Irrigation of trees and shrubs
Irrigation is required to maintain most urban landscapes in California where rainless weather prevails throughout much of the growing season. Poor water management is probably the biggest cause of problems in landscape trees and shrubs. Too much or too little soil moisture in the root zone can seriously damage and kill plants.
Irrigation time of day
Early morning or just before dawn is generally the best time to irrigate. Irrigating around dawn reduces water loss from soil evaporation and minimizes the length of time when foliage is wet, thereby discouraging the development of certain foliar diseases. Predawn irrigation improves sprinkler efficiency and the uniformity of water distribution because there is generally less wind and more water pressure.
Irrigating during late evening or night can minimize evaporation, but avoid overhead sprinkling if foliar diseases are a problem because leaves will remain wet longer than with irrigating around dawn. After plants are established, apply the water around the drip line (beneath the outer canopy), not near the trunk where the root collar should remain dry. Learn and follow any irrigation rules of your water supplier.
Frequency and volume of irrigation
The appropriate irrigation frequency and the volume of water to apply during each watering vary greatly. Considerations include
- drainage patterns
- irrigation system efficiency and type
- moisture demand according to plant species and season
- rooting depth
- soil properties
Water availability to roots is affected by soil properties including depth, structure, and texture. Methods of estimating irrigation needs include monitoring evapotranspiration or soil moisture and observing plants for symptoms of water stress.
When irrigating overhead, make sure that enough water is applied at each irrigation to penetrate mulch and wet the underlying soil to the average depth of the root zone. Periodically check the placement of drip emitters and the direction of spray heads to be sure they are applying water where it is desired and to a depth of about 12 to 18 inches in the entire root zone of trees and shrubs.
Environment, plants, and soil effect irrigation need
Rooting depth varies according to plant age, species, and type and soil properties and moisture. Plants generally have shallower roots when they are young or receive relatively frequent and light irrigations or are growing in compacted or poorly drained soils. Roots are usually deeper in older plants and plants growing in well-aerated soils with good drainage and less-frequent but deep irrigation. About 90% of tree and shrub roots are usually in the top 3 feet of soil.
Water demand and loss depend on the environment and plant species. Water depleted from the soil through a combination of evaporation from the soil surface and transpiration by plants is called evapotranspiration (ET). A plant's demand for water generally increases when weather is hot, sunny, and windy and when humidity is low.
Learn the characteristics of your soil to help guide irrigation practices. Sources of this information include the soil survey maps available online from the Natural Resources Conservation Service and SoilWeb by the University of California. Use a soil sampling tube or dig a 1-foot-deep hole and compare your topsoil with survey maps to reveal whether your soil matches the maps. If the topsoil differs from published surveys, for example because it has been disturbed, deeper soils may also differ.
Learn whether irrigation should be modified because soil has poor permeability and infiltration of water (slow drainage). One method to assess infiltration rate of topsoil is to conduct a percolation test.
For more information, see Conserve Water in Landscapes, Estimating Irrigation Needs, Irrigation Methods, Irrigation of Trees and Shrubs, Irrigation Scheduling Using Evapotranspiration (ET), 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).