Climate Control

By Karen E. Szabo


Air temperature affects a Cannabis plant’s metabolism, growth rate, and bud production. Maintaining proper air temperature and relative humidity levels in the growing environment is essential for a successful harvest, although they must fluctuate during the various stages of growth. To fully understand where air temperature and relative humidity levels differentiate, it’s necessary to start at the beginning. For simplicity, all references to temperature in this article use the Fahrenheit scale


Stage I Cloning. Clones require a higher relative humidity level than during any other stage of growth and so the goal at this stage is to create a rainforest atmosphere. Humidity plays a vital role in developing healthy roots on clones. If the humidity level fluctuates even as slightly as 10 percent, it could result in clones failing to develop properly, which could eventually cause them to mold, wilt or die. Maintaining a rather high relative humidity level, between 75 and 85 percent, while maintaining air temperature between 72 and 75 degrees for the first three days is absolutely vital to the success of the clones. Due to the fact that clones have not yet produced roots, allowing a cloning medium to become too hot will cause freshly cut clones to sweat, and they are unable to replace the water lost. Temperature fluctuations affect clones in different ways as well. When clones become too cold, it takes longer for them to produce roots, while clones that get too hot will most likely die from dehydration. Temperature-stressed and wilting clones are more subject to rotting due to reduced resistance to pathogens. Fluorescent lights are ideal for cloning, producing enough light without adding unnecessary heat.


Stage II Vegetation. During the weeks of the vegetation stage, air temperature should be maintained ideally at 78 degrees, although during daytime hours – what’s known as “lights on” – anything between 72 and 82 degrees is within the safe. Daytime relative humidity is ideally at 50 percent, but is safe within a range of 40 – 60 percent. Several measures can be taken to help increase humidity levels. To introduce moisture into the air, use a humidifier to achieve necessary relative humidity levels. Misting the plants several times a day can help to aid in producing moisture as well. If CO2 is being used during daytime hours, it’s best to increase the air temperature to between 79 and 85 degrees, but no higher than 90. Without the excessive heat emitted by daytime lighting, it’s obvious during nighttime hours – or “lights out” – that the air temperature is going to drop. Air temperature should range between 65 and 70 degrees during lights out, although the drop from daytime temperature should not exceed 10 degrees. To prevent stress, never allow air temperature to drop below 60 degrees during any stage. When lights go out, the relative humidity in the room will climb, that is just one of many reasons why ventilation fans and proper air flow are important.


Stage III Flowering. In this section, elongation – stretching – is mentioned, please note that some strains stretch more than others, but can be managed with temperatures, as described.  Lower humidity levels help the Cannabis plant transpire C02 and reduce the risk of acquiring bud mold during the flowering stage.

During daytime hours, air temperature should be maintained between 78 and 82 degrees with a relative humidity of 35 – 45 percent. It is preferable during the flower stage to have a nighttime air temperature drop of no more than 10 degrees to stimulate flowering hormones and reduce stem elongation. Relative humidity is normally lowered to a steady 35 percent during the last two or three weeks of flowering, decreasing chances of acquiring bud mold, which is known to form during the flower stage as well as while curing. Lower relative humidity also increases resin production by the plant as it attempts to prevent excess moisture loss by sealing itself off from the drier environment it’s now being exposed to while, at the same time, raising the THC level. Using a dehumidifier occasionally,  or during the entire flower time, may be necessary. Be sure to empty the reservoir on the dehumidifier when full or it will defeat the purpose of using one at all. Continuous air flow, along with use of a dehumidifier, will help achieve desired relative humidity levels. When using C02, the same rule applies during flower as with vegetation, increase the air temperature to between 84 and 88 degrees. At this stage it is not necessary nor advised to increase relative humidity.

While the lights are off during nighttime hours, photosynthesis stops. In its place, the Cannabis plant uses its stored sugars and starches for energy and tissue building. Plants fair very well during the nighttime hours when the air temperature is decreased during that time. The photosynthesis and growth rates slow in an environment where the air temperature is below 65 degrees; if the lower temperature occurs rarely, growth reaction is not readily noticeable. If air temperatures are between 50 and 55 degrees, it will halt the plant’s growth. When air temperatures fall into the 40-degree range, it causes slight, but reversible, tissue damage. However, if air temperatures are below the 40-degree range, there is a good chance damage caused from the cold air temperature will be irreversible.

Cannabis plants grown in an environment with consistent air temperature are more likely to grow sturdier stems, stouter stalks and denser buds. Even if maintaining a constant temperature is not possible, plants flowering in an environment where the temperature does not deviate more than 10 degrees generally avoid stem elongation.


Stage IV Learning from the past

Growing Cannabis is a constant learning process. It is possible each grow might differentiate from the previous one, even if you are growing the same strain repeatedly. There are steps you can take to avoid frustration. Consider maintaining a detailed growing journal, preferably with photos, which allows you to document the problems you encounter as well as the solutions you find. Such detailed documentation makes it easier to avoid repeating the same mistakes and improve your methodology so you can enjoy large harvests of premium bud while minimizing frustrations.

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