Just like humans, plants only thrive within certain temperature ranges. Outside of these ranges we both tend to shut down and try to “ride out” the too hot / too cold conditions. So, when we’re striving to give our plants optimal temperatures what exactly are we talking about—the temperature of the air, the soil or growing media, the roots, the nutrients, or the plant itself?
So, you've invested in a quality min/max thermometer. (In addition to displaying the current temperature, this handy gadget keeps a record of the maximum and minimum temperatures that have occurred since it was last reset. Find out more about why Min/Max thermometers are so useful.) But where exactly do you put it in order to obtain proper readings?
First, look out for a combined Min/Max thermometer and hygrometer to measure relative humidity too. You can pick these up for around $15. Be sure to choose a model with a remote probe-this allows you to accurately measure conditions at different points in your indoor garden while keeping the display component of the unit in a convenient location for viewing.
"Generally speaking, the optimum temperature for photosynthesis is 77°F (25°C)."
So, where to put the probe? Well, the most important place to measure temperature is your plants' canopy-the point where light is hitting the tops of your plants-this, after all, is where all the action should be taking place! Photosynthesis, the conversion of light energy into plant-usable energy, is concentrated around this area because it is receiving the most light energy.
The optimum temperature for photosynthesis is 77°F (25°C). (Plants growing in a CO2 enriched environment thrive in slightly warmer conditions-82°F (28°C.)) Rates of photosynthesis drop off sharply if temperatures rise above these points. It also falls gradually if temperatures are cooler.
Temperatures too high? Try these remedies:
- Raise or dim your lights
- Increase your ventilation (keep an eye on your relative humidity though as this can cause it to drop.)
- Choose a more insulated / cooler location for your grow space and / or air intake (basement or north-facing side of your home)
- Run your grow lights at night
- Invest in an air conditioning unit (reasonably priced, portable, self-install units are available.)
- Hydroponics: temporarily dilute your nutrient solution to ½ normal strength. Your plants will thank you for the extra water needed for transpiration.
If you're using a digital Min/Max thermometer with a remote probe, don't place the probe directly beneath a grow light. Use a little cable tie to tuck it in just underneath one of the upper leaves or flowers-affording it some shade from the glare of the grow lights-remember, you're following a similar principle to always measuring temperatures in the shade outdoors.
Thermometers with remote probes also come into their own during the propagation stage. Growers should always feed their probes through the ventilation holes of their propagator lids so they can be sure to monitor conditions inside the propagator itself-after all, this is what your young seedlings or cuttings are experiencing!
Some growers also choose to invest in an Infrared Thermometer. These handy, pocket-sized devices measure the temperature of any surface by shooting out an infrared beam to anything you point it at! They are a little more pricy, at around $55 but are particularly useful for checking on the surface temperature of the leaves themselves. Ideally the readings should be no more than 77°F (25°C) when the lights are on and no less than 64°F (18°C) when they are off.
The optimum temperature for active metabolism in most plants' root zones is around 64°F (18°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.