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Horticulture Care

Drought Tolerance

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Drought is a common stress experienced by plants during periods when they are unable to access and move enough water to support healthy growth and maintenance.

Drought stress may be the result of dry soil conditions, high air temperatures, or both. The frequency and severity of drought varies from mild and moderate short-term droughts, which often occur for a few weeks during summer, to more extreme and/or extended drought periods lasting a month or longer.

Most tree varieties used for landscaping or urban plantings are able to tolerate mild to moderate droughts. However, many are susceptible to more extreme or extended droughts that may lead to drought stress—negative impacts including reduced growth, leaf wilting, leaf drop, and tree mortality. Proper watering and care during drought can help maintain healthy trees in your yard and along our streets for many years to come.

To find drought tolerant trees, use The Morton Arboretum’s Search Trees and Plants webpage. Under “More filters,” select “Tolerant” or “Moderately tolerant,” depending on the conditions of your planting site. This will show a list of trees that have been assessed to be drought tolerant.

How Trees Cope with Drought

Trees take up water from soil through their roots and transport it upward through stems into the leaves where it is eventually released to the atmosphere from tiny pores on the leaf surface. This happens at the same time that they take in carbon dioxide for use in photosynthesis. Drought stress occurs when any part of this process is disrupted. Plants may use different strategies in their roots, stems, and leaves to avoid or tolerate this stress.

Drought responses of trees are therefore determined by adaptations in roots, stems, and leaves together. Trees may avoid drought stress by accessing more soil water, potentially with deeper or more efficient roots, or by reducing water loss from leaves by closing the tiny pores on the leaf surface where water is released.

Alternatively trees may tolerate drought stress by constructing strong “pipes” to safely move water up stems and/or by accumulating solutes in their leaf tissues that enable them to maintain healthy growth and maintenance, even under harsh conditions.

Trees may use one or several of these different strategies as they cope with different droughts, ranging from short-term dry conditions and mild droughts to longer and more severe droughts.

Determining Drought Tolerance

Describing drought tolerance of individual trees can be difficult because of the many factors that combine to determine the severity of the drought (e.g., available soil water, air temperature, air humidity), as well as the different strategies that trees can use to cope with drought (e.g., different root, stem, or leaf adaptations).

We use the designations of sensitive, moderately sensitive, moderately tolerant, and tolerant to broadly indicate which species are more at risk during drought (e.g., sensitive or moderately sensitive). Plants that are more at risk during drought may need supplemental watering sooner, compared to those that are designated more resistant to drought (moderately tolerant or tolerant), which can go without supplemental water for a longer period of time and likely do not need supplemental watering once established in good sites.

Important Considerations

All trees, regardless of their reported tolerance level, will be sensitive to drought immediately after planting.

This is because they need time to establish their new root systems. This means that newly planted trees must be cared for and watered regularly for at least the first several months, and in some cases for a few years after planting.

Trees planted where the rooting space is restricted may be more susceptible to drought stress.

For example, trees along roadsides, in planting pits, in pots, or any other situation where the ability for root systems to grow is restricted as the limited space for root systems also means limited amounts of soil water available to those plants. These trees may need additional watering during periods of drought.

Trees need to be in their right place.

Putting a shade-loving tree in bright, open sun will likely lead to greater drought sensitivity.