Heat Stress and Plant Growth

botanymaintenance

Jul 8

Gardeners in the Southeast know from experience that summer heat can be punishing. We also know—sometimes the hard way—that it's best to avoid installing new plants or building out beds during the hottest months. Anyone who’s planted something in July only to watch it fade quickly knows the feeling (I’ve done this more than I care to admit).

In this article, I’d like to take a deeper look at why plants suffer in summer heat—what’s happening inside the plant—and what that stress is trying to tell us. Understanding the why behind heat stress can help us better care for both established plantings and any new additions, even if they go in during less-than-ideal conditions.

How Plants Grow: A Quick Primer

Before diving into heat stress, we need to briefly review three key physiological processes in plants: photosynthesis, respiration, and transpiration. These are the basic systems that drive growth and survival.

Photosynthesis

This is the process plants use to make their own food. Water (taken up by the roots), CO₂ (taken in through stomata—tiny pores on the undersides of leaves), and sunlight (captured by chloroplasts) are combined to produce glucose (sugar) and oxygen.

Water + CO₂ + sunlight → Glucose (C₆H₁₂O₆) + Oxygen (O₂)

The glucose produced is distributed throughout the plant to fuel growth and development.

Respiration

Respiration breaks down those sugars to release energy needed to keep plant cells functioning.

O₂ + Glucose → Energy + CO₂ + Water

It’s often described as the “opposite” of photosynthesis. It occurs both day and night and has two forms:

Maintenance respiration, which keeps existing cells alive
Growth respiration, which supports new leaves, stems, and roots

When photosynthesis outpaces respiration, surplus sugars can be stored (often as starch). This energy reserve powers new root growth over winter and supports a burst of top growth in spring.

But if respiration exceeds photosynthesis—especially under stressful conditions—plants consume their stored sugars just to stay alive.

Transpiration

To absorb CO₂ for photosynthesis, plants must open their stomata. This also allows water vapor to escape. Transpiration is energetically costly—about 98% of the water absorbed by roots is lost this way. But it serves vital roles:

Water transport: Loss at the leaf surface pulls water upward from the roots, much like sucking through a straw.
Nutrient distribution: Nutrients dissolved in water (nitrogen, potassium, etc.) move through the plant.
Cooling: Evaporative loss cools the plant, much like sweating in humans.

The rate of transpiration depends on:

Temperature: Higher temps increase water evaporation inside leaves.
Humidity: The drier the surrounding air, the faster water escapes.
Wind: Air movement speeds up evaporation.

How Heat Disrupts Plant Physiology

Now let’s explore what happens to these core processes under typical Southeastern summer conditions—high temperatures, elevated humidity, and often low rainfall.

1. Increased Transpiration and Water Stress

As air temperatures rise, plants lose more water through transpiration. This isn’t inherently bad—plants “sweat” more to stay cool, provided the roots can keep up.

But once the plant loses water faster than it can absorb it from the soil, internal reserves are tapped. Cells begin losing turgor pressure (the internal water pressure that keeps leaves and stems upright), and visible wilting occurs. To prevent further dehydration, the plant closes its stomata.

That choice has consequences. With stomata closed, water loss slows—but so does CO₂ intake. Photosynthesis halts. The plant is now in survival mode.

Note: Even with stomata closed, some water still escapes through the leaf cuticle. If soil moisture is adequate, plants can rehydrate overnight once transpiration stops and root uptake resumes.

2. Reduced Photosynthesis Efficiency

When stomata are open, the plant wants to absorb as much CO₂ as possible while losing as little water as necessary. This balance is known as water use efficiency.

C3 plants (most perennials, vegetables, and shrubs) photosynthesize most efficiently between 65°F and 75°F.
C4 plants (like warm-season grasses such as Muhly or Bermuda grass) peak between 75°F and 85°F.

Beyond those thresholds, CO₂ uptake becomes less efficient. To make the same amount of sugar, the plant must “spend” more water—if it can. With heat-induced stomatal closure, this balance collapses. Photosynthesis slows or stops, and energy reserves shrink.

3. Increased Respiration and Sugar Burn

High heat doesn’t just reduce sugar production—it increases sugar consumption.

As temperatures climb, respiration rates increase, especially at night. Ideally, plants prefer a 10–15°F drop between daytime and nighttime temperatures to reduce overnight sugar use. Without that break, respiration may exceed photosynthesis, forcing the plant to consume stored sugars. Growth slows or halts entirely.

This is why plants often look like they’re just “sitting there” during the hottest months—even if the soil is moist.

High Humidity: A Double-Edged Sword

At first glance, high humidity seems beneficial—it slows transpiration, conserving water. But it creates new problems:

Reduced cooling: Less evaporative water loss means leaves stay hotter.
Slower nutrient transport: With less water flow, nutrients move more slowly from roots to shoots.
Increased disease pressure: Fungal and bacterial pathogens thrive in warm, moist conditions.
Still air: High humidity often comes with stagnant air, which limits convective cooling and increases disease risk.

Heat Damage to Plant Tissues

Even without dehydration, extreme heat can cause tissue damage. Most of the sunlight a plant absorbs turns into heat. Normally, transpiration helps dissipate this energy. But when stomata close, heat builds in the leaves.

The result? Cell death, often seen as:

Browning along leaf edges
Yellowing or browning between veins
Premature leaf drop

How to Protect Your Plants from Heat Stress

Understanding these processes allows us to take practical steps to support plant health through the summer:

Water deeply and consistently, especially for new plantings (herbaceous perennials <1 year old, trees/shrubs <2 years). Don’t let the soil dry out several inches down.

If a plant is wilting and the soil is dry, don’t wait—water immediately. Wilting means the plant is already under stress and pulling moisture from its own cells to survive.
That said, for established plants, I tend to be more conservative with watering. Plants naturally size their top growth in proportion to their root systems and available moisture. Excessive watering can encourage overly lush growth, which is more prone to flop and less resilient under stress. If a plant has been consistently irrigated and then that water source is suddenly removed, its large canopy—out of sync with its root capacity—can make it more vulnerable to drought stress.

Water in the morning to raise humidity around the plant and reduce daytime losses.
Use mulch to conserve soil moisture and slow runoff during watering.
Avoid fertilizing during heat—fertilizer salts reduce a plant’s ability to draw water from the soil.
Don’t prune in heat—it triggers new growth, which has thinner cuticles and is more vulnerable to heat.
Delay planting or transplanting—summer plantings establish slowly due to slow growth leading to their roots being confined to the potting soil they came in, which dries out faster than native soil.

Final Thoughts

Summer heat creates a perfect storm for plants: they need more water and energy precisely when their ability to produce food is compromised. Understanding these processes helps explain why even well-watered plants can struggle in extreme heat—and why timing your planting work around the seasons makes such a difference.

Additional Reading:

botanygarden maintenance

Douglas Leister