When we experience a heatwave, our first instinct is to look for shade. But for the trees that provide this very coolness—acting as true natural air conditioners for our environment—a heatwave can be a deadly event. Scientists and arborists are increasingly using a descriptive term to explain a devastating phenomenon: the “hairdryer effect.”
Although plants are resilient, extreme heatwaves create a physiological “perfect storm” that can lead to rapid decline, structural failure, and death. Understanding this mechanism is the first step toward protecting our urban and natural canopies.
What Is the “Hairdryer Effect”?
To understand this phenomenon, we must look at how a tree manages its water. Trees regulate their temperature through a process called transpiration. To absorb the carbon necessary for photosynthesis, they open tiny pores on their leaves called stomata. In doing so, water escapes in the form of vapor. This evaporation cools the leaf, just as sweat cools human skin.
The “hairdryer effect” occurs when ambient air temperatures soar and humidity levels plummet. The atmosphere then acts like a gigantic heating appliance aimed directly at the foliage.
When the air is exceptionally hot and dry, it extracts moisture from the leaves much faster than the tree can replace it through its roots. Faced with this danger, the tree finds itself caught in a tragic dilemma:
- If it leaves its stomata open to continue cooling itself, it empties its reserves and risks a fatal rupture in its sap circulation. This phenomenon of vascular system failure, known as cavitation embolism, is being studied extensively by researchers at the Joint Research Unit for Physics and Physiology of Trees (PIAF) at Clermont Auvergne University.
- If it closes its stomata to block water loss, its air conditioning system shuts down.
Without cooling, the leaf temperature explodes. The foliage dehydrates, “cooks” under the sun, and ultimately burns. This leads to leaf browning, branch dieback, and, in severe cases, the death of the plant.
The Conditions Required for Damage
A heatwave does not always cause immediate damage; it is the combination of several environmental stressors that triggers the hairdryer effect:
- Extreme Vapor Pressure Deficit (VPD): This is the most critical factor. VPD measures the “thirst” of the air. High temperatures combined with low humidity create a high VPD, meaning the air has an immense capacity to suck moisture out of plant tissues.
- Soil Moisture Depletion: If the soil is already dry from a prior drought, the tree no longer has a “backup reservoir” to compensate for the water lost through transpiration. INRAE’s research on forest resistance to drought demonstrates that depleted soil drastically accelerates the breaking point of tree species.
- High Solar Radiation: Intense, direct sunlight raises leaf temperatures beyond what the plant can manage through its internal cooling.
- Wind as an Accelerator: While we often think of a breeze as refreshing, during a heatwave, the wind acts like bellows on a fire. It sweeps away the thin layer of humid air that naturally stagnates on the surface of the leaves (the boundary layer), accelerating water loss and intensifying the action of the “hairdryer.”
Is This Also a Problem for Shrubs?
Yes, and sometimes in an even more sudden manner.
Shrubs are just as susceptible to the hairdryer effect, particularly those with thin or large leaves. While they may occasionally escape the stronger winds of the high canopy, they face two major challenges:
- Ultra-rapid Desiccation: Due to having significantly less biomass than a large tree, shrubs have very limited internal water reserves. They can lose all their moisture in just a few hours during a peak in VPD.
- Soil Temperature Spikes: Shrubs are located closer to the ground, where radiation and reflection from the earth (or asphalt in cities) create extreme temperatures. This heat can literally “cook” the fine, shallow roots, which are nonetheless the only ones capable of absorbing water.
How Can We Help? The Best Protection Strategies
We cannot control the weather, but we can increase a plant’s “safety margin” against heatwaves through proactive management.
1. Deep, Slow, and Nighttime Watering
Surface watering is ineffective during a heatwave because the water evaporates before reaching the roots.
Strategy: Water late in the evening or during the night. This gives the soil and the plant time to absorb moisture without losing a drop to evaporation. Use drip irrigation or soaker hoses to deliver water deep into the root zone to build a true underground reservoir.
2. Mulching: The Protective Blanket
Mulch is your best passive defense against the hairdryer effect. A thick layer (10 to 20 cm) of organic mulch (ramial chipped wood, dead leaves, straw) around the base of trees and shrubs keeps soil temperatures significantly lower, blocks direct evaporation, and preserves shallow root activity.
3. Absolutely Avoid Fertilizer During Heat Peaks
It is tempting to want to “feed” a stressed plant, but this is a dangerous mistake. Fertilizers stimulate the growth of very tender new leaves. This young vegetation is water-hungry and extremely sensitive to heat, which accelerates the overall dehydration of the plant. Wait until the heatwave passes and the plant shows signs of recovery.
4. Strategic Planting (Anticipating the Future)
As heatwaves become more frequent, plant selection must adapt. For the design of our cities and gardens, Ademe (the French Agency for Ecological Transition) recommends prioritizing “climate-ready” species: local or adapted species that tolerate high VPD and periodic droughts. Avoid installing water-hungry plants in areas exposed to full sun or near reflective surfaces (white walls, patios).
Building Tomorrow’s Gardens Together
At Vallonchêne, we have been working toward this goal every day for twenty years. Backed by this extensive field experience, we rigorously select, test, and prioritize plant and shrub varieties renowned for their resilience and high resistance to elevated VPD levels and prolonged droughts.
Every garden is unique, as is its exposure to heat. We are entirely at your disposal to offer advice, analyze your land, and put our two decades of expertise to work to guide you toward the best “climate-ready” species. Do not hesitate to contact us or visit us to design a sustainable, cool, and vigorous landscape together.
To deepen your knowledge on the vital role of our canopy, discover our full article: Why Trees Are Our Best Natural Air Conditioners.
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