| Maintaining Optimum Grow Room
Temperature
Marijuana plants are very hardy and survive over a wide range
of temperatures. They can withstand extremely hot weather - up to
120 degrees - as long as they have adequate supplies of water.
Cannabis seedlings regularly survive light frost at the beginning
of the season.
Both excessively high and low temperatures can slow marijuana's
rate of metabolism and growth. The plants function best in
moderate temperatures - between 60 and 85 degrees. As more light
is made available, the ideal temperature for normal plant growth
increases. In high temperature and moderate light conditions, the
plant's stems elongate. Strong light and low temperature
conditions will decrease stem elongation. During periods of low
light, sem elongation can be decreased by lowering the
temperature.
Temperatures below 50 degrees slow growth in most varieties of
marijuana. When the temperature drops below 40 degrees, the plants
may experience some damage and require about 24 hours to resume
growth. Low nighttime temperatures may delay or prevent bud
maturation. Some equatorial varieties stop growth after a few 40
degree nights.
A sunny room or one illuminated by high wattage lamps heats up
rapidly. During the winter, the heat produced may keep the room
comfortable. However, the room may get too warm during the summer.
Heat rises, so that room temperature is best measured at the
plants' height. A room with a 10 foot ceiling may feel
uncomfortably warm at head level but be fine for plants 2 feet
tall.
If the room has a vent or window, an exhaust fan can be used to
cool it. Totally enclosed spaces can be cooled using a water
conditioner which cools the air by evaporating water. If the room
is lit entirely by lamps, the day/night cycle can be reversed so
that heat is generated at night, when it is cooler out.
Marijuana is low-temperature tolerant. Outdoors, seedlings
sometimes pierce snow cover, and older plants can withstand short,
light frosts. Statistically, more males develop in cold
temperatures. Low temperatures, however, slow down the rate of
plant metabolism. Cold floors can lower the temperature in
containers, thereby lowering root temperature and slowing
germination and plant growth. Ideally, the medium temperature
should be 70 degrees.
There are several ways to warm the medium. The floor can be
insulated using a thin sheet of styrofoam, foam rubber, wood or
newspaper. The best way to insulate a container from a cold floor
is to raise the container so that there is an air space between it
and the floor. Overhead fans, which circulate the warm air
downward from the top of the room also warm the medium.
When the plants' roots are kept warm, the rest of the plant can
be cooler without danger. Heat cables or heat mats, which use
small amounts of electricity, can be used to heat the root area.
These are available at nursery supply houses.
When watering, tepid water should be used. Cultivators using
systems that recirculate water can heat the water with a fish tank
heater and thermostat. If the air is cool, 45-60 degrees, the
water can be heated to 90 degrees. If the air is warm, over 60
degrees, a 70 degree water temperature is sufficient.
Gardens using artificial lighting can generate high air
temperatures. Each 1000 watt metal halide and ballast emits just a
little less energy than a 10 amp heater. Several lights can raise
the temperature to an intolerable level. In this case, a heat
exchanger is required. A venting fan can be used to lower
temperatures.
Greenhouses can also get very hot during the summer. If the sun is
producing too much heat, opaquing paint may lower the amount of
light and heat entering the greenhouse. Fans and cooling mats also
help. Cooling mats are fibrous plastic mats which hold moisture
and which can be purchased through nursery supply houses. Fans
blow air through the mats, lowering the greenhouse temperature.
They are most effective in hot, dry areas.
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