Chapter 15

      Temperature, movement, humidity and content of the air all affect plant growth.
      Unlike warm-blooded animals, which can function regardless of the outside temperature, plantsí rate of metabolism, how fast they function and grow is controlled by the temperature of the surrounding air.
      At low temperatures, under 65 degrees, the photosynthesis rate and growth are slowed. The difference in growth rate is not readily apparent if the temperature dips once in a while or the low temperatures are not extreme. However, temperatures under 50-55 degrees virtually stop growth. Temperatures in the 40ís cause slight temporary tissue damage. When temperatures dip into the high thirties tissue damage which takes several days to repair may result, especially in older plants.
      When temperatures rise above 78 degrees, cannabisí rate of growth slows once again as the plant uses part of its energy to dissipate heat and keep its water content constant. The rate of growth continues to slow as the temperature rises. Photosynthesis and growth stop somewhere in the 90ís.
      When the lights are off, photosynthesis stops. Instead, the plants use the sugars and starches for energy and tissue building. The plants do best when the temperature is lower during this part of the cycle. The fact that the lamps are off will lower the temperature quite a bit, and ventilation can be used to cool the space down.
      Looking at a marijuana leaf under a magnifying glass, a viewer will notice that there are small "hairs" covering it. These appendages form a windbreak which slows air movement around the leaf. This helps to modify the temperature by holding air which has been warmed by the tissue surface, similar to the way hair or fur keeps warm air trapped near the skin.
      Since plants transpire water, the air surrounding the leaf surfaces is more humid than the air in the surrounding environment.
      Outside, there is usually a breeze so that air is ventilated from the surface. The breeze removes waste gasses and humidity and brings fresh air containing CO2 in contact with the surface.
      Indoors, air movement is easily achieved using fans. The movement should be swift but not forceful. Leaves should have slight movement. Oscillating fans are convenient means gardeners use to provide an air stream to all sections of the garden. A draft which is too strong can be buffered against a wall so that the current reaches the garden indirectly.
      Marijuana functions best at a humidity of 40-65%. Higher humidity causes problems in two ways. First, fungi which attack marijuana become active at higher humidities. They affect all parts of the plant, but especially the buds, which contain moisture holding crevices, are dark and have little air movement. The other problem with high humidity is that plants have a hard time dissipating water transpired by the stomata (plant pores).
      The humidity level is a measure of how saturated the air is with moisture. That is, how much water the air is holding as a percentage of its water holding potential. The warmer the air the more moisture it can absorb, so that when the temperature rises the air becomes less saturated and the humidity goes down, even though the same amount of water is dissolved in the air. The reverse happens when the temperature declines. The same amount of water may be in the air, but the airs water holding capacity is lower so the humidity rises.
      There are several ways to maintain the proper temperature and humidity. The easiest method gardeners use to rid a space of excess heat or moisture is to vent the space. Small spaces such as a closet or shelf are easily vented into the room because of the large surface area in contact with the general space. Room temperature and humidity conditions are similar to those needed by the plants. Heated rooms may be a little low in humidity, but the moisture level in the micro-environment surrounding the plants is usually higher. This is caused by evaporation of water from the medium and by plant transpiration.
      Since hot air rises and cool air sinks, a fan placed above the plants pulls out the heated air. Squirrel fans and other ventilation fans make these setups a snap. Experienced gardeners choose fans with the capacity to move the roomís cubic area every 10 minutes. As an example a fan in 200 cubic foot grow space moved 20 cubic feet per minute.
      Increasing the rate of air change using a fan has beneficial effects besides controlling temperature and humidity. A breeze which causes some movement of the stem increases its strength. When a plant moves in the wind, small tears develop in the tissues. The plant quickly grows new tissue, thickening and strengthening the stem. A breeze also increases the amount of CO2 available to the plant. This is covered in depth in chapter 17 - CO2.
      Sensible growers know that open windows are not as good a solution as fans for several reasons. They present a new problem regarding detection, both by light and odor, and plant pests living outside might use the passageway to find new indoor feeding grounds.
      Some growers use a closed system. The air is cooled using an air conditioner, the humidity is lowered using a dehumidifier and the CO2 is supplied using a tank. Each of these units is connected to a sensor so that they go on and off automatically. In temperate areas the air conditioner remains on only a small part of the time, except during the summer when it may be called on for heavy duty work. The air conditioner also dehumidifies the room. A small sized dehumidifier can keep a room at desired humidity when the temperature is within the acceptable range.
      Grow spaces located in basements or attics may get cool during the winter. An electric or gas heater designed for indoor use is often used to increase the temperature. Electric heaters raise the temperature, but decrease the humidity of the room because no additional moisture is added to the air. Gas heaters vented into the grow space provide CO2, moisture and heat to the plants.
      Plant roots are very sensitive to cold temperatures. Containers placed directly on a cold floor lose their heat. To conserve warmth the units are set on a pallet or the floor, or it is covered with a layer of styrofoam sheet, which is both an excellent insulation material and light reflector. Heat mats and heating cables which are thermostatically regulated to keep trays and soil in the mid-seventies are sold in many garden shops. Water in reservoirs is often heated using aquarium equipment.


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