Are you interested in hydroponic plant cultivation, or have you already been doing it for some time? If you are struggling with mould in hydroponic systems, or if your plants are not growing as quickly as they should and the quality of the harvested flowers or crops is not worth mentioning, you may have underestimated the importance of the optimal irrigation temperature. With the correct and stable temperature of the nutrient solution, you can prevent these risks.

If you read our blog regularly, you know that this is far from the first time we have emphasised quality irrigation in every respect, starting with what water you use to water your plants. Especially in hydroponics, the quality of the water and the nutrients dissolved in it plays a completely essential role and is the basis of successful cultivation both in indoor and outdoor environments. In addition to water filtration, we monitor pH and EC of the nutrient solution, which is made possible by pH and EC metres. (You can also read the articles: Which pH of the nutrient solution to choose and Electrical conductivity of the solution – EC). Filtration, pH, EC – but what about temperature? Many growers still forget how much stability matters.

Irrigation temperature vs. dissolved oxygen

This applies twice as much to recirculating hydroponic systems such as (NFT, Ebb&Flow, Atami Wilma, etc.). In these hydroponic methods in particular, practically every degree Celsius outside the ideal temperature range represents a problem that later has an adverse effect on the results of your efforts.

We will come to the ideal temperatures shortly. For now, let us explain the important relationship between water temperature and the amount of oxygen dissolved in it, and as is well known, plants need as much of it as possible for healthy and vigorous development. As you can see in the graph, there is a direct relationship between water temperature and the amount of dissolved oxygen.

The following factors cause less and less oxygen to dissolve in water:

• Rising temperature
• Amount of dissolved substances
• Atmospheric pressure

Unlike pressure, we can easily influence the temperature that affects the resulting oxygen saturation of the water most, together with the amount of substances (nutrients) dissolved in it. And that is a good thing, because if the temperature of the nutrient solution is too high or, conversely, too low for a long time, the plants will probably be quite weakened and therefore will not be able to use their potential to the full. That is why we recommend avoiding, thanks to the correct irrigation temperature, the consequences that are unavoidable in most cases and that lead only to an average harvest.

Consequences of a long-term high irrigation temperature:

• The amount of oxygen dissolved in the water decreases. An insufficient supply of oxygen will negatively affect the development of the plants’ root system, their growth, overall health and final quality.
• Irrigation, whose temperature remains above the optimum level for a long time, significantly increases the risk of fungal or bacterial diseases in plants.
• A high irrigation temperature combined with intense lighting, whether artificial or sunlight, often results in algae growth. The algae then consume even more oxygen.

Consequences of a long-term low irrigation temperature:

• Slow plant growth.
• Plants may become undernourished, as they are unable to absorb the necessary amount of nutrients because the nutrient solution is too cold.

Watch out for temperature fluctuations, or stability first and foremost

In terms of water temperature, or rather the nutrient solution (irrigation), we have discussed in detail the risks associated with both extremes. Another serious and common problem when growing herbs, agricultural crops or other types of plants, however, is frequent changes in water temperature, which cause unnecessary stress to plants, or rather shock.

Therefore, avoid temperature fluctuations at all costs! Try to keep the temperature of the nutrient solution with as little variation as possible so that your plants can grow steadily thanks to a stable irrigation temperature and remain vigorous until harvest.

With this important warning, we have touched on the topic of plant stress, to which we have devoted a separate article: Stress also harms plants, or plant abiotic stress.

What irrigation temperature should be maintained in hydroponic systems?

The water source and the surrounding environment determine this.

First of all, we measure the temperature in the water tank. The tank may be external or part of a recirculating hydroponic system. When measuring, do not forget the well-known saying “Measure twice...” and, to be safe, measure the temperature in the tank several times. The measured values may differ depending on the current time of day. Combined pH and EC metres, which can also measure temperature, will become a reliable helper for frequent measurement of the nutrient solution temperature, so you will always have all these important irrigation values under control. For measuring the temperature itself, a classic water thermometer will also suffice.

Combined measuring options including temperature are offered, for example, by the very popular metres in a practical case from the renowned manufacturer Milwaukee. We recommend, for example, the waterproof metre set Milwaukee KIT – Martini pH55 + EC60. For the purposes of hydroponics, it is also perfect thanks, among other things, to its ability to measure TDS values (Total Dissolved Solids), that is, the total amount of salts dissolved in the nutrient solution.

If you would appreciate a more technically advanced combined device that already bears the designation monitor, we recommend the Bluelab Guardian Monitor for monitoring acidity, conductivity and irrigation temperature. Its undeniable advantage is the option to set an alarm that automatically alerts you whenever the values deviate from normal.

We now come to the heart of the matter, namely the recommended nutrient solution temperatures for hydroponic plant cultivation. We would like to give you exact figures, but even if you looked elsewhere on the internet, the recommended values for hydroponics will differ slightly. The experience of professional growers therefore speaks clearly. Above all, it is important to monitor how your plants are doing and respond flexibly to changes. However, you can use the following information as a starting point.

• Nutrient solution temperature in recirculating systems: approximate minimum 15 °C, approximate maximum 26 °C.
• A range of 20–25 °C provides the correct temperature for healthy root system development in hydroponic plant cultivation. And as you probably already know, a healthy root system is the basis of every proper harvest.
• As a general rule, temperatures above 25 °C also lead to the development of parasitic fungi (oomycetes) of the genus Pythium.

What should you do if you measure incorrect temperatures?

If your thermometer, metre or monitor shows values significantly outside the stated range, action is needed. The temperature of the nutrient solution can be adjusted either directly or indirectly.

Indirect adjustment of the nutrient solution temperature

Indoor growers have it a little easier than growers who practise hydroponics in outdoor conditions. In a grow tent, there are several options for changing the temperature in the growing space and thereby influencing the temperature in the tank. Here they are:

• Change to a different type of lighting as needed depending on heat output.
• LED lights, CFL energy-saving lamps or CMH lamps emit less heat.
• HPS and MH lamps emit considerably more heat.
• Use heating inside the grow box or, conversely, an air cooler.
• Bring cold or warm air into the growing environment using a fan. 

Direct adjustment of the nutrient solution temperature

A straightforward solution is clearly more effective when adjusting the temperature in the irrigation tank, because you do not have to change the other parameters of the growing environment in a complicated way.

• Easy optimisation of the water temperature in the tank is made possible by digitally controlled tank cooling, or a chiller. This is a water flow-through device regulating temperatures on a scale from 18 °C to 35 °C. So whether you are dealing with low or high temperatures, you only need to connect the chiller to the tank or hydroponic system and a constant nutrient solution temperature will become a matter of course for you! Chillers from the traditional Boyu brand are also not particularly noisy and, with reasonable handling, will please you with a long service life. They are available in four variants, and we recommend choosing primarily according to water flow and the recommended tank capacity, in addition to price.

• The proven BOYU submersible tank heater is equipped with a built-in temperature controller (thermostat), so you can easily set the desired temperature by simply turning the control knob. Depending on your needs, you can choose a version with an input of 100 W, 200 W, or 300 W.

Whether it is a choice of cooling, heating or any other advice you would welcome in connection with the correct temperature of the nutrient solution or hydroponics in general, we look forward to your questions at the well-known e-mail address info@higarden.eu.

We also recommend these products:

• TECO HY-500 - tank cooling and heating
• Atami WILMA Small - 4x6 L, 60x60x20 cm
• BioNova Hydro Supermix

What to read next:

• Torus Hydro perfectpH: revolutionary pH stabiliser
• TBOX: a new product that pays off!
• Hydroponics made easy: drip irrigation