Explains the ideal environmental conditions for growing marijuana and how to correct problems with temperature, humidity and air quality.
The ideal temperature range for growing marijuana is 68º-85º F (20º-29ºC). The best place to take a measurement is directly above the canopy.
Specific ideal temperatures for various stages of growth:
|Germination||70º-70º (21º -25ºC)|
|Vegetative (grow)||68º-82ºF (20º -27ºC)|
|Flowering (bloom)||68ºF-85ºF (20º-29ºC)|
|Cloning||75ºF -83ºF (24º -28ºC)|
Different strains of marijuana prefer slightly colder or slightly hotter temperatures. Sativas are native to regions around the equator so they typically thrive on the upper end of the suggested range. Indicas are native to colder regions so they typically thrive in the colder end of the suggested range.
Cold temperatures below 60ºF (15ºC) will not harm the plant, but will cause growth to slow. Below 40ºF (4ºC) will cause damage to the plant tissue.
Cannabis is very hardy and can withstand heat up to 120ºF (49ºC), for short periods, without damage so long as it has access to enough water. Temperatures above 90ºF (32ºC) cause growth to slow, as the plant routes its energies toward self preservation when its that hot.
Light intensity and temperature
Temperatures and intensity of light should be proportional. Intense amounts of light should have higher temperatures, just like in nature. A bright, sunny day would also typically be warm. Low amounts of light should have lower temperatures, just like a cloudy day in nature would typically be cooler. If there is a mismatch, growth is affected. Intense light and and low temperatures slow growth. Lower light and high temperatures cause the stems to elongate.
At 40-50 watts per square foot (430-540 watt per square metre) plants should be kept at the low end of the recommended temperature range.
At 60 watts per square foot (650 watts per square metre) or higher, plants should be kept at the warmer end of the recommended temperature range.
When lights are off temperature can be kept 10ºF (5ºC) cooler (just like a temperature drop at nighttime in nature) without the plant suffering any adverse effects. But problems with stem elongation, slower growth, and delayed flowering will occur if the swings are any more severe than that.
Steady temperatures produce ideal growth.
How to control temperature indoors
The temperature in your grow room is influenced by the ambient temperature in the building, the number and intensity of lights, and heat being generated by other equipment.
If the temperature is too hot, here’s how you can reduce it:
If the heat of the grow room rises above 90ºF (30ºC) you can lower your water temperature to 70º F (65ºC) to help ease plant stress.
Cold temperatures are usually less of a problem since the lights generate a fair bit of heat, but it could still be a problem, particularly if the floor is making the roots too cold.
The ideal temperature for the growing medium is 70ºF (20ºC) If medium is too cold because of the floor, add an insulting layer such as a piece of styrofoam under the planters.
You can also cool the water you give to the plants to help control temperature. Heat the water no more than 80º F (27º C). When heating water add hydrogen peroxide or use air stones to supplement lost oxygen because warm water does not hold oxygen as well as cold water.
40-55% humidity is ideal for growing cannabis. Drier conditions will result in wilt and necrosis of leaf tips. More humidity is better tolerated than dry, but can create an environment conducive to mold.
It is hard to create conditions that are too humid growing indoors. Water evaporation from plants in enclosed spaces usually provide more than enough humidity.
If you have a problem, its probably too much humidity. First try opening a window or adding a ventilation fan. If that doesn’t work, add a dehumidifier starting with an hour a day and increasing the time its on until the correct level of humidity is reached.
Clones needs a very high level of humidity, 75%-80%. Use a humidity dome for the first week when rooting clones.
Poor air circulation leads to slow growth and weak stems. Use fans to vent air out of your grow room, draw new air in, and create air currents inside the grow room that moves the stems and leaves of the plants. Fans ensure your plants receive CO2, and causes them to grow stronger stems.
Dust build up on leaves can block a lot of light and hinder growth. There a couple of things you can use to get rid of dust in your growroom: