IMO ..the lower the better...when I grew outdoors, I sometimes got periods of only 10-15% humidity...and my plants love it...as long the containers stay moist.
lower humidity and good airflow will also keep you away from any budrot or other mold/mildew issues.
I cant remember where, but I do remember reading that mj likes low humidity, and if it gets too high, the leaves cant breathe properly.
however, you got me to thinking about this issue, and I got to researching and I came up with this from hightimes.com
Q: Our mother and grow room humidity levels are in between 20 to 30 percent. Does it have to be 50 percent or more?
-- Tom
A: Humidity must be extreme for it to seriously affect plant growth. Although 20% is low, if you have good ventilation and the plants are kept properly watered, you shouldn't have a problem. If you do however, try misting the garden once or twice a day. Moistening the foliage will allieviate some of the problem. Should more drastic measures be called for, you can add humidity to the air in a couple of ways. The easiest, but most costly way is to simply get a humidifier. Set it up in or near the place where fresh air is being taken into the garden. Don't put it in the garden itself. The fresh air intake will draw the moistened air into the room. If that's a little beyond your budget, you could set up a home made air humidifier. Get a small aquarium pump and create a waterfall inside the garden. The constant churning of the falling water will add moisture to the air.
