One of the challenges of maintaining an optimal growth environment for your plants over the summer months is mitigating the effects of excessive heat. Whether your plants reside in a polyhouse, greenhouse or warehouse, high temperatures can have negative consequences for produce quality, yields and profitability. However, there are strategies to implement to ensure that crops remain vibrant throughout the season.
How much heat is too much?
Simply put, if a human would be happy in the environment, plants will likely be happy. However, just as some people would prefer a sunny beach with 85-degree temperatures and a breeze and others would prefer a cool forest with canopied shade, a plant has its own environmental preferences and tolerances. Crops that prefer a relatively hot setting include tomatoes, peppers, melons and eggplants. Beets, cauliflower, lettuce and other leafy greens generally prefer a cooler ecosystem.
Under extreme conditions, the plant’s stomata will close and the process of transpiration stops. At this point, the plant is using all its energy to preserve life, and it ceases to grow, flower or fruit. Remember, we can get a measurement of transpiration by using a laser thermometer to measure the leaf temperature and comparing it with the air temperature.
Controlling heat in the air
The solutions for excessive heat are somewhat dependent on the type of building in which the plants are grown.
If growing produce in a polyhouse, ventilation may be increased by rolling up the plastic on the sides or by adding additional fans. Another solution is to use shade cloth. Shade cloth is classified by the amount of light it blocks out, expressed as a percentage of total light available. Shade cloth likely won’t interrupt the crops’ photoperiod, as the plants are still exposed to the sun, just to a lesser extent.
When using a greenhouse, the same options exist. Additionally, you can invest in an automatic system that opens and shuts the windows based on temperature. The opener has a cylinder containing a mineral that expands when heated. As the mineral swells, it pushes a piston that opens a vent.
In a closed environment, increasing CO2 levels is a helpful tool. By increasing the ambient CO2 levels from ambient levels of approximately 400 ppm (0.04 percent) to optimal levels of around 1,000 ppm (0.1 percent), we can extend a plant’s heat tolerance, or comfort zone, by 5 to 10 degrees Fahrenheit.
In any situation where high-intensity discharge lighting is used in an indoor setting, an option is to simply switch up the light cycle so the lights run at night, when the outdoor temperature is much cooler. In this scenario, ventilation may be enough to solve the problem.
We could also increase the distance between the bulb and plant canopy, reduce the power to adjustable ballasts (turn down the 1,000W to 750W or 500W) or use a light mover. A light mover will move the reflector and bulb back and forth along a rail above the canopy, increasing an individual light’s coverage area and reducing the overall number of lights required.
You may be asking, “Won’t that decrease my yields?” Possibly. If all other aspects of your growing environment are optimized and the amount of light is reduced, yields may be lower. However, when excessive heat is the limiting factor, managing the high temps will result in higher quality and quantity of produce (relative to a situation where maximum light is used but excessive heat is occurring).
You could also use an air conditioner to reduce the temperature. However, the use of an A/C unit can be relatively expensive to purchase and operate due to its high electricity costs. Additionally, running an A/C unit will lower the humidity in your ecosystem.
An evaporative cooler, also known as a swamp cooler, is a viable alternative for greenhouses and closed environments. It requires electricity and a water source to operate. A high-efficiency fan blows “hot” air from the environment over an evaporative pad, which is fed by water pumped internally through distribution lines. This results in evaporation of water, which lowers the air temperature and increases humidity in your growing environment. Since it takes energy to convert liquid water to vapor, and energy cannot be created or destroyed (just transferred), hot air travelling through a swamp cooler must give up some heat energy to evaporate the water. The remaining air has lower energy (the molecules aren’t traveling as fast or far), resulting in a lower temperature.
Controlling heat in the root zone or rooting environment
A nutrient solution’s available oxygen, or dissolved oxygen content, is inversely correlated with the temperature of the solution. The presence of nonbeneficial microorganisms (fungi, molds), which thrive in high temperature and low oxygen environments, may lead to root rot and other deficiencies. Root rot will turn your roots a brownish-black color, and they will feel slimy to the touch. Although air stones and the use of beneficial bacteria (those that feed on the bad microorganisms) may help, the use of a reservoir chiller offers a lasting solution to the problem.
A reservoir chiller is like air conditioning for your nutrient solution and thus the root zone. It is a useful tool for aeroponic, deep water culture, nutrient film technique and ebb-and-flow systems. It is more cost efficient than an air conditioner unit to purchase and operate, and is more of a targeted solution, as it addresses the specific problem of the roots being overheated, without having an effect on ambient air temperatures or humidity levels. In hydroponics, a plant can often thrive in a hot, humid environment, as long as its roots are kept cool.
Use of a medium should be considered. For example, Coco-coir maintains oxygen levels regardless of temperature, and it is often lack of oxygen in the rooting environment (a result of the heat), and not the actual heat, that causes problems for a hydroponic producer. Additionally, the use of a medium introduces a level of insulation for the roots. Although the medium’s temperature will fluctuate, it will do so more slowly than the air surrounding it.
A final tip is to decrease the strength (lower the ppm) of the nutrient solution. Under stressful conditions, the processes of transpiration and evaporation increase, leading to a higher demand of water, while nutrient demand remains the same. So, if nutrient solution is left unchanged, ppm will increase over time.
Implementation of these techniques should help to combat the negative effects of excessive heat. Stay cool and enjoy a plentiful harvest.