Irrigation mismanagement occurs simply because few people know when to begin irrigating. Some wait until plants begin to wilt before adding water, while others are convinced that watering frequently benefits the plants. There are several ways to determine if the landscape needs water.
1. Use a Screwdriver
Walk around the landscape once or twice a week with a standard screwdriver. Push the screwdriver 4 to 6 inches deep into the soil in several places in the lawn and flower beds, digging up a small amount of soil. Feel the soil for moisture – turn on the irrigation system if it feels too dry. If it feels relatively moist, irrigation is not required. If the soil feels quite wet, a drainage problem may need to be corrected.
This method has the added advantage of allowing you to “tour” your landscape regularly, seeing problems or potential problems you may not usually see. Some homeowners use this method quite well; others need another method to help maintain the landscape.
Figure 1. Footprinting.
(Image courtesy Dr. L.B. McCarty.)
2. Foot Printing
Walk across the lawn, then turn around and look for your footprints. If the lawn has adequate moisture, you cannot see where you have walked. You can see the footprints if the lawn is stressed due to lack of water (or some other condition). If footprints are readily visible, it is time to begin irrigation. If the lawn “ crunches” when you walk, you’ve waited too long!
Some portions of your yard lend themselves poorly to this method (flower beds, etc.). Use the screwdriver method in those areas to check the soil moisture. Footprinting the lawn will give a good preliminary indication of the moisture condition of flower beds and foundation plantings, but nothing replaces feeling the soil to make sure
3. Actual Plant Water Use
There are methods available to determine the actual daily water use of various plants. These methods (Penman-Monteith, etc.) are quite involved and require a large amount of information, including solar radiation, wind speed, relative humidity, and temperature, to name a few. The daily plant water use is calculated, and then the land is irrigated after the calculations show that a certain amount of water has been removed from the soil.
This method usually requires more daily work than the average homeowner cares to invest. An entire book on the United Nations Food and Agriculture website is dedicated to properly using the Penman-Monteith method (available at http://www.fao.org/docrep/X0490E/X0490E00.htm free of charge). Be aware that these methods are available but are not practical for the residential landscape.
Figure 2. A tensiometer.
(Image courtesy Irrometer Corporation.)
4. Measuring Soil Moisture
Since we are trying to maintain a certain soil moisture level to keep our plants healthy, one obvious way to determine when to irrigate would be to measure the soil moisture. There are many different types and models of soil moisture monitoring devices on the market, from inexpensive plastic devices of questionable accuracy costing a few dollars to fully automated, computer-controlled systems using radio links to turn on the irrigation system (and costing thousands of dollars). A tensiometer is a happy medium between these two options that might be useful to the homeowner.
It is straightforward for a plant to withdraw water from a soil that is saturated with water. As the soil begins to dry, the soil moisture is held more tightly to the soil particles and is increasingly difficult for a plant to remove. The tensiometer device measures the soil’s “soil moisture tension” and helps determine how difficult it may be for a plant to retrieve water from the soil.
A tensiometer is a hollow plastic tube with a porous ceramic tip on one end and a small water reservoir on the other. A vacuum gauge is attached to the side of the tube. The tube is pushed into the soil, with the ceramic tip first, until the tip is in the approximate center of the plant’s root zone (more involved methods use two tensiometers, but that is not necessary for our purposes). The tube and reservoir are filled with water, and a stopper is screwed into the reservoir to seal the tensiometer.
Water is drawn or “wicked” from the inside of the hollow tube through the porous, ceramic tip as the soil around the tip dries. Since the water reservoir connected to the tube is sealed with a stopper, this water removal creates a vacuum inside the tube, which registers on the vacuum gauge. As the soil becomes drier, the reading on the vacuum gauge climbs higher. When the vacuum gauge reading reaches a certain level (indicating a certain minimum moisture level in the soil), the irrigation system is turned on to replenish the soil moisture.
This is a relatively effective method of monitoring soil moisture and is inexpensive (tensiometers usually cost between $80.00 and $120.00). The tensiometer must be checked at least once daily (twice daily is a better option during the hot summer months).
The tensiometer may be a tool some homeowners would like to use, but most of us will use the screwdriver method – it works pretty well and is inexpensive.
Plant Water Needs Vary
Some plants require much water; others require a small amount. A mature pecan tree may use between 140 and 320 gallons of water daily. A prickly pear cactus is adapted to an arid climate and will require very little water. We may not have the research to provide estimated water needs for all landscape plants, but we can tell from our experience which plants seem to use more water and which plants use less.
It would be best if you considered the relative water needs of the plants you plan to use before designing the landscape. If a pecan tree and a prickly pear cactus are planted in the same bed, neither will be irrigated correctly. If the bed is irrigated for the needs of the pecan tree, the cactus will “drown.” If the bed is irrigated for the needs of the cactus, the pecan tree will wither and die. Plan bed plantings with an idea of the plant water requirements and the color and foliage to use your irrigation system best and ensure a healthy landscape.
Summary
Management is the key to any irrigation system. We cannot expect a timer or controller to “think” for us but must tour our landscape regularly, making adjustments and changes as needed. This will help us provide our plants the best growing environment all year.
Adapted from the 2007 South Carolina Master Gardener Training Manual.
Document updated 4/24 by Rob Last and Bryan Smith.
Originally published 05/08
