Irrigation methods
Basin, sprinkler, and low-volume drip or soaker irrigation are used where irrigation is required to maintain most urban landscapes in California during the dry season.
Basin irrigation
A basin is formed by creating a berm of soil several inches high that encompasses the drip line of the shrub bed or young tree. Water is provided within the berm by installing an irrigation head, using a hose or tank truck, or after plant establishment by relying on precipitation and runoff. Do not irrigate so frequently that topsoil within a basin is constantly wet. Either plant on a central mound or slope soil within any basin to keep water away from plants' root collar. Break down berms during prolonged rainy weather to prevent water from ponding around trunks.
It may be best to avoid basins around trunks, except for about the first year after planting. If basins are used beyond the first year or two after planting, periodically move the berm farther from trunks to increase the area of irrigated soil as plants grow larger.
Sprinkler irrigation
Sprinklers wet the soil and can also wash dust from plants and increase humidity in landscapes. However, sprinklers may distribute water unevenly and waste water especially in windy conditions. They can also compact the surface of bare soil, increase weed germination and growth, and promote certain foliar diseases by splashing fungal spores and wetting foliage. Irrigating with reclaimed water can injure foliage if the water has high salinity and is applied with sprinklers that wet leaves.
In comparison with low-volume irrigation (e.g., drip systems), high-volume sprinkling is less efficient because water is dispersed widely, making it suitable only for relatively large plantings with uniform water needs, such as turfgrass.
Many trees and shrubs solely dependent on lawn-irrigation sprinklers to meet their water needs during the dry season. If turfgrass is removed, lawn watering is curtailed, or water is turned off to lawns, the nearby trees and shrubs that depend on water applied to turfgrass may decline or die without supplemental irrigation.
Drip irrigation
Low-volume systems emit water directly on or below the soil surface using drip or emitter nozzles or soaker hoses to wet only plants’ root zone or a limited area of soil. They waste comparatively little water and avoid or reduce compaction, foliar-pathogen problems, and the extensive growth of weeds associated with high-volume, sprinkler irrigation.
Low-volume systems can be more expensive to install than sprinklers. They may require more maintenance and skill to use, especially to develop appropriate irrigation schedules and manage salinity. Be aware that it can be difficult to monitor and maintain systems installed belowground or beneath mulch. When using a low-volume system, shrubs in dry areas may need to be occasionally washed of dust to keep them healthy.
Solutions
For more information, see Conserve Water in Landscapes, Estimating Irrigation Needs, 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. Also consult publications such as Drip Irrigation in the Home Landscape and "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).
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Drip system before a mulch covering.
Sprinkler, also watering weeds.
Trees may depend on lawn sprinklers, or be killed by them.
Basin irrigation.
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