Humidity harm

Dear Ed,
My indoor garden is in the second week of flowering. The humidity in the room ranges between 65 and 75%. Is this too high? If so, how should I lower it?

Montreal, Quebec

Humidity in a flowering room should be kept below 55%. High humidity is harmful in at least two ways.

First, it slows plant growth. As part of the plant’s metabolic processes, it transpires water and gasses. As the humidity rises, slow evaporation becomes a problem, especially in still air. This slows growth.

Second, high humidity increases the chance of mold infecting the plants, especially during the later stages of flowering. Mold spores are in the air everywhere, so there is no way to stop them from coming in contact with the plants. But they only become active when they find ideal conditions. Gray mold (botrytis), the fungus most likely to attack growing bud, enjoys cool, humid, dark spaces with an acidic pH. The crevices of a tight bud during the dark period could be just the situation it seeks. An entire garden of nearly ripe bud can be destroyed overnight as many gardeners have lamented.

There are several solutions to the problem. Ventilation might be the answer. If the outside air is drier than the grow room’s, you might find some way to exchange it. Usually indoor air is drier than outdoor, so you might be able to draw it from the house. Dehumidifiers are quite efficient at drawing water from air. They do use a bit of current and add heat to the air. If heat is a problem, you might try a window or portable air conditioner. Air conditioners draw water from the air as they cool it.

Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St, San Francisco, CA 94117, USA.
You can also email Ed at [email protected] and send queries via his website at
All featured questions will be rewarded with a copy of Ed’s book, The Big Book of Buds.
Sorry, Ed cannot send personal replies to your questions.