Water Deficits Lead to Stressed Plants - September 24, 2003
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Long-term weather forecasts say that drought conditions are likely to continue through the coming winter with above normal temperatures and below normal precipitation. This column is to help us prepare for what's ahead. Water deficit is the number one cause of death in trees and shrubs in north central Arizona. To help avoid losses this winter, lets explore how prolonged drought affects our landscape and orchards.
Deficits occur when plant water loss exceeds supply or, as plant geeks say, transpiration losses are greater than water uptake. Drought tolerant plants can reduce their rate of growth to compensate for limited water supplies. Conversely, water loving plants may become diseased, colonized by pests, or simply die when water deficits occur. The key lies in knowing the moisture requirements of your landscape plants and anticipating their irrigation needs.
Water deficits may be acute or chronic. Acute deficits are short-lived and can range from hours to days. Chronic deficits range from a few days to several months and are often found in under-irrigated (or non-irrigated) landscapes during drought. Both can cause injury symptoms that range from mild to severe depending on the plant's innate ability to cope with the deficit.
The first symptom of acute water deficit is wilting leaves. If the deficit continues, plant tissues may dehydrate to the point that plant tissues die (necrosis). Rapid dehydration often causes the leaves to turn reddish-brown at the edges with a distinct delineation between live and dead tissue. If serious enough, leaf drop may occur or the whole plant may die.
Chronic water deficit often causes slow growth or stops growth altogether. Other symptoms are reduced leaf size, pale leaf color, premature leaf drop, or early fall coloring. Here it is important to understand what "normal" growth should look like for the species in question. Plants of similar age in similar growing conditions should be compared to see if the symptoms are real or imagined. In severe situations shoot and branch dieback, trunk bleeding, or plant death may occur. Chronically water-stressed plants are also more susceptible to insects and diseases. Can you say bark beetle?
Multiple factors often contribute to water stress. First, a water deficit causes a general loss of vigor in the plant. The plant compensates by reducing foliage or killing branches to conserve water. The loss of effective leaf area results in reduced photosynthesis leading to decreased energy production. The combined decreases of vigor and energy production reduces root growth, which reduces the ability of the plant to take up soil moisture. As you can see, this is a vicious cycle and the plant can easily go into a tailspin.
Other stress factors can combine with water deficit to further reduce the vigor of landscape plants. Reflected light from sidewalks or brightly painted buildings and radiant energy coming from paving and motor vehicles all can increase moisture demand. Impervious paving and parking lots reduce soil permeability so less water enters the root zone. Soil compaction from pedestrian or auto traffic also reduce soil water infiltration. Construction damage reduces the root system and hence the plant's ability to take up water. String trimmers and mower damage can increase stress of plants. Poor planting and staking reduces a plant's ability to develop a healthy, functional root system that takes up water from a large volume of soil and solidly anchors the plant. The list goes on.
We can also avoid many problems by selecting landscape plant species that are appropriate for the water availability and growing conditions that each site provides. I hope the above information provides food for thought that will help you keep your landscape healthy and water-wise.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: September 17, 2003
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