Today we will partly follow up on the article: High temperatures in indoor growing. How can you avoid them? We will proceed on the assumption that most growers who already have at least minimal experience are well acquainted in advance with the ideal temperature range that suits their plants. However, by no means does every grower also monitor the climate in the root zone separately. Do you know how many degrees are just right for your plants?

Do you carry out temperature measurement in the root zone? It is important, because the temperature in the area of the plant root system is governed by different laws than the temperature prevailing around the above-ground parts of the plant. So what is the ideal root temperature?

Heat is such a fundamental factor in plant growing, regardless of the conditions, because its intensity significantly affects the most important biochemical processes taking place in the plant body (photosynthesis, etc.).

However, growers do not perceive heat in the same way every time, because they distinguish between these types:

• Air temperature
• Temperature of the leaves
• Temperature in the root area

The first two points relate to the above-ground parts of the plant. However, different factors determine the temperature underground, i.e. in the immediate vicinity of the roots. As far as the above-ground parts of plants are concerned, it is generally known that most fast-growing plants thrive in a temperature range of 20–30 °C. At night it is better to move towards the lower limit, and in harmony with the sun’s rays, plants during the day prefer a warmer climate, which can approach the proverbial thirty. Many growers then automatically take these well-known rules and apply them to the underground parts of the plant as well. However, there is one catch. Do you already know what it is?

Roots are not capable of thermoregulation

Unlike the above-ground parts (stems, leaves, flowers), the underground parts of plants (roots) do not have the ability to thermoregulate. So what is the ideal temperature for them? The above-ground parts of plants use mainly the process of transpiration to regulate themselves, and thanks to this they can cool down even in distinctly summer temperatures, whether they prevail outdoors or inside a grow box. However, the roots of plants, as their underground parts, function best at temperatures around 20 °C. And let us not forget that the temperature of roots underground is influenced by different natural mechanisms than the temperature of plants in the above-ground parts.

Temperature may fluctuate during the day. But what happens underground?

The grower’s day is just beginning, so we have two possible scenarios here. While indoor growers have their grow tent and grow lighting switch on automatically, when growing plants outdoors we can, during sufficiently warm months, welcome the sun’s rays that we rely on to light our plants during the day. In both cases, however, a thermometer would show relatively low values at the start of the day, so there is no reason for cooling through transpiration. The rate of transpiration will, however, rise rapidly in connection with the increasing temperature during the day. As the day warms up, the energy and temperature in the plant tissues and in the air increase. For example, in summer it is quite common that half a day is enough for the thermometer to climb from 19 °C to thirty, and the plants therefore have to cope with a difference of more than 10 degrees!

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Underground, or rather in the root zone, we do not see such significant temperature fluctuations (see above). The temperature of the root zone (in the substrate) changes far less than the temperature of the air and plant tissues above ground. When growing plants in soil or substrate, the temperature difference over the whole day may therefore be only 1 °C, and the roots of plants will be perfectly happy if the temperature remains around the optimum throughout the day, which is usually considered to be 21 °C. What would definitely not suit them, on the other hand, are significant temperature fluctuations, which must be avoided.

You are probably therefore wondering what happens once the temperature around the roots does, after all, rise alarmingly. In such a case, oxygen dissolved in the water in the optimum amount is no longer available to the roots of the plants. As the temperature rises, its amount decreases. Higher temperatures in the root zone are also a major attraction for bacteria, as a result of whose action root rot and root dieback may occur. In practice, this problem is probably most closely associated with hydroponic systems, where it is beyond dispute that the temperature can be maintained in the range of 19–24 °C. (You can gather more practical information about hydroponics in separate articles such as “Hydroponics – what does it involve and how do you start?” or “Hydroponics made easy: drip irrigation - drip”.)

Classic soil and peat substrates have the advantage, in terms of roots and temperature fluctuations, that the roots of plants grown in soil are not always quite so sensitive to temperature increases. In any case, and regardless of whether it is a hydroponic system, a more or less pre-fertilised, or bio soil mix based on high-quality peat, or the increasingly popular coconut substrate, the above temperature range can be considered “golden”, or universally valid. In short, it is a temperature range in line with the ideal conditions for the development of your plants, regardless of which growing medium you are currently using.

While the air temperature around the above-ground parts of the plant rises noticeably over the course of the day, the situation in soil or substrate is quite different, and only slight changes in the ambient temperature