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How much water do your plants need...and how often?

How much water do your plants need . . . and how often? How much water do plants need, and how often do they need it? These questions puzzle Western gardeners as soon as they experience their first dry season. They're hard to answer--much depends on the climate, the soil, the wind exposure, the type of plant you're watering, even on how that plant has been watered in the past.

But here are some suggestions and guidelines that may help you water more efficiently--and conserve one of the arid West's most crucial resources.

First, know your soil

In spite of the existence of other variables, soil type is the most important factor.

There are three basic kinds: sand, clay, and loam. While yours won't match any one of them exactly, it will be more like one than another--and that will help determine how often and how much you have to water.

Sandy soils, composed of the largest particles, are generally well aerated. Water moves through them fast, they hold little water, and they dry out quickly. When you irrigate, the water seems to be absorbed in a moment. If you pick up a handful of moist sandy soil, it forms a cast but barely holds together.

Clay soils, made up of very fine particles, are usually poorly aerated and absorb water slowly. When you irrigate, the water puddles up fast. Once wet, such soils hold a lot of water. If you pick up a handful of wet soil, it makes a slippery ball that oozes through your fingers in ribbons; when you let it go, it doesn't crumble. Clay soils give up moisture slowly, turning crusty and often cracking when dry.

Loam soils, with an ideal balance between large and small particles, are well aerated, easy to wet, and dry out at a moderate rate. When you pick up a handful of loam, it forms a pliable ball that breaks apart with a gentle touch.

Some soils change textures at different depths. A layer of clay or compacted soil under sandy soil or loam can restrict drainage and keep the upper level unusually wet. To check drainage, dig a 2-foot-deep hole and fill it with water. After it drains, fill again. If it hasn't drained again in 48 hours, your soil probably has a drainage problem. The solution is to install drainage tiles, drill through the obstruction with a soil auger, or plant in containers or raised beds.

The photograph at the top of this page, done in cooperation with soil scientists from the University of California at Davis, shows how the basic soil texture influences water penetration. Tests have shown it takes about 3/4 inch water to wet sand to a depth of 12 inches and 2-1/2 inches to wet clay to the same depth.

Deep watering for deep roots

Each plant has a maximum depth to which its roots will grow. But they only reach that depth if soil is moist and contains enough air and nutrients. Deep watering encourages deep rooting, giving the plant a greater soil reservoir from which to draw water. The advantage? Deep-rooted plants can go longer between waterings and withstand drought better.

To water effectively, you have to know how much water it takes to wet your soil to the depth you expect roots to grow. The table below shows maximum rooting depths, under ideal conditions, of several popular plant groups. To give you an idea what deep watering means, the illustrations on page 150 show what you have to do to wet the root zone of a fruit tree or a 4- by 5-foot bed of various soil textures.

Your soil probably won't match the table exactly. To find out how deeply you're wetting it, use a soil probe. Some gardeners just push a metal rod (about 1/4 inch in diameter) into the ground; it should move easily through moist soil and be harder to push when it hits the dry area.

How often should you water?

Very few plants grow well in soil that is constantly saturated; it should be allowed to dry partially between waterings. This replenishes oxygen in the soil, which is necessary for healthy growth.

Many native Western plants are so suited to dry summers that they routinely survive without summer watering.

For plants that need a steady supply of moisture, such as annual flowers and vegetables, fruit bearers, or plants you'd like to grow vigorously all summer, water when the top few inches of soil (3 to 6, depending on root depth) go dry.

Your plants will give visible signs when the soil's too dry: leaves lose their luster and may pick up a blue or gray cast, and growth slows; eventually, they wilt. But it's best to water before the onset of such stress: it can make plants lose leaves, vegetables turn bitter, tomatoes develop blossom-end rot, fruit trees drop fruit.

Weather and time of year influence watering frequency. In warm or windy weather, plants transpire faster and soils dry out more quickly. In some wet climates or wet seasons, you may need to water only to supplement rainfall. However, a light rain does no more to wet soil than does a light sprinkling with a hose. Recent transplants need frequent light watering until their roots become established in surrounding soil. In hot, dry weather, small flower and vegetable transplants may need water almost daily. With a plant previously grown in a container, the rootball may be dried out but unable to extract water from surrounding soil of a different texture. For newly planted trees and shrubs, some gardeners construct a double basin--an inner one to water the rootball frequently, an outer one to water surrounding soil as needed. Container plants dry out quickly because of lightweight soils, evaporation through porous walls, and heat buildup in pots. Rootbound container plants are hard to wet and quick to dry out: tightly knit roots restrict water penetration (proper transplanting can help).

Water container plants until water flows out the bottom of the pot; this may necessitate several refills. When a plant gets too dry, the rootball may pull away from the container, so water gushes through without wetting soil. In such cases, place the pot in a pan with several inches of water so roots can soak it up through the bottom.

Prevent wasteful runoff

Two ways to save water are to direct it to plant roots by building soil basins and furrows and to apply it at a slower rate.

A third (not necessarily easy) way is to improve soil texture by adding ample amounts of organic matter. This is particularly important in clay soils that tend to crust over. But it is very difficult to work organic matter any deeper than 12 inches.

Furrows are most useful with vegetables, berries, and other row plants grown on land that is relatively flat. Before planting, use a hoe to dig small trenches about 6 to 8 inches deep. Slightly slope each trench so water will flow slowly but evenly from one end to the other. You can space furrows as far as 24 to 36 inches apart, with one or two rows of plants in between, or set them nearer for closely spaced crops.

Basins are circular ridges of soil that concentrate water at the roots of trees and shrubs. They can also be used for vegetables that need wide spacing, such as squash, melons, and tomatoes.

Build them before or after planting. As the plant grows, keep extending the basins to just outside the plant's drip line--this way, you water the entire root zone.

You can construct your basin so it's easy to measure how much water you've applied. For instance, when filled twice, a basin with a 4-inch-high rim should wet clay soil to a depth of about 36 inches.

Slowing the flow can be as simple as turning down water pressure. One of the best ways to deep-water a tree is to let a hose dribble water into its basin for several hours. Or irrigate every half-hour or so, letting the water soak in before you turn the hose or sprinkler back on.

Drip systems save water by applying it directly to the root zone at a slow, even pace, eliminating runoff and reducing evaporation. You can plan a system that applies water at varying rates to meet the needs of different plants growing close together. To ease watering chores, you can also hook drip systems to automatic timers.

Use a mulch

Mulches reduce evaporation from the soil and can lengthen time between waterings. They also cut down on weeds, which compete with desirable plants for water. And when organic mulches such as compost and shredded bark break down, they improve soil texture, possibly increasing water penetration.
COPYRIGHT 1986 Sunset Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1986 Gale, Cengage Learning. All rights reserved.

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Date:Aug 1, 1986
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