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In this article, I want to talk about drought stress in trees, specifically broadleaf trees. While drought stress is a broad topic, I want to focus on broadleaf trees dropping their leaves to conserve water. I will go deeper into drought and how it affects trees long-term in a future article.

Leaf Drop

Every year, we get customers who bring in a picture of a tree—most often a sycamore—that has lost a lot of leaves or has a canopy filled with yellow leaves.

This is often the result of a lack of water combined with summer heat, which causes plants to use and lose more water. When trees don’t get sufficient water at the roots, they may compensate by getting rid of some—or a lot—of their leaves.

In botanical terms, this process is called abscission. It is a natural defense response that helps the tree conserve water by reducing moisture loss through transpiration.

Heat can make the problem worse. Prolonged high temperatures increase water demand and water loss. Eventually, an imbalance develops between the amount of moisture available in the soil and the amount of water the tree is using. When that happens, the tree may begin shedding leaves to reduce demand.

What to Do

The best approach is to address the root cause. Make sure the tree is getting consistent, deep watering, ideally through a slow drip or deep soak. Mulching around the root zone can also help retain soil moisture.

Avoid causing any additional stress, such as unnecessary pruning, and monitor the tree as it recovers.

The temptation is often to add fertilizer or mistake the discolored leaves for a fungal problem and apply fungicide. Neither should be your first response to drought-induced leaf drop. The yellow or dropping leaves are not necessarily the result of a fungal disease, and fertilizing a severely stressed tree may create additional stress by encouraging new growth when the tree is already struggling to support its existing canopy.

Make sure the tree is getting an appropriate amount of water. Overwatering can also be a problem and may lead to poor root conditions and root rot. Soil type plays a big role in how often and how much you should water.

To help a drought-stressed tree, provide a deep, slow watering of approximately 1 to 2 inches of water over the root zone. Apply the water primarily beneath the canopy and around the drip line using a soaker hose or drip irrigation. Avoid concentrating all of the water immediately around the trunk.

Liquid seaweed can also be applied as a soil drench as part of a stress-recovery program. Apply according to the product label, preferably after the tree has been properly watered. Early morning or late evening applications are generally best during extreme summer heat.

Once the tree begins recovering and the drought stress is under control, you can evaluate whether fertilizer is actually needed. If fertilizer is appropriate, use a balanced, slow-release fertilizer.

As always, never use a weed-and-feed fertilizer anywhere near your trees.

Drought-induced leaf drop is a normal response to low soil moisture and high water demand. Give the tree some extra attention, correct the watering problem, and in many cases it will recover just fine.

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