Estimating irrigation needs
Inappropriate irrigation can adversely affect most any aspect of plant appearance, development, and growth and cause plant death. Schedule irrigation by observing plants in combination with monitoring evapotranspiration or soil moisture in the root zone. These techniques assume that plants are correctly planted, well rooted, and have been growing well.
Observe plants
Examine plants regularly for symptoms of water stress. Early, drought-stress symptoms exhibited by broadleaf plants include foliage that becomes dull, faded, or grayish, but normally is shiny green. Wilting in the afternoon, such as growing tips becoming limp, and recovery by the next morning also indicate water deficit. As water deficit becomes more severe, plants may not recover from discoloring and wilt. As symptoms progress, leaf margins or interiors turn brown or yellow and foliage dies and drops prematurely. Twigs then large branches and eventually the entire plant may die.
Certain plants may exhibit symptoms first because of drier soil (e.g., due to inefficient irrigation) and topography or because they are less-drought-tolerant species. Inspect these plants more frequently and use them as indicators of drought stress and irrigation need.
Be aware that foliar symptoms resulting from overirrigation and poor drainage can resemble the symptoms from underirrigation. Because roots require oxygen for growth and the uptake of water, insufficient oxygen in the root zone (aeration deficit, sometimes called waterlogging) results in root asphyxiation and sometimes plant death.
Monitor soil moisture
Schedule irrigation by monitoring soil moisture. The frequency of monitoring varies greatly depending on the factors discussed above. Soil around young plants during hot weather may need to be monitored daily; every few weeks may be adequate when monitoring around mature trees during more favorable weather. Sample soil from the root zone in several different areas of the landscape to assess overall irrigation needs and determine whether water is being applied deeply and uniformly enough.
Monitor soil moisture by digging a shallow hole with a trowel or other small digging tool that minimizes root injury. Alternatively, use an auger, soil probe, or soil sampling tube. Examine soil moisture in the rooting zone to a depth of about 1 foot. Soil lightens in color and crumbles more easily when it is dry.
Tensiometers. You can use tensiometers to monitor soil moisture if you properly install and maintain them. These devices are buried so their bottom contacts soil particles in the root zone, perhaps about 1 foot deep. Some tensiometers can be wired into irrigation-system controllers to assure watering occurs only when soil reaches a preset, dryness threshold. Be sure to locate tensiometers in areas representative of plants’ irrigated root zone and use the measurement in the driest part of the landscaped area to trigger the need for irrigation.
A properly operated irrigation controller can help you avoid over- or underwatering plants. Irrigation can be scheduled using “smart” irrigation controllers with sensors that measure moisture-dependent electrical properties of soil, including capacitance sensors, electrical-resistance or soil-moisture blocks (gypsum blocks), and granular-matrix sensors.
Monitor evapotranspiration
Irrigation can be scheduled by monitoring evapotranspiration (ET), the combination of evaporation of water from soil surfaces and transpiration from plants. Some "smart" irrigation controllers use weather-dependent, ET data to automatically adjust the watering schedule. Weather-based controllers can use historical monthly averages of ET, on-site ET sensors, or remote data that are broadcast, such as via radio signals. See monitoring evapotranspiration for how to use ET and schedule irrigation without specialized equipment.
For more information, see Conserve Water in Landscapes, Irrigation Methods, Irrigation of Trees and Shrubs, Irrigation Scheduling Using Evapotranspiration (ET), Soil Properties and Water Availability to Roots, and Water Deficit and Excess. Irrigating Fruit and Shade Trees and Shrubs provides an index to more resources. See also "Water Management" in the California Master Gardener Handbook.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Wilting can be due to water deficit, or excess.
Monitoring soil moisture.
Tensiometer, a soil-moisture gauge.
ET monitoring, the information is online.
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