Environmental control has long been an integral part of maximizing yields in high value crops. Studies have shown that regulating root temperature can ameliorate the effects of sub-optimal air temperatures, increase water transport from the rhizosphere to the leaves, increase stomatal conductance, and increase the dry shoot weight, leaf area, and fruit development. These benefits alone warrant looking at root zone temperature just as seriously as any other environmental factor, but keeping root temperatures in an ideal zone can also promote an environment where beneficial microbes can flourish. More research continues to point to the benefits of having a robust beneficial biology, particularly in the root zone. Reviewing the results of a few studies we can see what root zone temperatures have performed well for a couple different crops. Then an informed decision can be made about what your best strategy will be for controlling your root zone temperature.
Multiple researchers have found that plants are able to withstand fluctuations in ambient temperature with a constant root zone temperature. In layman’s terms, keeping our roots in a stable temperature allows the plants to tolerate more extreme heat and cold. Aeroponically grown peppers had their root zone held precisely to a range of 18 to 20 degrees celsius or 64 to 71 degrees fahrenheit while the ambient temperature fluctuated from 25 to 40 degrees celsius or 77 to 104 degrees fahrenheit. The plants with a a controlled root zone temperature showed improved growth as well as a higher transport of water from the reservoir . The level of stomatal conductance, which is essentially a measurement of how open stomata become and their efficiency at releasing vapor, was higher in the regulated root zone temperature plants than in the control except in cases where there was a high vapor pressure deficit. For those instances, the stomatal conductance was similar.
This comes as no surprise for anyone who has run hydroponics or aeroponics as it is common place knowledge that your system will experience issues quickly when the water gets too warm. In ambient temperatures that get colder similar benefits can be seen by keeping the root zone warm. In another aeroponic experiment with cucumbers, the results in this experiment showed that moderately warming the root zone increased the net photosynthesis of mature leaves in the colder temperatures up to the same level as the cucumbers in an optimized ambient environment by alleviating both diffusive and metabolic limitations. In new growth however the colder climate increased leaf surface area with a heated root zone but did not match the photosynthesis of the optimized climate. Furthermore, the heated root zone did not match the same level of new leaf production as in the optimized climate. The same study noted that when the ambient air was optimized the energy cost of heating the root zone was reduced by 15% to 40%. In summary, having the entire environment controlled where both air temperature and root zone temperature are held in the desired range is the best practice. If that is not a viable option, you get more proverbial bang for your buck out of controlling the root zone. However, the energy cost of root zone temperature control is reduced when air temperature is also controlled.
So what are the ideal temperatures for roots? This will of course depend a bit on the crop, so lets review a study done on tomatoes. In a study of 6 week old tomato plants the researchers tested five different root zone temperatures under 4 different lighting situations. The five temperatures tested were
12, 18, 24, 30 and 36 degrees celsius or 54, 64, 75, 86 and 97 degrees fahrenheit. The root zone temperature of 24 degrees Celsius performed the best under the brightest lighting. These tomatoes exhibited greater dry shoot weight, leaf area, and fruit development. However, when plant growth was limited by available light the same root temperature of 24 degrees celsius exhibited reduction in day shoot weight, leaf area, and fruit development. This makes sense from the other studies we have reviewed, those showed us an increased metabolic process with optimized root temperature, so of course it could be inhibited by a limiting factor such as low lighting. However, looking at it from the other side, keeping root temperatures at 24 degrees Celsius allowed the plants to take full advantage of the available light, resulting in larger and healthier plants.
Root zone temperature can also have effects on the microbes in the root zone. Healthy soil contains a plethora of microbial life. The majority of microbes are usually bacteria. The next most plentiful microbe is the actinomycetes, a subset of bacteria. Fungal microbes fall next on the list with soil algae and cyanobacteria behind that. Protozoa are the final microbe on the list. Most of these microbes thrive in temperatures of 15 to 30 degrees Celsius (60 to 85 degrees Fahrenheit). There are exceptions, many of the beneficial bacteria can be found in compost piles that are well above those temperatures ranging all the way up to 77 degrees Celsius (170 degrees Fahrenheit). Of course this is far too hot for a healthy root system. So to simplify things we will look at the range where most microbes can thrive. What is important to remember in these cases is that the temperature can affect how quickly the microbes grow and populate. Remembering that bacteria is the most plentiful we should think about its role in nutrient cycling. The higher the temperature the more quickly the bacteria will consume carbon in the soil. The cooler the root zone the slower the bacteria will breakdown available nutrients for the plant to consume. In an organic system, retarded growth could be a sign that the root zone is not warm enough for the microbes to provide an optimized level of nutrition. Though there are complexities to the soil food web and entire books have been written on the subject, we can use soil temperature as a tool to control the pace of nutrient cycling. For example, if you are noticing a retarded pace of growth in an organic system, one of the first things to check should be the root zone temperature. It may very well be the case that the organic nutrients are not being properly cycled by the microbes. Luckily, beneficial microbe health and growth will be able to keep up just fine at the same temperatures that optimize plant growth (24 degrees Celsius in tomatoes) and tolerance of suboptimal air temperatures.
From these studies we can determine that the best course of action is to have an optimized air temperature as well as a controlled root zone temperature. Many hydroponic setups already incorporate a rigorous control of water temperature, however this can be an energy intensive method. For applications where irrigation or fertigation is done in an environment with a controlled air temperature it would make sense to take advantage of those temperatures by having your reservoir filled well prior to feeding and placed in the controlled environment so the process of feeding and watering does not swing the temperature of the root zone beyond optimized levels. Absent a rigorously controlled ambient air, or even for those looking for more control in a soil or soilless medium grown in either the ground or beds, pipes can be plumbed through the medium to run either cold or hot water through them as a means of tightly controlling the root zone temperature. Another method is to use electric heating pads or wires. Even simply insulating containers or the crown of plants grown in the ground can make a difference. Employing these strategies will reap the well documented benefits of controlling the environment of the rhizosphere, bringing your garden one step closer to optimized.