Temperature
Marijuana Horticulture
by Jorge Cervantes
An accurate thermometer is essential to measure temperature in all grow rooms. Mercury or liquid thermometers are typically more accurate than spring or dial types, but ecologically unsound. An inexpensive thermometer will collect basic information, but the ideal thermometer is a day-night or maximum/minimum type that measures how low temperature drops at night and how high it reaches during the day. The maximum and minimum temperatures in a grow room are important for the reasons explained below.
Under normal conditions, the ideal temperature range for indoor growth is 72-76F. At night, the temperature can drop 5-10F with little noticeable effect on growth rate. The temperature should not drop more than 15F, or excessive humidity and mold might become problems. Daytime temperatures above 85F or below 55F will slow / stop growth. Maintaining the proper, constant temperature in the grow room promotes strong, even, healthy growth. Make sure plants are not too close to a heat source, like a ballast or heat vent, or they may dry out, maybe even get heat scorch. Cold intake air will also stunt plant growth.
Cannabis regulates its oxygen uptake in relation to the ambient air temperature rather than the amount of available oxygen. Plants use a lot of oxygen; in fact, a plant cell uses as much oxygen as a human cell. The air must contain at least 20 percent oxygen for plants to thrive. Leaves are not able to make oxygen at night, but roots still need oxygen to grow. A plant’s respiration rate approximately doubles every 20F. Root respiration increases as the roots warm up, which is why fresh air is important both day and night.
Temperatures above 85F are not recommended even when using CO2 enrichment. Under the proper conditions, which are very demanding to maintain, higher temperatures step up metabolic activity and speed growth. The warmer it is, the more water the air is able t hold. This moist air often restrains plant functions and decelerates growth rather than speeding it. Other complications and problems result from excess humidity and moisture condensation when the temperature drops at night.
Heat buildup during warm weather can catch growers off guard and cause serious problems. ideal grow rooms are located under ground, in a basement, taking advantage of the insulating qualities of Mother Earth. With the added heat of the HID and hot, humid weather outdoors, a room can heat up rapidly. More than a few American growers have lost their crops to heat stroke during the Fourth of July weekend, since it is the first big holiday of the summer, and everybody in the city wants to get away t enjoy it. There are always some gardeners that forget or are too paranoid to maintain good ventilation in the grow room while on vacation. Temperatures can easily climb to 100F or more in grow rooms that are poorly insulated and vented. The hotter it is, the more ventilation and water that are necessary.
The cold of winter is the other temperature extreme. Montreal, Quebec, Canada, growers will remember the year of the big ice storm. Electricity went out all over the city and surrounding areas. Water pipes froze, and heating systems failed. Residents were driven from their homes until electricity was restored some days later. Many growers returned to find their beautiful gardens wilted, stricken with the deepest, most disgusting green only a freeze can bring. Broken water pipes, ice everywhere! It is difficult to combat such acts of Gd, but if possible, always keep the grow room above 50F and definitely above freezing, 32F. If the temperature dips below this mark, the freeze will rupture plant cells, and foliage will die back or, at best, grow slowly. Growth slows / stops when the temperature dips below 55F. Stressing plants with cold weather conditions is not recommended; it may yield a proportionately higher THC content, but will reduce plants overall productivity.
A thermostat measures the temperature, then controls it by turning on or off a device that regulates heating or cooling, keeping the temperature within a predetermined range. A thermostat can be attached to an electric or combustion heater. In fact, many homes are already setup with electric baseboard heat and a thermostat in each room.
A thermostat can be used to control cooling vent fans in a but the coldest grow rooms. When it gets too hot in a room, the thermostat turns n the vent fan, which evacuates the hot stale air. The vent fan remains on until desired temperature is reached, then the thermostat turns off the fan. A thermostat controlled vent fan offers adequate temperature and humidity control for many grow rooms. A refrigerated air conditioner can be installed if heat and humidity are a major problem. Just remember, air conditioners draw a lot of electricity. If excessive heat is a problem, but humidity is not of concern, use a swamp cooler. These evaporative coolers are inexpensive to operate and keep rooms cool in arid climates.
Common thermostats include single stage and two stage. The singe-stage thermostat controls a device that keeps the temperature the same both day and night. A two-stage thermostat is more expensive but can be set to maintain different temperatures during day and night. This convenience can save money on heating, since room temperature can drop 5-10F at night with little effect on growth.
Many new grow room controllers have been developed in the last ten years. These controllers can operate and integrate every appliance in the grow room. More sophisticated controllers integrate the operation of CO2 equipment, vent, and intake fans. Relatively inexpensive computerized controllers are also available for grow rooms. If temperature and humidity regulation are causing cultural problems in the grow room, consider purchasing a controller.
Uninsulated grow rooms or grow rooms that experience significant temperature fluctuations require special consideration and care. Before growing in such a location, make sure it is the only choice. If forced to use a sun-baked attic that cools at night, make sure maximum insulation is in place to help balance temperature instability. Enclose the room to control heating and cooling.
When CO2 is enriched to 0.12-0.15 percent, a temperature of 80F promotes more rapid exchange of gases. Photosynthesis and chlorophyll synthesis are able to take place at a more rapid rate causing plants to grow faster. Remember, this higher temperature increases water, nutrient, and space consumption, so be prepared! Carbon dioxide enriched plants still need ventilation to remove stale, humid air and promote plant health.
The temperature in the grow room tends t stay the same, top to bottom, when the air is circulated with an oscillating fans. In an enclosed grow room, HID lamps and ballasts keep the area warm. A remote ballast placed near the floor on a shelf or a stand also helps break up air stratification by radiating heat upward. Grow rooms in cool climates stay warm during the day when the outdoor temperature peaks, but often cool off too much at night when cold temperature sets in.
Sometimes it is to cold for the lamp and ballast to maintain satisfactory room temperatures. grow rooms located in homes are usually equipped with a central heating and / or air conditioning vent. The vent is usually controlled by a central thermostat that regulates the temperature of the home. By adjusting the thermostat to 72F and opening the door to the grow room, it can stay a cozy 72F. However, using so much power is expensive, and could cause a security quandary. Keeping the thermostat between 60 and 65F, coupled with the heat from the HID system, should be enough to sustain 75F temperatures. Other supplemental heat sources such as inefficient incandescent light bulbs and electric heaters are expensive and draw extra electricity, but they provide instant heat that is easy to regulate. Propane and natural gas heaters increase temperatures and burn oxygen from the air, creating CO2 and water vapor as by-products. This dual advantage makes using CO2 generator economical and practical.
Kerosene heaters also work to generate heat and CO2. Look for a heater that burns its fuel efficiently and completely with no tell-tale odor of the fuel in the room. Do not use old kerosene heaters or fuel-oil heaters if they burn fuel inefficiently. A blue flame is burning all fuel cleanly. A red flame indicates only part of the fuel is being burned. I’m not a big fan of kerosene heaters and do not recommend using them. The room must be vented regularly to avoid buildup of toxic carbon monoxide, also a by-product of combustion.
Diesel oil is a common source of indoor heat. Many furnaces use this dirty, polluting fuel. Wood heat is not the cleanest either, but work well as a heat source. A vent fan is extremely important to exhaust polluted air and draw fresh air int the room heated by an oil furnace or wood stove.
Insect populations and fungi are also affected by temperature. in general, the cooler it is, the slower the insects and fungi reproduce and develop. Temperature control is effectively integrated into many insect and fungus control programs.