Extreme heat challenges plants, pushing them past optimal growing temperatures, typically ranging between 59°F and 86°F. When temperatures consistently exceed 90°F, growth slows, and sustained exposure above 104°F can be severely damaging. Heat stress operates through transpiration failure and molecular breakdown. Plants attempt to cool themselves by opening tiny pores called stomata to release water vapor. When the air is too hot or dry, they close these pores to conserve moisture, stopping the cooling process and causing internal temperatures to rise. This article provides actionable steps to protect your garden from destructive heat, focusing on water management, shade provision, root insulation, and recognizing distress signals.
Strategic Watering Practices
The timing of water application is crucial for helping plants manage extreme heat. The most effective time to water is in the early morning, ideally before 10 a.m. This minimizes water loss to evaporation before the sun reaches its peak intensity, ensuring the plant has a full supply of moisture to meet the day’s high transpiration demands.
Applying water directly to the soil surface around the base of the plant is superior to overhead sprinkling. Watering the foliage can increase the risk of fungal diseases, especially if the leaves remain wet into the evening. Midday watering is inefficient because high evaporation rates mean much of the water is lost before it can penetrate the soil and reach the roots.
The technique of deep watering is far more effective than frequent, shallow sprinkling. Deep soaking means applying water slowly over 30 to 60 minutes to ensure moisture penetrates at least six inches into the soil. This practice encourages a deeper, more resilient root system that can access cooler, more stable moisture reserves below the surface. Slow application methods, such as drip irrigation or soaker hoses, prevent water runoff and deliver the water precisely to the root zone, maximizing absorption and reducing waste.
Providing Temporary Shade and Physical Barriers
Reducing the intensity of direct sunlight and ambient heat effectively lowers a plant’s internal temperature. Temporary shade structures can be created using shade cloth, a woven polypropylene fabric designed to block solar radiation. A 50% blockage rating is a common choice for many vegetables and flowers, as it can reduce the temperature underneath by approximately 10°F to 15°F while still allowing sufficient light for photosynthesis.
For plants sensitive to heat, a higher percentage cloth, such as 70%, may be necessary. White shade cloth is the most effective color for reflecting light and radiant heat. These barriers should be installed above the plants, allowing for good airflow, rather than draped directly onto the foliage, which can trap heat and humidity.
Container plants are vulnerable because their roots are exposed to heat from the pot sides and surrounding air. Move potted plants into a naturally shaded area, such as under a deck, a large tree, or against a north-facing wall, for the duration of the heatwave. Grouping containers tightly together also provides mutual shading and helps reduce moisture loss by creating a slightly cooler, more humid microclimate around the plants.
Insulating Roots and Soil Health
Insulating the root zone is a complementary action to watering, as it addresses the temperature and moisture stability of the soil itself. A layer of mulch acts as a protective barrier, preventing direct sun exposure from heating the soil and dramatically reducing the rate of water evaporation from the surface. This insulation helps maintain a consistently cooler temperature around the roots, minimizing the stress they experience.
Organic mulches, such as wood chips, straw, or shredded bark, are effective because their light color reflects some solar energy, and their loose structure traps air for superior insulation. The mulch layer should be spread evenly to a depth of two to four inches across the entire root zone of the plant. Leave a small, clear ring of soil a few inches wide around the base of the stem or trunk, as mulch piled directly against the plant can trap moisture and lead to rot or create an entry point for pests.
Recognizing and Responding to Heat Stress
Observing signs of distress allows for a rapid, focused response to prevent long-term damage. The most common sign of heat stress is wilting, where leaves droop due to a lack of water pressure within the plant cells. If a plant wilts during the hottest part of the day but recovers completely by evening or the next morning, it is likely a temporary, non-damaging stress response.
Other visual cues include leaf rolling or cupping, which plants use to minimize the surface area exposed to the sun and reduce moisture loss. In severe cases, heat can cause leaf scorch, appearing as dry, brown edges on the foliage, or sunscald, which manifests as discolored or hardened areas on fruit and leaves. Plants may also drop flowers or small fruits to redirect energy toward survival.
If signs of stress are observed, the immediate action is to supply water directly to the roots, even if it is outside the optimal morning window. Do not apply fertilizer during a heatwave, as the plant cannot effectively absorb the nutrients, and the concentrated salts can further dehydrate the roots. Once the extreme heat has passed, prune away any visibly damaged or scorched foliage to encourage fresh, healthy growth.