Managing Heat Stress on High Value Crops

Hot summer temperatures can place significant stress on crops. While the most effective way to prevent heat stress is to grow exceptionally healthy plants from the start, there are also several in-season strategies that can help reduce its impact. Practices such as shade paint, overhead irrigation, and targeted nutrient applications can all play an important role in helping crops withstand extreme heat.

How Heat Stress Affects Plants

Heat stress can have a major impact on plant growth and productivity because it depletes energy reserves that would otherwise support development and yield.

When plants experience excessive heat, their first response is to conserve water. They do this by closing their stomata. Unfortunately, once the stomata close, carbon dioxide uptake slows dramatically, causing photosynthesis to essentially stop.

As a result, the plant is no longer producing sugars and carbohydrates, which serve as its primary energy source. Until temperatures cool and the stomata reopen, the plant cannot generate additional energy.

What Happens When Energy Reserves Run Low?

Healthy plants store surplus energy in the form of fats and lipids. As long as those reserves are available, temporary periods of heat stress generally do not cause major problems.

The real trouble begins when those energy reserves are depleted.

The plant still requires energy to function, but it is no longer producing carbohydrates through photosynthesis and has exhausted its stored fats and oils. At this point, the plant begins breaking down proteins that were previously built during periods of healthy growth.

Proteins are essentially carbohydrates and nitrogen combined together. When the plant separates and uses the carbohydrate portion for energy, excess nitrogen remains behind. Nitrogen by itself is unstable and will often revert back to nitrate or ammonium forms.

Then, almost as if by magic, insects and spider mites begin to appear.

How to Build Resistance to Heat Stress

The foundation of heat stress resistance is an abundance of plant energy.

Building energy reserves requires efficiency. Rather than relying on a constant supply of soluble nutrients, we want soil biology delivering the majority of plant nutrition.

Microbial nutrition requires significantly less energy for the plant to absorb than individual nutrients. As plants become more energy-efficient, they begin storing more energy as fats and oils.

One visible sign of this process is the development of glossy, waxy leaves.

Why Glossy, Waxy Leaves Matter

Plants with glossy, waxy leaf surfaces tolerate high temperatures much better than plants without them.

Under normal conditions, there is typically about a 5-degree difference between air temperature and leaf temperature. However, when plants develop these healthy waxy coatings, the difference can become much greater.

Instead of a 5-degree difference, the leaf temperature may be 20 degrees cooler than the surrounding air.

For example, if the air temperature reaches 95 degrees, an exceptionally healthy plant may still maintain a leaf temperature of only 75 degrees. Since plants can continue photosynthesizing up to approximately 76 degrees leaf temperature, these healthier plants are able to keep producing energy long after stressed plants have shut down.

The Key to Long-Term Resilience

The key point I want to emphasize is that we need aggressive soil biology. We need exceptional levels of photosynthesis. We need to build resilience before we have a problem, not react after the problem appears.

In-Season Strategies for Managing Heat Stress

Even healthy crops can benefit from additional support during periods of extreme heat. The following practices can help reduce heat stress and maintain productivity throughout the growing season.

Improve Airflow

Good ventilation helps keep leaf temperatures lower.

Use fans, vents, and other airflow management strategies as needed to reduce heat buildup around the crop canopy.

Irrigate Early in the Morning

Plants require a consistent supply of water to manage heat effectively.

Maintaining adequate soil moisture helps reduce stress, and early-morning irrigation ensures plants are fully hydrated before the hottest part of the day arrives.

Use Mulch or Living Covers

Mulches and living covers can dramatically reduce soil temperatures during the summer months.

This helps conserve soil moisture while also protecting soil microbial activity. As soil temperatures increase, microbial populations become less active, reducing nutrient cycling and overall soil function.

Apply Shade Paint

Shade paint is widely used in greenhouses and high tunnels.

Compared to shade cloth, shade paint often does a better job of reducing heat while still allowing maximum sunlight penetration and absorption.

Consider Overhead Irrigation

Overhead irrigation and misting systems can provide significant relief during periods of extreme heat.

In tomato greenhouses, running misters for approximately 10 minutes during the hottest part of the day has reduced temperatures by as much as 20 degrees, with the cooling effect lasting one to two hours.

In the situations I observed, shade paint was also being used and likely contributed to the overall cooling effect.

A common approach is to run misters around 11:00 a.m. and again around 1:00 p.m. However, additional experimentation is warranted. I suspect shorter, more frequent applications may be even more effective.

For example, misters could be automated to activate when temperatures reach approximately 85 degrees and run for only a minute or two before shutting off. Systems could also be manually disabled on cooler or cloudy days.

Use Foliar Applications

Foliar nutrition can improve a plant’s ability to withstand environmental stress.

Applications of potassium, EnSoil Algae, and a carbohydrate source such as Root Primer can significantly improve stress resistance during the growing season.

For best results, apply foliar sprays early in the morning, preferably before 6:00 a.m.

Maintain Adequate Potassium Levels

Potassium is one of the most important nutrients for heat stress management.

When potassium levels are inadequate, plants become much less efficient at regulating and utilizing water, making them more vulnerable to high temperatures.

Apply Silica Before Heat Events

Silica can provide substantial benefits for stress reduction, but timing is important.

Results are generally best when silica is applied before the heat stress event occurs, with at least two applications completed beforehand.

Silica helps prevent contraction of the plant’s vascular system, ensuring water and nutrients continue moving efficiently throughout the plant.

Heat Management Consulting and Products

Many growers are already implementing several of these practices, particularly airflow management, irrigation, mulching, and shade paint. However, overhead misting and targeted foliar nutrition may deserve a closer look.

Heat stress management is not just about reacting to hot weather. It starts with building resilient plants through strong soil biology, efficient nutrient delivery, and exceptional photosynthetic activity.

The team at Keystone Bio-Ag is here to help guide you on the proper strategies for developing healthy, active soil, and providing the products to get you there. 

Experiment with different approaches, observe the results, and don’t hesitate to reach out to a consultant if you need additional perspective or guidance.