Temperature Humidity and C02

Hydroponics—Dealing with High Temperatures

It can catch even the best grower off guard. Whether you grow indoors or outdoors, a sudden rise in ambient temperatures due to a heat wave or just regular seasonal variations can put a lot of additional stress on your plants. So what’s a grower to do? Fortunately there are a whole host of strategies—some more obvious than others!

First, regard this magic number: 77°F (25°C). Keep this in mind as you read on. Even tropical plants thrive around this temperature, providing relative humidity is around 50%-60%. In fact, this temperature is damn near perfect for most plants. 

Learn more about optimal temperatures for plants

Bolivian Rainbow Pepper suffering from high temperatures and overly concentrated nutrient solutionSo why are so many growers' gardens running at higher temperatures if this ultimately means less photosynthesis and reduced metabolic rates, elongated growth patterns (making plants more difficult to illuminate with grow lights) and lower quality produce overall?

The answer is often-too many grow lights, not enough ventilation and cooling!   New indoor gardeners often underestimate just how much fresh air their plants need, both for continual access to carbon dioxide and for temperature and humidity regulation.

But first, let's look at the common sources of heat problems in the indoor garden-starting with the one we just mentioned, the number one culprit of heat problems indoors!

Heat Source 1: Grow Lights

The best way to combat high temperatures in your grow room is at their source. High intensity discharge (HID) grow lights are the main cause of temperature problems in indoor gardens. A 1000W HID generates nearly 4000 BTUs of heat per hour. Common sense or, if you prefer, the laws of thermal dynamics, dictate that temperatures will steadily rise in your indoor garden unless you do something with this heat (i.e. vent heat-charged air out the garden or counteract the heat with air conditioning.)

Perhaps the most straightforward technique of minimizing the inevitable heating effect caused by high intensity discharge horticultural lighting is to house the lamp itself in an air-cooled reflector. Ideally you should duct the coolest air you can into the reflector and vent it away directly out of your room via insulated ducting for minimal thermal return. Whichever route you choose, remember the air you use to cool your grow lights needs to be room temperature or less.

Learn more about air-cooled reflectors

Heat Sources in your Indoor Garden



1000W HID Grow Light (Lamp only)


1000W Magnetic Ballast


1000W Electronic Ballast


600W HID Grow Light (Lamp only)


600W Magnetic Ballast


600W Electronic Ballast


BTU stands for British Thermal Unit. It is equivalent to just over 1,000 joules of energy. 

Heat Source 2: Ambient Temperatures Outdoors

You can mitigate the effects of higher temperatures outdoors by running your grow lights at night-often electricity is cheaper at this time too. Whether you run your lights during the day or night, try to avoid positioning your garden in a room with poorly insulated, sun-facing walls. The extra heat generated by the sun's rays beating on exterior walls and roofs will only make environmental control more difficult and energy intensive in the long run. Many growers report their best results from grow rooms located in basements due to the natural insulating properties of the earth, although extra care has to be taken to deal with humidity in subterranean environments. Remember-it's best to draw air from a cool shady location, whether indoors (better) or outdoors.

Heat Source 3: Ballast Location

If possible, try to house your ballasts next door to your grow room, not inside it. Not only do you offer more protection from accidental splashes, but ballasts also generate a significant amount of heat-both magnetic 'core and coil' and electronic / digital models. Remember, ballasts must not overheat so do not enclose them in an unventilated cupboard-they need some ventilation too!

Heat Source 4: Dehumidifers, CO2 generators, Pumps and You!

Remember, all electrical equipment in your indoor garden contributes. Even if you are doing some light-work in your garden (such as repotting) your body can produce upwards of 500 BTUs per hour! A CO2 generator providing CO2 for a 20' x 20' room will produce upwards of 7500 BTUs per hour (equivalent to an additional 1000W light and ballast in your garden!)

Some Great Strategies for Combating Excessive Heat in your Indoor Garden

1)    Reduce Nutrient Concentration
Highly recommended for hydroponic growers! This is a tried and tested commercial technique. In hot conditions your plants are transpiring more moisture than normal. This means that your plants are using a higher water : nutrient ratio. The net effect is a concentration of your nutrient solution which then places cumulative stresses on your plants. Try reducing the concentration of your nutrient solution by 25%. (E.g. If you are running an E.C. of 2.0 (~1400 PPM) try reducing it to 1.5 (~1050 PPM) in hot conditions.)

2)    Temporarily raise your Grow Lights
By positioning your grow lights further away from your plants, (an additional 6 - 12 inches) the canopy will be exposed to less radiant heat emitted from the lamp. Of course, if your grow lights are too far form your plants for a prolonged period of time your plants will inevitably begin to stretch in reaction to insufficient light levels, but this is a good strategy for a heat wave lasting just a few days.

3)  Dim your Grow Lights
Some models of electronic ballasts now carry a dimming or dial-a-watt feature affording growers the option of running their HID grow lights at 75% or 50% full power and thus, producing less heat. Growers should note that the spectral distribution of some lamps can change in dimming mode (i.e. some parts of the spectrum dim more than others.)

4) Power Down!
It's much, much more preferable to simulate a cloudy, overcast day than a scorched desert. Don't be afraid to switch off half of your grow lights for a few days during a heat wave.

5) Add Air Conditioning
An A/C unit is the ultimate way to control temperatures in your indoor garden. Reasonably priced, portable, self-install units are available. 

6) Keep Your Roots Cool
The optimum temperature for active metabolism in most plants' root zones is around 64-68°F (18-20°C). Ensure that your water or nutrient solution is at this temperature too. If using tap water, be sure to add just enough from the warm tap so that the water feels tepid-not warm, not cold, just silky to the touch. Better yet, invest in a nutrient thermometer or a thermostatically controlled nutrient heater.