Urban Ecology

Planting Water-Wise

by Diane Boudreau

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

Stabler studies the water use efficiency (WUE) of desert plants. Water use efficiency describes how much a plant will grow for every unit of water applied. When a plant can grow a lot using a little bit of water, it has high water use efficiency.

Water is scarce in the desert. Improving your yard’s WUE helps the environment by conserving water. It also can save you money, time, and effort.

Some types of plants need less water than others. But other factors affect WUE as well. Stabler studies Texas sage and oleander, two shrubs popular in desert landscapes. She wants to know how irrigation and pruning affect these plants’ WUE.

In the Phoenix area, these shrubs are often planted close together and pruned often. Many times, you can see Texas sage trimmed into square or rounded shapes. Keeping these shapes neat requires a lot of work.

Oleander is often used as a natural fence because it grows very tall. The plants are placed close together to create a “wall.” These walls also need frequent pruning.

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

Stabler set up an experiment at the Desert Botanical Garden in Papago Park. She created 14 mock landscapes. Each was set up to look like a typical Phoenix yard. Each 100 meter by 100 meter plot included trees, shrubs, and ground covers.

Stabler divided the plots into groups based on how often they are pruned. Some were pruned every six weeks, others every six months, and others once a year or never. Then she measured the WUE of plants in each of the groups.

“We found that plants given low volumes of irrigation water and left unpruned had high water use efficiency. Those given lots of water and pruned often had very low water use efficiency,” says Stabler. “A fully mature leaf is good at using the sun’s energy to make the plant grow. 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.”

Not all pruning is bad. “Pruning is a rejuvenating process,” she says. Occasional pruning is healthy for a plant.

Stabler says that people often buy new houses and program their irrigation systems for young trees and plants. Of course, young plants need more water than older plants. Then they forget to change these levels once the plants mature.

These people are wasting water. They 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,” Stabler says.

Spacing of plants is another important part of the equation. Many people place plants so close together that they don’t have room to spread out. Then they have to prune them more often.

A water-efficient yard is also low-maintenance, says Stabler. “In Arizona you always see landscapers out with their power hedgers and leaf-blowers,” she says. These machines are quick and easy from a business standpoint. They are 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.”