Regional Gardening Reports :: National Gardening Association

In the Garden:
Southwestern Deserts
October, 2012
Regional Report

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Native plants in this well-designed xeriscape will survive mainly on rainwater once established.

Xeriscape Part IV: Irrigation: Effective Watering Techniques

Xeriscape design includes a set of seven principles to guide you in the creation and long-term maintenance of a colorful, earth-friendly landscape that suits your unique needs. Previous reports covered design and site analysis, whether to install turf, varied plant selection subjects, and factors influencing plants' water use that affect how frequently water should be applied.

This report continues the important topic of effective watering. In addition to watering frequency, understanding how deep to water and where to apply water is essential.

Water Deep with Each Application
Start by memorizing the 1-2-3 Rule: Annuals, perennials, cacti, succulents and groundcovers typically have root systems about 1 foot deep. With each irrigation, apply sufficient water to moisten soil 1 foot deep. Most shrubs have root systems that are 2 feet deep, so water to soak that deep. Most of a tree's feeder roots, the ones that absorb moisture and nutrients, spread horizontally through the top 3 feet of soil. So apply water to soak 3 feet deep for trees.

And now for the exception to every rule: if irrigating turf, water does not need to soak as deep as 1 foot. Recommendations vary for different types of turf. Winter ryegrass has a shallow root structure, so 4 to 6 inches watering depth is adequate. Bermudagrass has a deeper root system, so some experts suggest water should soak 6 to 10 inches. The goal is to water deeply through the root zone, which keep roots growing roots deep and protected beneath the soil, where they will develop some drought tolerance. Even in the heat of summer, it should never be necessary to irrigate lawns daily!

Determine How Far Water Penetrates
So, how do you know how far water soaks in? You may recall from my last report that 1 inch of water penetrates about 1 foot through sandy soil but just 5 inches through clay. Water penetration will vary widely depending on your soil characteristics. To gain an idea of how far water penetrates in your landscape, perform a simple test. Run your irrigation system (or let a hose trickle slowly) for a set amount of time, for example 30 minutes for a groundcover or 1 hour for a shrub. Wait an hour or so for water to continue penetrating through the soil. Stick a soil probe (any long, pointed slender piece of metal) into the ground. A long screwdriver works for shallow-rooted plants. The probe will move through moist soil but stop at hard, dry soil. Continue watering in timed increments until you know how long your system needs to run to apply enough water to soak 1, 2 or 3 feet deep for the varied plants in your landscape.

Water Far and Wide
The 1-2-3 Rule covers how deep water should penetrate, but where water is applied is equally important. Apply water around a plant's entire circumference, at the canopy edge (also called the dripline) and just slightly beyond. This is where feeder roots that absorb water and nutrients are located. As a plant matures, feeder roots grow outward horizontally, keeping pace with the expanding canopy. Thus, drip emitters (or whatever delivery system you use) must be moved outwards as a plant grows. Emitters left at the base of a maturing plant are of no use because the feeder roots that absorb water have moved far beyond. Irrigating near the trunk area is just wasting water.

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