Once established, cannabis plants are often quite hearty and resilient, able to withstand a fair amount of abuse and variation in atmospheric conditions. Nevertheless, there are optimal ranges of humidity and temperature that will promote the healthiest, most lush and delicious growth (and certainly circumstances that will hinder or hurt your plant's path toward greatness!). This document will review these optimal constraints, and include a few tips for achieving them.
First and foremost, you will want a digital thermometer / hygrometer in order to sample the environmental conditions in the atmosphere of your grow room. Combination units with a remote probe are inexpensive and a smart investment for the serious grower. Some of the more elaborate meters will note the high and low extremes for the temperature and humidity in your room, so that you will know if your room is getting too hot or dry during the peak of the light cycle, or too cold and wet during the dark cycle, even if you are not around at those times to check in yourself.
40 - 60% rH during flowering (lower rH helps to reduce the risk of fungal contamination on your already moist dank buds).
80 - 90% rH for rooting clones and cuttings (high humidity is a must as these new plants may not have roots to draw moisture through).
More Details and Commentary:
Night temperatures should be kept above 60 degrees to prevent stress. It is preferable during flowering to have a night temperature drop of 10-20 degrees to stimulate flowering hormones and reduce stem elongation.
If you are supplementing your grow space with additional carbon dioxide (CO2), your target daytime temperature should be between 80 - 90 degrees (F), until the last two weeks of flowering when daytime temps should be kept about 10 degrees lower and CO2 can be reduced to adjust for the lower metabolic rate of the mature plant.
While one should always make an effort to maintain temperatures below 85 degrees (F), cannabis can handle higher temperatures and you're probably not going to ruin your crop if your grow room peaks in the mid to upper 90's during the day. Just make sure that you maintain excellent air circulation to prevent the heat from building up in hot spots and burning your plant more than necessary. Placing a fan in line with the top of your canopy and blowing toward your light can help prevent excess burning during peak temperatures. If you do notice serious burn occurring on the tops of your plant, you're going to have to either add more ventilation to your grow room, an air conditioner,or just move your plants farther away from the light.
If low humidity is a problem, you can mist your plants during the cloning and vegetative growth stages, but you should refrain from misting during flowering as it can surely encourage fungal growth! There are also humidifiers available for purchase, or you can place a large dish of moist perlite in your grow space to help transpire more moisture into the air. When using perlite, it should not be swimming in water, and it would be advised to mix a dilute solution of hydrogen peroxide in with the water each time you moisten it, in order to help prevent mold growth. If this isn't enough, you may need to fully dry the perlite on occasion, which can be accomplished by spreading it onto a foil lined baking dish and placing it in the oven for a while.
Low humidity (below 60% during vegetative growth or below 40% during flowering) can stress plants. However,some growers swear by lowering the humidity as much as possible during the last several days of flowering, as this may promote increased resin production by the plant as it attempts to prevent excess moisture loss by sealing itself off from the dry surrounding environment with the good THC-packed sticky stuff.