Drought stress in trees: what extension services describe

This page describes what the named extension services and the Missouri Botanical Garden say. It does not diagnose a tree, say whether any tree is safe or unsafe, or tell anyone how to water, treat or remove one. Drought stress is one of the most common abiotic causes of tree decline, and the sources agree that its symptoms are easy to confuse with other problems.

What the sources say it looks like

The Missouri Botanical Garden says water needs vary greatly with species, age, soil and exposure, and that drought symptoms can look like overwatering or even like pest and disease damage. For woody plants it lists yellowing and wilting leaves that turn early fall color, burning or scorching along leaf edges, and dropping some or all leaves so the plant looks dead. Under long-term water stress it says plants may stop growing, or die, and that stressed plantings show the effects of weeds, insects and diseases more.

Utah State University Extension (Ornamental Pest Guide) lists leaf wilt, folding or rolling, scorch, yellowing, early leaf drop or death, and overall decline, and says trees not adapted to drought will be stressed without extra water during the growing season in Utah's dry climate. It notes some drought symptoms resemble salt damage, since salt reduces a plant's ability to take up water. The Utah State forestry specialists (Darren McAvoy and Mike Kuhns, July 9, 2021) say wilting and scorching are symptoms of water stress, and describe an under-watered tree as one that may die and take decades to replace.

How the sources say trees differ from lawns and shrubs

The Utah State forestry specialists say trees and shrubs need deeper, more extensive root systems than turfgrass, and that tree roots extend a distance close to the tree's height when water is available. Colorado State University Extension (Watering a Home Landscape During Drought) says the water-absorbing roots of a tree sit mainly in the top 12 inches of soil. Missouri makes the same point from the other direction: the soil surface dries first and is not a true sign of what is happening near the roots, and many established woody plants recover once watered. It says a plant that has dropped its leaves may still be alive and describes scraping the outer layer of a twig or bark to look for a green layer.

Which trees the sources put first

Missouri and Colorado State both put newly planted trees first in a drought. Missouri says young plants have not had time to build deep roots and depend on surface water. Colorado State gives young or newly transplanted trees a limited root system and top water priority, says establishment in its climate takes a minimum of one year per inch of trunk diameter, and calls for extra observation of trees with restricted root zones in sidewalk planters, near streets and near construction. It also says drought-tolerant plants are not drought tolerant until their roots are established.

What the sources list about water

The sources describe watering in general terms. Missouri and Utah State both favor deep, less frequent watering over frequent shallow watering, which Utah State says may not get past competing turfgrass roots. Missouri adds mulch kept off stems, organic matter in the soil, native or drought-tolerant plant selection, and checking soil with a trowel or probe. Colorado State describes probing and moisture meters, warns that clay soils can stay waterlogged and short of oxygen below a dry-looking surface, and notes that sandy soils drain quickly. Colorado State also says to follow the rules of the local water provider, which in drought can limit when and how landscapes are watered. Utah State says drip systems may be unable to deliver the volume large trees need.

Look-alikes and linked problems

Missouri says drought symptoms can be similar to those of overwatering and of some pest and disease problems, and Utah State says they can resemble salt damage. See the pages on road salt, leaf scorch and flooding and waterlogged soil. The page on sunscald sets out where the extension services disagree on a link between water stress and trunk injury.

The other pages in this group are listed on the weather, soil and chemical damage page. For how roots take up water, see how a tree works.