Humidity partially controls the moisture that's lost from a plant. Plants growing in dry rooms lose moisture, which can harm many plants. Not all houseplants need the same levels of humidity. While desert houseplants such as cacti need about 30 percent humidity, others native to tropical rain forests require much higher humidity levels, as high as 90 percent. Plants needing higher humidity do best when grown in closed containers such as terrariums.
Rex begonias are common houseplants that can brighten a room with their striking colors. It's not the flowers, but the brilliant foliage, that is this plant's main attraction. The plant's foliage comes in unusual patterns and colors--such as metallic silver and spiral shapes. According to FlowerShopNetwork.com, they need high humidity, with ideal daytime temperatures around 70 degrees Fahrenheit and a night temperature of around 60 degrees Fahrenheit.
Caladiums are houseplants known for colorful foliage. Unable to tolerate air conditioning, they're delicate plants that need protection from drafts and cold. They grow best in diffused sunlight as their leaves can be easily scorched. The Peacock plant (Calathea) is a type of caladium that's grown for its foliage. This plant does best grown in bottle gardens and terrariums because of its need for high humidity.
All ferns are moisture lovers that need high humidity. Ferns with tough leathery foliage, such as the Boston fern, adjust well to household conditions, while more delicate ferns such as maidenhair ferns are not as tolerant. They need well-drained soil high in humus content. Because drafts and direct sunlight can damage foliage, they make ideal houseplants. Common houseplant ferns include holly fern (Cyrtomium), rabbit's foot (Davallia), hare's foot (Polypodium) and sword fern (Nephrolepsis).
Humidity is vital to orchids, which need a relative humidity level of around 40 to 80 percent. A gravel tray with water placed under an orchid helps increase humidity levels for the plant. The bottoms of the pots should be above the level of water; daily misting also helps. The frequency of watering for orchids depends of a plant's size, as well as the type of pot and potting soil. Usually, orchids need to dry out between watering. Roots kept wet for more than a few days can rot.
The Areca Palm, a native of Madagascar, has upright fronds, adding a tropical touch to a home. It's known by other names, including Golden Cane Palm and Madagascar Palm. Because of its high need of humidity, these plants need daily misting and heavy watering. The plant grows slowly, giving off large amounts of moisture for creating the high humidity environment that it craves.