Recognizing Water Stress in Plants

The Arboretum at Flagstaff Extension Bulletin No. 91-01

By Robert Wilson, Horticulturist

    Although watering plants may seem like a menial chore to the average gardener, it merits considerable attention in order to supply the plants with the appropriate amount of this necessary nutrient. In places like Flagstaff where water costs to maintain a large landscape can be excessive, judicious irrigation is important. It is with this in mind that gardeners must learn how much water their plants need in order to optimize growth and provide the satisfaction that the purchase was intended to provide.

    Water stress, which is caused by insufficient soil moisture, is among the chief causes of poor growth or poor health in plants. It is responsible for slow growth and, in severe cases, dieback of stems. It also makes plants more susceptible to disease and less tolerant of insect feeding.

    The first signs of wilt appear in the afternoon, when the weather is hottest and driest, and may disappear at night. For plants that do not wilt easily, the presence of this symptom indicates a good time to water.

    Wilting will occur in some plants with adequate soil moisture on hot, dry days. Cucumber and squash may wilt during the day because they cannot draw up water through the roots fast enough to replace that which is lost through the leaves. For these plants it is best to wait until wilting occurs in the morning before irrigating.

    In some plants, the leaves produce enough woodiness to support the leaf when moisture is not adequate. In this case other symptoms of water stress need to be sought.

    Some plants that have a glossy leaf surface will lose their glossy appearance when dry. This is very comparable to wilting, but the leaves may not droop. A similar effect is seen in succulents where the leaves acquire a wrinkled appearance.

    Many desert and dry-climate plants have developed mechanisms to cope with times of low water availability. These provide some symptoms of low soil moisture and include folding of the leaves and in severe cases, leaf drop. Both mechanisms are intended to reduce leaf area which is the surface from which plants lose nearly all water.

    Although not usual, some plants will turn noticeably yellow when the soil is dry. This is more common in plants that require a lot of water, although some conifers will change color slightly in response to water stress. Yellowing is more frequently caused by nutrient deficiencies.

    If any symptoms of drought stress are apparent, it is best to apply a deep watering which allows roots to grow deeply. Shallow watering encourages plant roots to grow near the soil surface making them more prone to stress from low soil moisture.

    Close attention to plant water needs can provide the gardener with both healthier plants and lower water bills.