Like anything else, it depends. Some orchids need very high relative humidity. Others don't. In general, orchids are happier and happier as relative humidity rises. Fungus infection can become a problem when humidity is over about 70%, in plants otherwise weakened.
Most hybrid Cattleya and Phalaenopsis survive and bloom in the 30% range, but are happier when it's up to 60%-75% or more. Some species Phals don't do well under 50%-60%. Other orchids are similar. Most of the miniatures need 70%-80% plus, which is why most people grow them in a terrarium.
The most effective ways to raise humidity are to have more mammals and plants in the space. Large, leafy house plants that need a lot of watering put a lot of water into the air. Even in dry areas, a house with 2-6 mammals will rarely drop below 30% humidity. A humidifying machine is also quite useful.
You can read about where your orchids originate, and study the climate of those surroundings. Some areas are wet all year. Many species Phalaenopsis, Masdevallia, Pleurothallis and Vanda come from areas like this. Other areas have distinct rainy and dry spells. Most Cattleya relatives (Brassavola, Encyclia, Epidendrum, Laelia, Sophronitis) come from areas like this, though they rarely see dry weather with low humidity. Some areas have dew at night even when there is no rain. And a few tropical orchids come from areas with extended dry spells with very low relative humidity. Brassavola nodosa is an example.
At first (and for sanity, for a long time thereafter), it is a good idea to figure out what conditions you can provide, then select orchids that fit those conditions. I just can't grow cool-growing orchids, since my house is quite warm all summer.
Edit:
When relative humidity is lower than plants expect, they transpire more water than usual. Sometimes making more water to the roots - watering more often - can compensate for this. People in areas with low humidity might have to water small Cattleya seedlings every day.
But sometimes a plant will never have a large enough root system to compensate for too-low humidity. There is no way I could grow a Boston fern (Nephrolepis exaltata) outside here in summer, no matter how often I water it.
Each of us without a carefully-controlled greenhouse has to learn what we can and cannot grow. Orchids are amazing; many can grow in circumstances far different from what they had back home. A lot of learning is by trial and error.
What I might suggest is to begin with some of the easier-to-grow orchids that tolerate lower humidity, and then branch out. Most of these plant groups can be grow in lower than ideal humidity if watering is increased.
This would include Phal hybrids in the Phal amabilis group (not the jungle plants with star-shaped flowers, like Phal. bellina and violacea); many Cattleya alliance plants; Australian ancestry Dendrobiums; Vandas; Catasetum relatives (which have dormancy requirments not found in other kinds of orchids) and many Cymbidiums. Don't fall in love with a photo online and buy something without a lot of research. Buying something that will not grow for you is not only disappointing, it's expensive.
Last edited by estación seca; 01-26-2018 at 02:27 AM..
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