When growing indoors it’s important to realize the comprehensive control the environment you create has over your cannabis garden. New growers often overlook temperature as a potential source of issues, and some advanced growers overlook the potential to use differences in day and night temperature to regulate the growth and size of a plant for their benefit.

Due to the variables involved, no single temperature scheme will work for everybody. To grow successfully, and without consuming too much electricity, you need mesh environmental factors (humidity, outside temperatures) with the plant’s requirements for healthy growth.

As long as temperatures don’t dip below 60 °F at night, they don’t go over 95 °F during the day, and the day/night fluctuation doesn’t exceed 20 °F, your plants will still be healthy. Due to the heat that high-intensity discharge (HID) lights produce, indoor growrooms normally have at least a 5 degree difference between day and night, and while this is in fact may be closer to its natural environment, it may do a disfavor to indoor growers for several reasons.

For starters, budding plants become susceptible to mold if temps drop too low during lights-off or fluctuate too much from the lights-on temp. Hot air holds a lot of moisture in the form of water vapor, but if the temp drops, water will precipitate out covering your plants in dew. Many growers let their growrooms cool at night to induce vibrant colors of purple and pink, but have to tread carefully so as to not harbor mold growth like powdery mildew. Running a dehumidifier helps, but maintaining higher temps at night will also prevent this.

What other ways can you manipulate cannabis with temperatures? In the 1980’s researchers from the Michigan State University discovered how day and night temperature differences regulate plant growth. They found that keeping daytime temps lower than nighttime temps, a situation they called “negative DIF,” made plants have lower internodal length. “Positive DIF,” keeping daytime temps higher than nighttime temps, caused stems to grow longer. More recently researchers have discovered this effect comes from differential expression of enzymes that degrade gibberellins, a group of hormones that regulate plant growth.

After the discovery at Michigan State University it didn’t take long for ornamental flower growers to adopt the technique of keeping a negative DIF. This temperature manipulation provided a straightforward way of producing plants with a desirable compact shape without the use of toxic chemical plant growth regulators.

For an indoor cannabis grower, the prospect of cooling a growroom during the lights-on period to the point that it’s cooler than during lights-off is quite daunting. While it may be possible for growers in certain climates, for most it would be prohibitively expensive. Could there be a way of taking advantage of their discovery without running more air-conditioning during lights-on in addition to heating during lights-off?

Further research discovered that most of the stem-elongation caused by positive DIF occurs during the first couple hours of the morning. Lowering the growroom temp below nighttime temps during the first two hours of lights-on is sufficient to retard the stem-elongation otherwise induced by a positive DIF.

This technique, called a “cold-air-dump,” is relatively simple. Say, for example, your growroom normally reads 80 °F during lights-on dips to 75 °F during lights-off. Simply run a small air conditioning unit, or vent cold air from outside, to bring the temp down to 70 °F for the first two hours of the lights-on period. This precise temperature manipulation will produce shorter plants that don’t crowd each other out, producing a homogenous crop of dense flowers.

Before you experiment with finely adjusted temperature schemes, make sure to have to proper equipment first. To provide adequate cooling during the cold-air-dump you’ll need a good freestanding air conditioner. Another option is using cold air outside; use a vent fan with a thermostat and make sure it works well. However, before you make any of those investments you’ll need a hi-low thermometer that can tell you how cold it gets at night. An even more advanced option would be data logging thermometer that can tell you how the temperature changes throughout time.

Photo Credit: Oregon Breeders Group