Dry air poses a threat to plants, and to human throats and noses as well. To boost humidity, set out a tray about five inches deep on a radiator or fireplace and fill it with plain gravel, pretty pebbles or last summer's collection of seashells. Cover it with an inch or so of water. Misting substitutes nature's nocturnal dew. It also increases humidity and is especially useful for plants with smooth leaves.
Some indoor gardeners believe potted plants are happier when huddled close together.
Whatever the truth, close proximity of foliage does increase humidity. Another vital element is temperature. All houseplants grow better when night temperatures drop 5 to 10 degrees from daytime levels. Few if any houseplants can live with temperatures permanently over 80 degrees. In their native tropical habitats, they can survive heat waves soaring beyond 100 degrees over prolonged periods, but higher humidity and greater light intensity balance that out.