Air and fertigation water temperatures impact the growth rate, health, and harvest weight of your marijuana plants. In indoor grow ops, you can totally control these temperatures. In outdoor growing, you have a harder time controlling these factors, although I’ve worked with outdoor growers who chill their nutrients water and (in greenhouses) use climate control to control air temperature.
My focus today is on the many impacts of fertigation water and root zone temperatures on the health and productivity of your marijuana plants. Cannabis roots evolved in nature, where below-ground temperatures are cooler than the aboveground ambient temperatures. Water entering a natural outdoor root zone comes from rain, which is often cooler than prevailing air temperatures.
When you’re growing cannabis and providing the water your plants’ roots intake, your plants benefit most when you deliver water that’s 67-68°F. This temperature has been shown to protect roots from heat stress, aid in maximal uptake of nutrients elements, and is the temperature at which water oxygenation is very high.
Water and root zone oxygenation is an important growth and vitality factor for your marijuana plants. One reason you see incredibly fast and robust root growth (especially during cloning) in pure hydroponics systems such as deep water culture and aeroponics is that the root zones are filled with an extraordinarily high amount of oxygen compared to root zones using soil and other solid media. Growers are wise to use pumps, bubblers and air stones to oxygenate water in pure hydroponics, and in drip irrigation and other hydroponics systems.
The use of professional root zone materials such as Grodan rockwool and the highest grades of coco coir and soilless mix often increases oxygenation in solid-media root zones. Growers using generic soil and peat-based mixes should add extra-coarse perlite so that it is at least 10% if not more of total volume. This increases porosity, drainage, and oxygenation of soil and peat mixes, which otherwise tend to pack down and become overly dense and waterlogged, reducing oxygenation, impeding root growth, and decreasing plant health and productivity.
Fertigation water temperatures above 68°F can decrease root zone oxygenation and create other problems. The warmer the irrigation water is, the less dissolved oxygen it can hold. Water that’s too warm and stagnant can even become anaerobic, which is deadly for your marijuana roots.
Another problem is that warmer irrigation water is an ideal environment for harmful microbes that rot roots and rob the root zone of oxygen. Some growers run their indoor grow rooms above recommended temperature range when carbon dioxide is added to indoor grow room air during lights-on cycle because in C02 grow ops, you can allow marijuana grow room ambient temperature to creep higher than the usually recommended 74-78°F– added C02 helps cannabis plants use higher ambient air temperatures for accelerate metabolism that produces faster growth and bigger buds.
Regardless of what the optimum air temperature is for your grow op, the temperature of the root zone and fertigation water still matters. I urge all growers to use a chiller in the root zone. If the grow room is hot, and the fertigation water is inside the grow room in a hydroponics reservoir without a chiller, the water is most likely way too hot too.
Quality water chillers cost several hundred dollars, and require some hardware and properly-tight seals to prevent leaks, but they pay for themselves many times over. In hot grow rooms with or without C02, chilled fertigation water frequently recirculated in the root zone greatly reduces plant stress. Plants grow faster and are more productive because chilled water holds more oxygen, is far less likely to contain pathogens, and relieves root heat stress. Some growers have discovered that using chilled fertigation water allows them to reduce air conditioning costs, especially in grow rooms run hot because added carbon dioxide makes it useful to do so.
Too-warm water isn’t the only problem—fertigation water and root zones that are too cold can also harm plants. Growers who have their plants on pots directly on the cold floor of a basement grow op or in other too-cold places risk slow root growth, decreased root mass, and ineffective root function. The same happens when fertigation water is below 65°F, and some cannabis strains will be severely damaged or even die if root zone and/or ambient air temperatures go below 59°F for even short periods of time.
A simple aquarium heater is used to warm water to the correct temperatures. When grow lights are off and grow room temperatures drop by several degrees to dip near 59-60°F, you may consider warming your root zone with grow room heat mats, rather than heating fertigation water. Most grow room heat mats are too small and generic, made for seedling or clone trays. They often lack precision temperature control—they’re either on, or off, offering no ability to program specific temperatures. Get professional size mats unless you have a very small grow space.
In all cases when you’re purchasing marijuana grow room temperature-altering gear, I recommend programable or at least digital gear that allows you to set specific, precise temperatures. This kind of gear can be hard to find, so you may have to contact several hydroponics equipment and greenhouse supply stores before you find what you need.
In all cases, I recommend placing a bubbler into fertigation water reservoirs to increase oxygenation, and using only reverse osmosis water.
As with many facets of advice about perfect growing conditions for marijuana plants, every strain is different. This means you should use the recommendations we just talked about as guides, but closely monitor your plants’ growth rate, overall health, root health, and root development to see how your temperature control strategies are working. Monitoring root development and health is easiest if you’re operating pure hydroponics systems such as deep water culture and aeroponics, of course.
If you’re growing a strain with genetic influences originating in a region with cold ambient and/or ground temperatures, you may find that a slightly lower root zone and fertigation water temperature works better than the temperatures I’ve recommended. If you’re growing a strain with genetics from hot climates, root zone and air temperatures might be best if they’re slightly higher than my recommendations. However, I caution you that fertigation water warmer than 72-72°F can create problems, no matter how heat-adapted your strain’s genetics are.
By keeping your grow room ambient air and fertigation water in the proper temperature range and by oxygenating fertigation water, you’re ensuring optimal growth, nutrients uptake, and harvest rewards from your marijuana garden. The total control that indoor growers can have over factors such as water quality, temperature, oxygenation, and root zone conditions, especially in pure hydroponics water culture systems, is a true blessing for your plants and your grow room goals.