How is plant health related to root temperature?

Today we partly follow up on the article: High temperatures during indoor cultivation. How to avoid them? We'll assume that most cultivators who already have at least minimal experience are well acquainted with the ideal temperature range their plants prefer. Far from every grower, however, pays special attention to the climate in the root zone. Do you know what temperature is just right for your plants' roots?

Do you perform temperature measurements in the root zone? This is important because the temperature in the root system area is governed by different principles than the temperature around the aboveground parts of the plant. So what is the ideal root temperature?

Heat is such a fundamental factor in plant cultivation under any conditions because its intensity significantly affects the most important biochemical processes occurring in the plant body (photosynthesis etc.).

However, a grower does not perceive heat in the same way at all times, because there are these distinctions:

• Air temperature
• Leaf temperature
• Temperature in the root zone

The first two points relate to the aboveground parts of the plant. What happens below ground, i.e. in the immediate vicinity of the roots, is determined by other factors. As for the aboveground parts, it is generally known that most fast-growing plants prefer a temperature range of 20–30 °C. At night it is better to be closer to the lower limit, and in harmony with sunlight the plants prefer a warmer climate during the day that can approach the notorious thirties. Many growers then automatically take these known rules and apply them to the underground parts as well. But there is a catch. Do you know what it is?

Roots are not capable of thermoregulation

Unlike the aboveground parts (stems, leaves, flowers), the underground parts of the plant (roots) do not have the ability for thermoregulation. So what is the ideal temperature for them? Aboveground parts regulate mainly through transpiration and can cool themselves even at decidedly summer-like temperatures, whether outside or inside a grow box. Roots, however, function best at temperatures around 20 °C. And remember that the temperature of roots underground is influenced by different natural mechanisms than the temperature of aboveground plant tissues.

Air temperature can fluctuate during the day. But what happens below ground?

A grower's day starts, and here we have two possible scenarios. While indoor growers will automatically switch on the pestebni stan’s grow lights, in outdoor cultivation we may welcome sunlight during sufficiently warm months, which we rely on during the daytime lighting of our plants. In both cases, a thermometer at the start of the day would show relatively low values, so there is no reason for cooling via transpiration. Transpiration rate will, however, increase rapidly as temperature rises during the day. As it warms up during the day, energy and temperature in plant tissues and the air increase. For example, in summer it is common that from 19 °C it only takes half a day to approach 30 °C on the thermometer, so plants must cope with more than a 10-degree difference!

, in outdoor cultivation we may welcome sunlight during sufficiently warm months, which we rely on during the daytime lighting of our plants. In both cases, a thermometer at the start of the day would show relatively low values, so there is no reason for cooling via transpiration. Transpiration rate will, however, increase rapidly as temperature rises during the day. As it warms up during the day, energy and temperature in plant tissues and the air increase. For example, in summer it is common that from 19 °C it only takes half a day to approach 30 °C on the thermometer, so plants must cope with more than a 10-degree difference!

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Now let's clarify which factors ultimately influence the temperature in the root zone the most. First and foremost it is the temperature of the surrounding air during day and night (light/dark cycles). In a closed grow area two things matter:

• the temperature of the intake air, i.e. the air brought into the grow area (grow box, indoor grow room, etc.)
• the type of artificial lighting used (see article: Cultivation under artificial lighting: HPS vs. LED)

To better illustrate the impact of the two factors above when growing indoors, here are three model examples:

1. Intake air temperature is in the range 19–21 °C and you illuminate plants with discharge lamps. This combination is absolutely fine. During the dark phase when discharge lamps are off the temperature is ideal, and when the grow lights are switched on it rises only to a healthy level (max. 30 °C). This suitable intake air temperature causes natural cooling of the substrate at night, which then does not have time to overheat beyond acceptable limits even during the light phase.
2. Intake air temperature is 24 °C. If this refers to air brought into the grow room at night, during the dark phase, we are already on the borderline of acceptable climate conditions compatible with optimal plant development. The problem is that roots do not have the same opportunity as in example 1 to cool down during the night to a lower temperature. And during the day, once the light phase begins, the air temperature in the grow area will start to rise mercilessly. If you still use classic discharge lamps inside the grow box, you should be more cautious than if you use LED lighting, which emits much less heat compared to powerful discharge lamps. On the other hand, if you grow heat-loving plant species, it will be easier to reach an ideal daytime temperature thanks to a higher intake air temperature.
3. Then there are extremes when intake air temperature is 25 °C or higher, or conversely when intake air temperature barely reaches 18 °C. In both cases you cannot achieve an optimal temperature range in the root zone. How to fix this? During cold nights use heating if you need to raise the temperature in the grow room. If the problem is the opposite and your plants cannot cool sufficiently, invest in air conditioning or at least a modern LED grow light that is energy-efficient, full-spectrum (so-called full-spectrum fixtures such as ViparSpectra PRO Series P600 etc.) and generates only negligible amounts of heat to the surroundings.

The nutrient solution and its temperature also play an important role

The temperature measured in the root zone is logically also influenced by the temperature of the nutrient solution. Therefore we recommend reading the separate article: Why and how to maintain a stable nutrient solution temperature? Again, we return to the fact that temperature ranges in substrate show much smaller changes than in hydroponic systems. Quality peat substrate maintains its own temperature range. In hydroponic systems and tanks for automatic irrigation, so-called flow chillers are sometimes used.

One more very important warning is left for the end of this article, as it also relates to the irrigation temperature. Some less experienced growers, once they feel that the root system of their plants is overheated or suffering from low temperatures, start to act impractically in good faith. They try to literally cool or heat the roots directly through the irrigation water (water + nutrients), but the irrigation must have the same temperature as the root zone (see the article referenced above). If you try to cool or heat the root system with irrigation water that differs by several degrees from the current root zone temperature, you will expose the roots to stress that can significantly slow their development.

Root temperature and plant health now depend only on you; you already know the options and the principles. If you would like to ask us anything or want advice on configuring your indoor garden equipment, we are here for you at the known email address info@higarden.cz.

We also recommend these products:

• LED Set 720W Lumii Black
• Biotabs Starterpack
• Secret-Icer, extraction bags

Further reading:

• VIPARSPECTRA and growing under LED – definitely a good idea!
• How to increase terpene content in the harvest?
• Aqua Master Tools - digital measuring instruments from the Netherlands