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Understanding the Thermal Environment in Cannabis Grow Spaces

To provide the optimal thermal environment for your plants through ventilation and oscillating fans, it is crucial to understand the nature of heat in grow spaces, how to measure it, and the common mistakes that can be made.

The Two Aspects of the Thermal Environment​

There are two aspects of the thermal environment that you need to be aware of:
  1. Air Temperature
  2. Radiant Temperature
While these two aspects are related, they represent separate phenomena, and plants have different tolerances for each.

1. Air Temperature​

Standard mercury/alcohol-bulb or digital thermometers are designed to measure the air temperature, not radiant heat. This is what growers typically refer to when advising to maintain a grow space between 16°C and 30°C.

However, many growers mistakenly place their thermometer in direct light, causing the radiant energy to warm the thermometer and provide a higher-than-actual reading.

To avoid this, place your thermometer halfway up a wall with a piece of cardboard over it to block the radiant heat. Ensure it is exposed to the mixed air in the grow room.

2. Radiant Temperature​

Radiant temperature is the result of heat transfer between objects at different temperatures without affecting the air in between. In this case, heat is transferred from the light and reflector to the plant's foliage through radiation, causing the leaves to heat up. Plants can withstand higher radiant temperatures (around 40-50°C) than air temperatures, but too much radiant heat can cause the plants to burn.

The Hand Test​

To prevent light burn, perform the "hand test." Place your hand where the tips of your plants will be for an extended period. If your hand feels hot, the lights are too close, and the radiant energy is too intense, which may result in light burn. Adjust your HID lights/reflectors accordingly and perform this test regularly to minimize light burn risks.

Monitoring and Adapting to the Thermal Dynamics of Your Grow Space​

Regularly monitor conditions in your grow space, as things can change rapidly. By keeping a close eye on the thermal dynamics of your particular grow space, you can optimize it for the best results.

Note: Be cautious when using high-intensity lights like 600s and 1000s, as light burn can easily damage clones and young plants. Symptoms of light burn include pale green leaves that eventually turn yellow and then yellow/brown. If plants outside the intense light zone appear greener and healthier, consider raising your lights to prevent damage.
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  • dcgolfer
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makes complete sense but never thought about it.
  • BuDGooDE
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Never knew about Radiant heat :-0 Amazing read, very informative. Love this site :-)
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