Plants Face Drought Stress
By Linn County Extension & Outreach Master Gardener, Lisa Slattery
This year's lack of rainfall, wind and high temperatures are contributing to drought stress in our landscapes. But what is drought stress and what are the symptoms? Experts consider a plant to be drought tolerant if it can withstand a moderate period of limited moisture. Drought resistant doesn't mean a plant prefers hot, dry conditions or that the drought won't adversely affect the plant. Woody plants like trees and shrubs are typically more tolerant of drought than herbaceous plants because they can store more energy in their roots and woody tissues. Herbaceous plants aren't as efficient at storing energy so your perennial flowers and vegetable crops will wilt much faster than your Maple trees.
When a plant’s roots have limited soil moisture, the plant actually signals the closure of stomata or plant cells that control movement of water, carbon dioxide, and oxygen into and out of the plant. Closing this pathway to the exchange of water, carbon dioxide, and oxygen results in a decrease in photosynthesis. We all learned in elementary school that without photosynthesis plants will starve and eventually die. Leaf growth is affected by lack of moisture more than root growth because roots compensate better for less moisture. Lack of leaf growth and stomata reduces water lost from a plant through its leaves (transpiration) which cools the plant by pulling water and nutrients from the soil throughout the plant. This all leads to reducing photosynthesis.
So what symptoms to look for? The effects of drought vary between plant species. The most common symptom of drought is wilt. When a plant has decreased water pressure inside its leaves they wilt and this will reduce growth on nearly any plant.
Short-term symptoms to drought include wilting, marginal leaf scorch (browning along leaf edges) and leaf drop which helps plants to conserve energy. Long-term symptoms can include twigs and branches dying back, a reduction in flower and fruit production and a decrease in leaf size. Younger and newly planted trees or shrubs are more susceptible to death from drought because they do not have an established root system. A bit of good news is that some plants, particularly turf grasses, will go dormant, and should be fine once moisture levels return.
Drought stressed plants are also more susceptible to attack by diseases and insects. However, sometimes plant disease won't become evident until a year or two after a drought. When moisture levels return to normal, disease organisms tend to infect the weakened plants first. Many canker-causing disease organisms on trees are fungi that like to infect stressed plants - these attack the bark saprophytes or organisms that live off of dead or decaying matter. A common soil based pathogen Verticillium will also attack stressed plants first, this is common on many fruit tree species and vegetables like tomatoes. Some insect species simply prefer hot, dry weather like the two-spotted spider mite which will invade field crops. When it comes to crops, drought can be the cause of lower yields and possible crop failure. In the home garden it is easier to maintain moisture levels with watering and irrigation. If you do find drought symptoms occurring in your yard and garden you can cut back the dried plant material and try to maintain a good level of moisture. Other than that, it simply needs to rain.
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