Water deficit and excess
Inappropriate irrigation is probably the most common cause of landscape plant damage. Too little or too much soil moisture can adversely affect most any aspect of the appearance, development, and growth of any plant.
Identification
Underwatering of the root balls is common in new plantings, which are shallowly rooted and dry out quickly between waterings. Many trees and shrubs depend on lawn irrigation to meet their water needs; if turfgrass is removed or lawn watering is greatly reduced or turned off, nearby trees and shrubs will decline in appearance and health and possibly die.
Overwatering (watering too frequently or applying excessive amounts during an irrigation) is especially common in established landscapes. Overirrigation in combination with poor drainage (slow infiltration and percolation) results in aeration deficit. Because roots require oxygen for growth and the uptake of nutrients and water, insufficient oxygen in the root zone (aeration deficit, sometimes called waterlogging) results in root asphyxiation and is a serious, often life-threatening problem for plants.
Injury from water deficit, especially chronic water deficit, can be confused with that from other disorders. Maladies that cause look-alike symptoms include
Damage
Inadequate soil moisture causes foliage to fade in color and wilt, die along margins and tips, and drop prematurely. Prolonged but mild moisture deficit results in dieback, fewer flowers and fruit, slower growth, smaller leaves, and increased susceptibility to plant-killing pests such as bark beetles and wood-boring insects. Inadequate soil moisture also increases the risk of plant damage from chilling (cold injury from temperatures above freezing) and freezing and frost because dry soil is less able to absorb and radiate plant-warming heat.
Overirrigation can cause foliar symptoms resembling those from underirrigation. Prolonged soil saturation causes roots to die and plants become unable to take up sufficient water. Excess soil moisture also increases plant susceptibility to certain invertebrate pests and root decay pathogens, especially nematodes and Phytophthora spp. See aeration deficit for more information on the damage symptoms caused by overirrigation and how to diagnose and remedy this problem.
Inappropriate soil moisture causes trees and shrubs to decline and possibly die. Declining and dead trees can be hazardous if they are located where limb drop or trunk failure (breaking or tipping) could injure people or damage property.
Solutions
Irrigation during at least the drought season is required to maintain most urban landscapes in California. The appropriate irrigation frequency and the volume of water to apply during each watering vary greatly. Considerations include drainage patterns, microclimate, moisture demand according to plant species and season, rooting depth, efficiency and type of the irrigation system, and the depth, structure, and texture of the soil.
A properly operated, irrigation controller can help you avoid over- or underwatering plants. With some types, you must periodically adjust the settings manually as plants’ need for irrigation changes with the weather and seasons. Some “smart” irrigation controllers use on-site, soil-moisture sensors or weather-dependent, evapotranspiration (ET) data to automatically adjust the watering schedule.
For more information, see Conserve Water in Landscapes, Estimating Irrigation Needs, Irrigation Methods, Irrigation of Trees and Shrubs, Irrigation Scheduling Using Evapotranspiration (ET), and Soil Properties and Water Availability to Roots. Irrigating Fruit and Shade Trees and Shrubs provides an index to more resources. See also Abiotic Disorders of Landscape Plants 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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Wilting, acute water deficit.
Necrosis, underwatered root ball.
Dieback from chronic water deficit.
Overirrigation, poor drainage.
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