How Drought Affects Plants

The current drought is having a major impact on trees and shrubs, both in our landscapes and in natural, forested land. Stressed and dying trees can be spotted in town and in the countryside. What is really needed is a return to more normal rainfall patterns that can recharge depleted soil moisture. Wednesday’s rainfall, while so very welcome, needs repeating many times.

Here is a short overview on how drought affects plants, so you can understand why this dry spell could continue to affect trees for months or years even after the drought ends.

One definition for drought is the absence of rainfall or supplemental irrigation for a period of time sufficient to deplete soil moisture and injure plants. Drought stress results when water loss from the plant exceeds the ability of roots to absorb water and when the plant’s water content is reduced enough to interfere with normal plant processes.

Water is the stuff of life. It is a major component of plant cells, and is the medium in which growth processes occur. Without adequate water, biological processes, such as photosynthesis, are greatly reduced. Reduced photosynthesis means reduced plant growth, including root growth.

Later is also the driving force of transpiration – the process by which a plant moves water from the roots throughout the plant and is lost through the leaves. Transpiration is the way dissolved mineral nutrients are transported from the soil to the rest of the plant. It is also how a plant cools itself (and us) by water vapor loss through its leaves.

In the root system of a tree or shrub, tiny, delicate root hairs extending from tender feeder roots at the extremities of the root system are responsible for the bulk of water uptake. Confined to the upper 15 inches or so of the soil profile, they are the first part of the root system affected by dry soil conditions. With the loss of root hairs, the water absorbing capacity of the plant is severely reduced.

Even after a drought has ended, it may take months or even years for a plant to repair damaged root systems and regain its growth capacity it had prior to the drought.

Besides the direct effects of drought, a plant under stress becomes more susceptible to insect and disease problems that can attack a weakened plant.

The time required for drought injury to occur depends on the water-holding capacity of the soil, environmental conditions, stage of plant growth, and plant species. Plants growing in sandy soils with low water-holding capacity are more susceptible to drought stress than plants growing in clay soils. A limited root system will accelerate the rate at which drought stress develops. A root system may be limited by the presence of competing root systems, or by site conditions such as compacted soils. Newly installed plants and poorly established plants may be especially susceptible to drought stress because of the limited root system or the large mass of stems and leaves in comparison to roots.

Whether or not you are under water conservation restrictions, it always makes sense to conserve this most valuable resource when it comes to irrigating our landscapes.  Get the best benefit out of the water you do apply.

The main strategy for supplemental irrigation during a drought is to apply enough water to wet the soil several inches deep. How much this requires depends on your soil type. Watering for a few minutes every day, or even every 2 or 3 days a week, does little more than barely keep the grass alive. It is better to water less frequently and more deeply, applying about an inch of water per irrigation to wet the soil 5 or 6 inches deep.

For important trees and large shrubs in your landscapes, you may want to periodically apply a larger volume of water to wet the soil deeply to get past the grass roots. This should be done slowly and over a long period of time to avoid wasteful runoff. This could be done with a soaker hose, or with a hose-end sprinkler under low water pressure. Irrigate for about an hour, and move the hose or sprinkler around the dripline of the tree until a large area has been watered.

Keep in mind that the root system of a tree extends well beyond the ends of the branches (the dripline), perhaps 2 to 4 times beyond.  It is impractical to adequately water the entire root zone – to do so would require thousands of gallons of water. Fortunately, wetting even a small portion of a tree’s root zone will greatly help reduce drought stress. Water applied at or near the trunk of a large tree has little benefit.

Even though we are entering the cooler time of year, when eventually our plants go dormant, they still require soil moisture to keep roots alive. Watering is required much less frequently, but in the absence of soaking rains, be ready to continue applying supplemental water whenever the soil profile dries out.

Comments are closed.