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Conserving Water in the Home Garden
By: Ben Faber, Farm Advisor, U.C. Cooperative Extension, Ventura
When to water? How much to water? These are two very important questions that need to be answered before watering lawns, shrubs, trees, and vegetable gardens if we want to provide the most beneficial use of this resource. Because of variability in plant size, weather (temperature, wind speed and direction, humidity, clouds and fog), types of soil and water, aspect and the method of application, it is difficult to give one recommendation that is true for all situations. Additionally, watering plants on a fixed schedule subjects them to periods of too much water and at other times to drought, because weather and plant size change. A young avocado tree in December may use only one gallon per day while a 20-foot tree in August might use 35 gallons per day.
The following are general principles. As plants increase in size, rooting becomes deeper, and the frequency of waterings can be decreased because more water is stored in the larger root zone. The time to water can be determined by the appearance of the plant, by the soil moisture content, or by estimating the amount of water used since the last irrigation.. In most cases, plants can wilt slightly at midday before it is necessary to apply water the following day. Alternatively, i in digging with a trowel or shovel down to a foot’s depth it is found that the soil is dry, then this might be used as an indicator of the time to water.
Soils and Water
In a similar climate, water use by plants generally is the same regardless of the soil from which it is taken; however, soil variability is important in watering. A sandy soil requires more frequent, short watering than a clay soil to prevent water loss beneath the root zone. With heavy soils, it is best to water less frequently because these soils will hold more water. Heavy soils typically do not absorb water rapidly, so to avoid runoff it may be necessary to split the watering times into two or more periods. Adding organic matter to clay and sandy soils will increase the rate of water penetration in day soils and the water-holding capacity of sandy soils.
Other than seedlings which are shallow rooted, water should be supplied to a minimum of a foot’s depth of soil (approximately one-inch depth of water on the surface of the soil will infiltrate down to one foot). An area ten feet by ten feet will require about 62 gallons of water to filter down to one foot. This would be similar to one inch of rain or lawn sprinkling on the same 100 square feet. Infrequent but deeper watering will result in a deeper rooting system, and the plant will be better able to sustain periods of high water demand. Less frequent watering will also minimize loss of water by evaporation from the soil surface.
During or after a watering, the depth to which soil moisture has been restored can be determined by probing the soil with a metal rod not more than 3/16 inch in diameter. A big screwdriver is also a good tool for probing. The force needed to push the probe will increase suddenly when it reaches dry soil. The length of time of sprinkler operation or amount of applied water that was used to achieve a certain soil depth can then be used as a standard for future waterings.
Trees and Shrubs
In a winter with adequate rainfall, the whole root zone is filled with water near the end of the rainy season; however, in dry winters, plants need supplemental watering. The amount of water to apply is the amount required to replace the water taken from the soil by roots and lost by evaporation from the soil surface since the last rainfall or watering. Water to a depth of two feet (approximately two inches of water or 125 gallons per 100 square feet) under the drip line (canopy) of the tree or shrub. With deeper-rooted trees, for every third watering, apply twice as much, or four inches (250 gallons per 100 square feet). This will insure that the deeper roots will be maintained and that various salts in the water are leached from around the roots.
Ideally, water applied to trees and shrubs should be ponded at the site by building berms around the plant. This insures that the applied water goes directly to the plant and is not wasted. It also makes it possible to visualize two inches of applied water. For trees it is best to build two berms, one around the trunk one foot away (six inches high) and a second one following the drip line of the tree. The interior berm is created to prevent diseases caused by water standing around the trunk of a tree or shrub. For trees located in lawns, &water bubbler at the base of the tree can be used to deep water the tree, without applying excess to the lawn.
Because roots of trees and shrubs often extend in all directions far beyond their longest branches and comingle, it may not make sense to water them individually. A more practical procedure may be to create dikes around a group of these plants. The total impounded area of each basin should not be greater than 50 square feet and the surface within should be as level as possible. The 50-square-foot area and levelness will encourage an even distribution of water to the various plants.
Depending on the hose diameter and water pressure, many household hoses give five gallons per minute. So to apply water with a subsoil irrigator to a tree with a ten-foot-diameter canopy, it is necessary to run the water for 25 minutes (two inches of water on 100 square feet = 125 gallons). The bubbler should be moved every five to ten minutes around the tree so that all the root zone is watered. Since the five-gallon-per-minute rate is an average, one’s own situation can be measured by filling a five-gallon bucket with your hose and timing how long it takes to fill.
Vegetables and Flowers
Vegetables or flowers can be grown in sunken beds or level basins that can be flooded in the same fashion as trees and shrubs. Sunken beds should not be larger than 50 square feet so that they can be filled rapidly to achieve uniformity with the depth of water to be applied. Furrows also can be used in growing flowers and vegetables. Spacing of the furrows should be such that water from the furrows wets the whole bed. The spacing of furrows will vary with soil type. Sandy soils need closer spacing to avoid loss of water out of the root zone when trying to wet the whole bed, while clay soils can have wider spacing. The amount of water applied by furrows is the amount needed to move across to the center of the bed. Then by probing with a slick or trowel in the bed’s center, it is possible to determine the depth of water infiltered and the amount of time needed to continue to run the water. Canvas soaker hose or drip tape are also good ways to water beds. The length of time to run them can be determined by using the calculation used for trees or simply by probing the soil to find the depth of water inifitered.
Studies by the University of California at Riverside have demonstrated that many turf grass species can get by with as little as 60% of optimum watering with little stress. Most lawns have areas that dry sooner than other parts of the lawn. Let these areas be your indicator for watering the rest of the lawn. When the grass in the dry area becomes dull colored and does not spring back when stepped on, water the entire lawn. The amount or length of water application should be enough so that a stiff metal rod or screwdriver can be pushed one foot into the soil. This depth for most soil textures represents about one inch of rain. Alternatively, cans can be set out in the lawn, the system turned on, and the length of time it takes to collect an average of an inch of water can be used in succeeding waterings. This test will also show how evenly water is being applied and can suggest ways to correct sprinkler performance.
Sprinklers should be run so that no runoff occurs. If water has not penetrated to a foot and runoff occurs, turn the system off for an hour then turn it back on to apply the needed amount. Spildng or aerating lawns can help water penetration.
Some Do’s and Don’ts
Select plants that are adapted to warmer, dryer climates.
Adjust sprinklers for uniform water distribution.
Fix leaky faucets and lines.
Water early in the day to reduce evaporative loss.
Mulch beds to reduce evaporation from the soil surface..
Shelter container plants from winds.
Don’t sprinkle during windy or hot periods of the day.
Don’t put the water on the street and sidewalk; put it on the plants.
Don’t use softened water (sodium treated), if it can be avoided; it will harm most plants.
Don’t put sprinklers on a timer that is not adjusted with the weather; failure to adjust your timer will assure you of wasting water.