Category: Gardening tips

In the height of summer, with the sun shining brightly, you might expect the garden to be bursting with vigor. Yet, as you have probably noticed, in July and August, the growth of many trees, shrubs, and plants seems completely frozen. Young shoots stop growing, and buds lock up.

Don’t panic! Your garden is not dying: it is simply taking its summer nap.

Let’s take a look behind the scenes at what is happening in nature’s great laboratory.

Estivation: The Survival Strategy Against Heat

Everyone knows about hibernation, the deep sleep that certain animals and plants share to get through the winter frost. But did you know there is a summer equivalent? This is called estivation, or summer dormancy.

For a plant, growing requires colossal energy and, above all, an astronomical amount of water. When the thermometer rises and rainfall becomes scarce, the tree must make a choice: continue to grow at the risk of dehydrating, or shut down the machinery to preserve its life. The choice is easily made.

The Secret Mechanism of Stomata

Under the leaves of your plants lie thousands of microscopic pores called stomata. Normally, they open to breathe and absorb carbon. However, by opening, they allow water to escape (this is transpiration).

During the intense heat of August, to avoid emptying themselves of water, plants seal their stomata. The direct consequence: sap circulation slows down, photosynthesis runs in slow motion, and growth stops. The plant enters ‘energy-saving’ mode.

Why This Break Is Excellent News for Your Plants

This visual stagnation sometimes worries gardeners, but it is actually vital. If a tree continued to produce tender, water-gorged young shoots in the middle of August, they would be instantly scorched by the sun or dried out by the hot wind.

By blocking its growth, the plant takes the opportunity to accomplish invisible but crucial work:

• Wood Lignification (Aoûtement): The shoots that grew in spring begin to harden, filling with lignin (the component of wood). This process allows the branch to resist future winter frosts.
• Root Development: If energy is no longer directed into the leaves, it is sometimes redirected to the deepest roots to seek out moisture wherever it hides.
  The Vallonchêne Nursery in spring

💡 Did you know? The trap of greenhouse plants raised ‘on life support’

This hardening phenomenon explains why plants from intensive greenhouse production often turn out to be so fragile once they arrive in your garden.

In the artificial environment of a greenhouse, protected from the wind and temperature fluctuations, and constantly kept on life support with liquid chemical fertilizers and scheduled watering, plants do not experience these natural cycles of stress and rest. Their tissues remain tender, heavily gorged with water, and their shoots never learn to harden off. Furthermore, having never been confronted with direct sunlight or drying drafts, they lack the reflex to regulate the closing of their stomata.

When they come out of their bubble to join the real soil of your garden, the shock is immense: the first real heatwave, or the first autumn or spring frost, is often fatal.

This is precisely why at the Vallonchêne nursery, we grow things differently:

• 100% Outdoor Cultivation: Raised in the open air, subjected to the whims of the weather, the wind, and real sunlight, our plants learn from a very young age to regulate their strength and stop their growth when nature demands it.
• ZERO Liquid Fertilizer: At Vallonchêne, we NEVER use liquid fertilizers that artificially boost plant tissue at the expense of their hardiness. We let the plant feed at its own pace.
• A Low-Peat Substrate: We strictly limit the use of peat in our potting soil mix. You can easily notice this by touch and handling: our pots are much heavier! This denser, more natural mix retains moisture better in the long term and provides a smooth transition when you replant into the real soil of your garden.

By accepting this natural and demanding rhythm, our plants develop unique cellular robustness and perfectly mature wood. When they arrive at your home, our trees and shrubs are already armed to face real garden life.

Good Habits for the Gardener in August: Do Nothing (or Almost Nothing)!

Since your plants are resting, you should do the same! It’s the perfect time to kick back and adapt your routine:

• No Fertilizer: This is the most common mistake. Seeing that the plant is no longer growing, one is tempted to give it a ‘boost’. This is a very bad idea. Fertilizer would force the plant to restart its growth when it does not have the water resources, which would exhaust it or burn its roots.
• No Severe Pruning: Pruning stimulates new growth. In August, these young leaves would not have time to harden off before winter and would weaken the plant.
• Keep It Cool: Simply monitor your watering (as explained in our dedicated article) and check that your mulch is thick enough to protect the soil from direct sunlight.

A Word from Vallonchêne:

The garden in August is not lazy; it is simply wise. It knows that to last for decades—just like the exceptional trees we love to help you discover every week—you have to know when to stop. So, set up your deckchair in the shade of a large tree, enjoy the freshness of its foliage, and let nature take its nap in complete serenity!

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.