Ultrasonic, with a cool mist option would be ideal. They put out a fog, versus a mist…..which is the main thing. The mist droplets are too big, and the potential is there for issues. The fog incorporates into the air much better.What type are they? You've piqued my interest.
personally…. I don’t run a humidifier. I’ve grown in the humid northeast, and the dry southwest. Currently my RH outside is about 17%. My thought is, do my best to make my environment as perfect as is in my control….and don’t worry about the Rh so much. You can get into a situation where chasing problems can cause more problems down the road. The plants are resilient…..look at the multitude of environments it grows in outdoors. They adapt. To be honest…. I’ve only really seen major issues from too high RH.