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Climate Control: What's the best temperature & humidity for an indoor grow?

Cannabis is a fair weather plant. Chances are if you feel hot, cold or damp in your garden, your plants do too. Indoor gardeners have complete control, and complete responsibility for the environment that the plant grows in. Keep the temperatures as close to 72°F with 45% humidity to optimize growth. A hot, cold, or wet garden is prone to fail.

Too Hot

Hot gardens incur the wrath of nature. Pest insects thrive in warm temperatures because warm weather increases their metabolism. For example, the life cycle for spider mites takes about three weeks when it is cool outside. Warm weather puts spider mites into overdrive. When temperatures exceed 80°F, a spider mite can hatch, reproduce and die within as little as five days! Cool temperatures help keep insect problems at bay.

Hot gardens also defy the laws of science --specifically Boyle’s Law and Henry’s Law-- that define the behavior of gases (air) in relation to temperature and pressure. Roots need oxygenated water. So do soil microbes. But the warmer water is, the less dissolved oxygen it contains—100°F water holds about half as much oxygen as 68°F water. Without oxygenated water, roots turn brown, slimy and sick. No amount of air stones or supplements will overcome the way gases behave as temperature increases. Cool temperatures keep roots healthy and white.

Conversely, warm air has the capacity to hold more water vapor. Therefore, as temperature increases, relative humidity decreases in an enclosed garden. The difference between the actual moisture in the air, and the saturation level (dew point) is the Vapor Pressure Deficit.

The greater the VPD, the faster water travels from the roots upward to be “sweat” out via leaf stomata-- openings that allow the leaf to “breathe.” High VPD is better than low. But when VPD is too high, plants close their stomata to retain water, and growth slows. High VPD is also bad for germination, propagation and transplanting. These tender plants do not have enough roots to support rapid water uptake under high VPD.

Too Cold

Frost kills at 32°F, but growth slows below 68°F. Photosynthesis and plant metabolism slow down when the temperature drops. Leaf stomata close too; plants essentially hold their breath in cold weather. All this means plants grow smaller slower in a cold grow room. And plants root very slowly into cold soil.

Cold air cannot absorb as much moisture as warm air. As temperature drops, the volume of water in the air stays the same while the saturation (dew point) decreases. High relative humidity and low VPD encourage mold. Use heat, dehumidifiers and exhaust ventilation to prevent nighttime humidity problems.

Just Right

Climate control gives indoor gardeners an edge because the conditions are always perfect for growth. Sunny and 72°F is a great temperature for cannabis; beware of too much humidity. Cannabis is forgiving of dry conditions, so keep humidity below 45%.

Greenhouse in snow.
This greenhouse has condensation from trapped moisture and dropping temperature.

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