Hey there! What’s the correct water temperature when watering my plants? I’m growing four Trainwreck autos in coco coir and can’t find any information on the topic. The reservoir I have is on the ground so I’m worrying the water may be too cold. Is this an important factor for plant health? Looking forward to your response!

Thank you for your question. The ideal water temperature for watering plants is between 62-72°F. Within this range, the water is not too cold and can still hold a good amount of dissolved oxygen. When discussing water temperature for plants, it is important to understand dissolved oxygen. Dissolved oxygen is one of the most important factors of water quality and it is directly influenced by temperature. As the temperature of the water increases, the water’s ability to hold dissolved oxygen decreases. As the dissolved oxygen level in water decreases, the likelihood of pathogenic organisms increases. Generally, beneficial microorganisms that live and breed in the soil are aerobic. In other words, they thrive in an oxygen-rich environment. These beneficial microbes have symbiotic relationships with plants and aid in nutrient uptake. If the water temperature is too warm, many beneficial microorganisms will not have access to the oxygen they need to live and breed and, therefore, will die off. When this happens, anaerobic pathogenic organisms (organisms that thrive in an oxygen-depleted environment) will take their place, causing countless issues for the horticulturist. The relationship between temperature, dissolved oxygen, and beneficial microorganisms is why cooler water is preferred and why many hydroponic growers implement water chillers and/or other techniques to maintain a cool water temperature.

(Read also: Dissolved Oxygen: The Hidden Necessity)

Although cooler water can hold more oxygen, water that is too cold can cause stress to the sensitive roots of a plant. This stress hinders a plant’s ability to uptake nutrients and grow healthily. This means horticulturists must find a balance between water that is too cold, which causes stress, and water that is too warm, which depletes dissolved oxygen. This balance ensures the best environment for both the beneficial microbes and the sensitive plant roots.

An inexpensive digital thermometer would be a worthy investment and could be used to monitor the water coming out of your reservoir to make sure it is not below 62°F. If the water temperature is too low, it could be heated with a submersible aquarium heater or moved into the grow space (or another area) where the ambient air can “warm” the water to the desired temperature before giving it to your plants. Using water that falls within the 62-72°F range will ensure your plants (and the microorganisms) have ample dissolved oxygen and their sensitive roots won’t be shocked, possibly hindering their growth. I hope this answers your question.