Water management and pest problems
Improper water management is probably the biggest cause of poor performance and survival of trees and shrubs. Too much or too little soil moisture damages or kills plants and favors many pest problems.
Overwatering (watering too frequently or applying excessive amounts during an irrigation) is especially common in established landscapes. Underwatering of the root balls is common in new plantings, which are shallowly rooted and dry out quickly between waterings.
Identification
Almost every aspect of plant growth and development can be affected by inappropriate water.
Water deficit. Insufficient soil moisture causes leaves to droop, yellow, wilt, or drop prematurely. Marginal necrosis in leaves is a common symptom from water deficit. Severe, prolonged water stress causes shoot and branch dieback and, in some cases, bark cracking and trunk bleeding. Sunburn, branch and trunk cankers, and overall slow plant growth can be due to insufficient soil moisture.
Water deficiency promotes certain pests. Mites, certain leaf-sucking insects, and most wood-boring insects including bark beetles, flatheaded and longhorned beetles, and clearwing moths cause more damage to plants stressed from drought; once trees become severely infested by borers, they decline and usually die. The Botryosphaeria fungus commonly causes cankers and branch dieback on drought-stressed giant sequoia.
Excess soil moisture. Overwatering and excess soil moisture excludes from soil the oxygen that roots need to survive. Aeration deficit, sometimes called waterlogging, is a serious, often life-threatening problem for plants.
Root decay pathogens such as Dematophora and Phytophthora spp. are present in many soils, yet usually become damaging only when excessively wet soil conditions favor them. Excessive water uptake can cause trunk splitting in palms and bark splitting in certain trees (e.g., × Chitalpa tashkentensis).
Water placement. Water dripping, ponding, or spraying near the basal trunk and soggy soil around the root collar are primary causes of root and crown diseases. Wetting areas where there are no desirable plants encourages germination of weed seeds. Splashing water spreads fungal spores and wets foliage, promoting foliar and fruit diseases such as anthracnose, brown rot, leaf spots, and rusts.
Seasonal timing. The seasonal timing of irrigation is important in disease development. For example, oak root fungus, Armillaria mellea, is present on dead or living roots in many soils and becomes active when soils are warm and moist. Native oaks and many other woody species are susceptible to the disease but usually are not damaged in California, where soils normally dry out as they warm during summer. However, when people water native oaks frequently during the summer or alter soils, moist roots and warm soils coincide, predisposing oaks and many other species to infection and death by Armillaria.
Solutions
Monitor plants' appearance, growth rate, and root zone soil moisture to determine whether irrigation amount and frequency is adequate, inadequate, or excessive and whether the system layout (e.g., location of emitters) is appropriate.
Learn the appropriate irrigation for trees and shrubs. Drought-adapted plants, such as certain eucalyptus, may benefit from deep, supplemental water at 1- or 2-month intervals during the summer, especially during years of abnormally low rainfall. Supplemental irrigation may also be appropriate if drought-adapted trees have been injured (e.g., by cutting roots) or soil moisture has been altered from historical levels (e.g., changes in drainage) and after consecutive years of abnormally low precipitation.
For California native oaks and other species adapted to summer drought, irrigation should simulate natural patterns and be applied mostly during the normal rainy season (late fall to early spring), tapering off during the dry season. Exceptions where summer watering may be warranted include planted oaks, which may be adapted to summer irrigation from the start, and in certain situations where soils have been disturbed. Be aware that certain eucalyptus and non-native oaks are not adapted to summer drought and require more frequent irrigation when planted in California.
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Shoot wilt from insufficient water
Marginal necrosis from water deficit
Dieback from prolonged water deficit
Irrigation water ponding around trunk
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