Planting Water-Wise

Laziness is a virtue. At least, it can be when it comes to maintaining your yard. For desert plants, less work can be better in terms of efficient water use, says ASU plant biologist Linda Stabler.

Efficient water use is important to the environment, especially in the desert. Improving your yard’s water efficiency can also save you money, time, and effort.

Stabler studies the effects of irrigation and pruning on a plant’s water use efficiency. She took a close look at two shrubs popular in desert landscapes: Texas sage and oleander.

In the Phoenix area, these shrubs are planted densely and pruned frequently. Landscapers often trim Texas sage into square or rounded shapes. Keeping these shapes neat requires a lot of maintenance. Oleander is often used as a natural fence between properties because it grows very tall. The plants are placed close together to create a “living wall.”

Both of these plants require little water. But Stabler found that many people get caught in a cycle of heavy watering and pruning. Plants that are watered heavily need to be pruned often to control size. Those that are pruned frequently use water less efficiently.

As part of her study, Stabler set up 14 simulated residential landscapes at the Desert Botanical Garden in Phoenix’s Papago Park. Each 100 meter by 100 meter plot includes trees, shrubs, and ground covers. The distribution and density of the plants mimics what commonly exists around homes throughout the Phoenix area.

The plots are divided according to pruning frequency: every six weeks, every six months, once a year, or never. Water use efficiency is measured in growth per unit of water applied. Plants that score high in water use efficiency can grow more per unit water applied than low-scoring plants.

“We found that plants given low volumes of irrigation water and left unpruned had high water use efficiency,” Stabler says. “The plants given high volumes and pruned often had very low water use efficiency.”

Why is this so?

“A fully mature leaf is good at using the sun’s energy to make the plant grow,” the scientist explains. “Immature leaves require a lot more energy to grow. If you’re constantly pruning a plant it never develops a lot of good mature leaves to provide energy for the rest of the plant. Also, it’s stressful on the plant to constantly shear it.”

But this does not mean that all pruning is bad.

“Pruning is a rejuvenating process,” Stabler adds. “Very mature tissue isn’t very efficient either, so occasional pruning is helpful.”

Stabler says that people often buy new houses and program their irrigation systems for young trees and plants, which require more water than established plants. Then they forget to change these levels once the plants mature. While wasting water, these people may also be harming their plants, causing problems such as root rot.

“When people see a plant failing they immediately think, ‘More water!’ But that might not be the solution,” she says.

Distribution is another important part of the equation. Many people position plants so close together that they don’t have room to spread out. As a result, the plants require more pruning.

A water-efficient yard is also a low-maintenance yard, says Stabler. “Around here you always see landscapers out with their power hedgers and leaf-blowers,” she says. These are quick and easy from a business standpoint, but not very good for the environment or for the customer’s pocketbook, Stabler notes.

“Fallen leaves are good for your yard. Your front lawn is not your kitchen floor—it doesn’t have to be spotless.”—Diane Boudreau