drainage and the plant roots to breathe. That’s the easiest solution if it’s possible.

The next easiest solution to this problem is planting on a hill or mound of soil. The size of this mound depends on the size of the plants; bigger plants require bigger mounds or hills.

If these are small plants such as perennial flowers that don’t get much above 18 to 24 inches tall, then a 12-inch-high and 18-inch-wide mound is high and wide enough. If the plants are small to medium-sized trees, then mounds 18 inches tall and 4 to 6 feet wide will be big enough.

Sculpting the soil or creating elevational changes in the landscape can be done when first landscaping. Elevating the soil improves soil drainage of the taller mounds.

Q: I think I have 10 older desert willow trees in my yard. Every year after flowering and the weather gets hot, the leaves turn brown during the summer and fall off. What can I do to prevent this?

A: If your tree is a desert willow, then the reason for its leaves turning brown and dropping is a lack of water to its leaves. But first, make sure it’s a desert willow. Take some small branches, with leaves and flowers attached, to a nursery and have them identified.

Leaf diseases are a possibility for desert willow, but usually that requires humid weather. Because it happens when it gets hot, the problem is most likely a watering issue.

There can be many reasons for the browning of leaf tips of plants and leaf drop, and they can be confusing: waiting too many days to water, watering daily, damage to the trunk by maintenance people or insects, disease problems, weed killers and others. The most common reason for it happening during the hot summer months is watering too often.

Desert willow is a small tree native to all of the deserts in the Southwest, found growing along washes such as arroyos. It doesn’t like daily watering or any type of frequent irrigations.

When it is watered, it prefers a lot of water all at once and not applied again for a week or so. That encourages its roots to grow deep in the soil. Deep roots help the plant endure the high temperatures of the desert. Ideally, it should be watered at the same time as other desert plants.

Some people like to give plants a little sip of water daily. Perhaps the irrigation system won’t allow much flexibility, and watering a variety of landscape plants and containers is complicated. This irrigation practice works in the short run but affects how and where the roots of the plants grow in future years.

Plants are like us; they are lazy. Plant roots grow best where they find it the easiest to take up water and have an abundance of air and fertilizer to boot. Watering with a little sip of water daily encourages the growth of shallow roots. Shallow roots are less tolerant of the high temperatures and drought common to the desert during the summer months.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.