Premature leaf drop of sensitive species such as yellow poplar is a common indicator of drought stress. (Credit: University of Maryland)

While dry weather during summer months is not a stranger to Missouri, the blistering temperatures over the past several weeks have added to the peril of landscape plants. The immediate effects of drought on smaller plants are evident as they yellow and die. Plants with a more extensive root system normally show less immediate symptoms even though damage still is occurring.

Drought is a meteorological event that can be defined as the absence of sufficient rainfall over a period-of-time to cause the depletion of soil moisture. At this time, soil moisture is said to have declined to the "permanent wilting point." The lack of soil moisture, in turn, causes damage to plants (especially their roots). The lack of moisture needed to declare that a drought event is underway varies by region, water needs and disciplinary approach.

To understand the impact of drought on woody plants, it is necessary to understand the role of water in plant growth. Water is the lifeblood of all living organisms. In the case of plants, a very small amount of water is used as a chemical reactant in the process of photosynthesis whose chemical formula is carbon dioxide plus water in the presence of chlorophyll and energy from the sun yields a carbohydrate plus oxygen.

The vast majority of water used by a plant is needed for the uptake and transport of nutrients as well as to maintain cell turgidity. When water is adequate and cells are turgid, pores known as stomata open for the intake of carbon dioxide. Unfortunately, open stomata also results in water loss. When water loss exceeds water uptake, cells loose turgidity and the stomata close. This defense mechanism that keeps plants from losing even more water unfortunately slows or stops photosynthetic activity.

Three-dimensional view of leaf showing open stoma. (Credit: Wikipedia.com)

Since roots are responsible for water uptake, root damage is one of the leading causes of drought injury to plants. A plant's root system is a complex network of support roots and feeder roots along with their root hairs. The feeder roots (responsible for water uptake) of most woody species are primarily located in the top 12 inches of the soil profile.

Illustration of the location of feeder roots on a tree. (Credit: University of California)

During a drought, the amount of water available in the soil declines to a point where the tree's roots aren't capable of absorbing adequate amounts of moisture. While most soils will still retain some moisture, the soil "holds on" to this moisture, making it unavailable to trees, shrubs and other landscape plants.

Illustration of soil field capacity and permanent wilting point. (Credit: USDA)

If these conditions persist, the fine feeder roots begin to die back. Under prolonged droughts, even the larger, more fibrous roots are lost. Once root loss has happened, it can take days to weeks for the trees to re-grow the root hairs necessary to take advantage of rainfall. This, of course, is contingent upon the replenishment of soil moisture via rainfall or irrigation.

Among the first woody landscape plants to be damaged from drought stress are the broadleaved evergreens. Rhododendrons, hollies or azaleas may or may not wilt immediately, but their shallow, fairly fine roots are easily killed during a drought. As a result, flower buds that normally form in midsummer will not develop. Leaf color may be poor, and the overall appearance of the plant is one of being unhealthy. If a severe winter follows. The weakened plant may be killed back or become more susceptible to other problems such as opportunistic diseases.

Needle leaf evergreen trees and shrubs (conifers) must be observed more carefully to detect drought injury. The thin needles do not loose water as fast as leaves of the broadleaf evergreens and may appear to be surviving quite well. However, without adequate soil moisture, damage also occurs.

Gradually, the older needles (leaves) may begin to drop while shallow roots are also dying in the dry soil. The tree may appear to be surviving quite well, but suddenly dies even though drought conditions may have ended several months earlier. The stress caused by drought apparently starts a cycle of decline in some plants from which they are never able to recover. Therefore, it might be years before the full affect of a drought become evident. Those who can recall the severe drought of 2012 can attest to the fact that drought-stress trees were lost for a period of years after the drought ended.

Drought injury on conifer. (Credit: Montana State University)

Many flowering shrubs that set buds during midsummer may have reduced flowering the following season if drought conditions persist. Shrubs such as lilac and forsythia, or herbaceous plants such as peony and Oriental poppy, may not flower or flower poorly after a spring and summer drought. Fruit trees such as cherry and peach may develop fewer flower buds if drought has been severe and growth poor.

Although many of these responses will not appear until the year following a drought, some perennial plants prematurely drop leaves during a drought to conserve moisture. Species such as sycamore, poplar and cottonwood commonly drop leaves during drought stress. Other species less well adapted may not be able to drop their leaves, but will show browning and tissue death along leaf margins which commonly is referred to as scorch.

Leaf scorch resulting from drought. (Credit: Virginia Tech University)

Not all drought responses that appear the following season of growth will be as subtle as reduced flowering. Dieback of twigs may be a carry-over result of drought. If drought persists, this type of damage will become increasingly severe, and the plant may die. This is particularly true of smaller plants that are in close proximity to larger ones and must compete with them for water. Additional stress from diseases, insects or mechanical damage tend to worsen drought stress, and can result in plant death.

Branch die-back may not be evident until the year following a drought. (Credit: Alabama Cooperative Extension)

Of course, the best way to prevent or minimize drought damage is to irrigate. Even limited (spot) watering can be helpful, depending upon the size of a plant's root system. This especially is true for recently planted trees and shrubs. In other cases, watering plants may not be possible because of their distance from a water source, or a water shortage. Other approaches for easing drought stress are well known and include mulching, shading or, perhaps, the use of anti-transpirant sprays for higher dollar value plants. Anti-transpirants are compounds that close or plug the stomata, thus (temporarily) curtailing water loss.

A very informative article on mitigating drought injury to landscape trees can be found at https://mailer.missouri.edu/42N5-HXTW-E0E8427C8CE0721117WAJC0D1EA7EAE568453C/cr.aspx.