We may all feel the effects of hot weather on our bodies, but few take notice of high temperatures on plant matter. Cold weather protection methods are commonly used to protect plants from the cold. After all, a plant will die if it gets too cold –especially at the roots. For most, it ends here with temperature control. Heat is an extreme issue, but it has less immediate warnings.
For example, on a day after an extreme freeze when sensitive, unprotected plants are clearly suffering from freezing temperatures. High temperature susceptibility is not as acute, with the damage effect taking some time to develop.
Root system controls a huge part of its health. It has been researched that substrate temperatures of 40°C over lengthy periods (five to six hours), can lead to indirect injury. This is also more insidious, as it stops the plant from growing and hinders hormone utilization (over time stunted growth), limits nutrient absorption, or creates greater disease susceptibility.
With much more excessive and non-therapeutic temperatures (as low as 20 minutes), direct injury – a far greater problem. Extended cold can create damage that is irreparable to root systems & may lead to poor health and appearance of plant materials. In cases of direct damage, symptoms associated with indirect injury are observed and the plants may start dying, especially in the roots.
For decades we have known that containerized environments can get very hot and stay relatively extreme for long periods of time, seriously damaging plants health and vigor as well as their quality. This problem , while worrisome , had not been given the proper attention in earlier years, as there were fewer immediate threats, and fewer possible solutions. But the growing number of heat stress and resulting crop loss across the country, even in cooler areas, is another indication that a warming planet keeps getting hotter. When we spoke to growers, there was a sense that they were refocusing on extreme temperatures.
Potential Solutions
There are many options available to mitigate extreme substrate temperatures, some of which are more practical than others.
Incorporating shadowing into your production is probably the easiest way; however, it can be costly, especially for larger areas.
There are some management practices that can be more easily implemented to mitigate extreme fluctuations in substrate temperature. Irrigation with temperature regulation in mind helps cool plants and the substrate. Periodic irrigation (i.e. spacing out watering throughout the day) has been proven to keep temperatures moderate. However, afternoon watering only limits the heat and, in extreme cases, only reduces temperatures by a few degrees. With exposed pipes, the water coming out will be hot and will have little cooling effect. Additionally, watering in the heat of the day is generally against recommended best practices. What’s the biggest concern? Using too much water to control temperature can have a negative effect on plant growth. Recent research from Auburn University has shown that decreasing the distance between plants can help protect them from direct sunlight and increase shade, thereby limiting direct sunlight access to their surface. Despite the benefits of spacing management, this option can be inconvenient because it may require plants to be periodically repositioned or moved over time.
Substrate temperatures of over 65°C are also recorded in hot years. More and more brands of plants are being produced, many of which come in brighter, bolder colored containers to stand out among the crowd of black potted plants.
These containers can mitigate substrate temperature extremes. In 2022, our team at the LSU AgCenter evaluated plants grown in white and black containers. On average, the black containers exceeded the critical temperature of 100°F for approximately 9 hours per day, while the white containers exceeded the critical temperature for less than 2 hours.
In a typical summer, substrate temperatures in these pots regularly reach over 37°C. However, substrate temperatures of over 65°C are also recorded in hot years. More and more brands of plants are being produced, many of which come in brighter, bolder colored containers to stand out among the crowd of black potted plants. These containers can mitigate substrate temperature extremes. In 2022, our team at the LSU AgCenter evaluated plants grown in white and black containers. On average, the black containers exceeded the critical temperature of 37°C for approximately 9 hours per day, while the white containers exceeded the critical temperature for less than 2 hours. Note that indirect damage is the result of the time the critical temperature was exceeded. Thus, while high temperatures may be unavoidable, efforts to reduce their duration are well worth the time. In fact, during the summer, the temperature in the white container exceeded 38°C for less than half the time as in the black container. This means a conservative 50% reduction in exposure to heat stress.
The white container also reduced the average daily maximum temperature by about 4°C. Of course, there were extreme events, but 4°C is a very big opportunity when studying temperature stress. Don’t believe it? Set your thermostat 9 degrees warmer and see if anyone notices.
What does this temperature difference mean for production? Healthier, stronger plant material.
There are an increasing number of white containers available from many suppliers. For growers who don’t have branded plant material, there are still empty white containers available that will be kept at the right temperature. Please note that if you use alternative containers, you should always use containers with a black lining. This lining limits light penetration into the root zone, which can hinder root growth and lead to algae growth.
Another method that is cheap and very useful is simple monitoring. As the saying goes, knowledge is half the battle, and knowing the temperature of the substrate certainly helps in making decisions.
There are several ways to monitor the substrate temperature: Handheld probes can be inserted into containers and random temperature readings can be taken. For the more tech-savvy farmer, sensors can be installed in containers. These can be wired or simply embedded in the container and can transmit temperature and other measurements wirelessly to a mobile phone or computer.
The agriculture industry is becoming increasingly technologically advanced and temperature monitoring, management and mediation is an exciting area to explore at low cost.
Original article by Jeb S. Fields, an Associate Professor and Coordinator of the Hammond Research Station at the Louisiana State University (LSU) AgCenter and Ashley Hickman, a graduate student in the Fields Lab; and Damon Abdi, an Assistant Professor at the LSU AgCenter.