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 try reducing it to 1.5 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 (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
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source: Just for Growers