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                 -+- The Merry Pranksters from Menlo Park -+-
                               10.1990.01.01.13

                 Marijuana Grower's Handbook - part 13 of 33
                               by pH Imbalance
                                "Temperature"

                                     from

                         Marijuana Grower's Handbook
                         [Indoor/Greenhouse Edition]
                                 Ed Rosenthal


   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 high and low temperatures slow marijuana's rate of metabolism and
growth.  The plants function best in moderate temperatures - between 60 and
85 degrees.  As more light is available, the ideal temperature for normal
plant growth increases.  If plants are given high temperatures and only
moderate light, the stems elongate.  Conversely, strong light and low
temperatures decrease stem elongation.  During periods of low light, strong
elongation is decreased by lowering the temperature.  Night temperatures
should be 10-15 degrees lower than daytime temperatures.
   Temperatures below 50 degrees slow growth of most varieties.  When the
temperature goes 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 rapdily.
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 the
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 the heat is generated at night, when
it is cooler out.
   Marijuana is a low-temperature tolerant.  Outdoors, seedlings sometimes
pierce snow cover, and older plants can withstand short, light frosts.
Statistically, more males develop in cold temperatures.  However, low
temperatures slow down the rate of plant metabolism.  Cold floors lower the
temperature in containers and medium, slowing germination and 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 kept
cooler with no damage.  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 degres.  If the air is warm, over 60 degrees, 70 degrees for the water is
sufficient.  The pipes and medium absorb the water down a bit before it
reaches the roots.
   Gardens using artificial lighting can generate high air temperatures.
Each 100 watt metal halide and ballast emits just a little less energy can 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 or misters
can be used to lower temperatures.  Misters are not recommended for use
around lights.
   Greenhouses can also get very hot during the summer.  If the sun is very
bright, 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.  Fans blow air through the mats which
lowers the greenhouse temperature.  They are most effective in hot dry
areas.  They are available througn nursery supply houses.




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