We call our green indoor friends houseplants, but as Mitch Baker, horticulturist at the American Plant Food nursery in Bethesda, Md., points out, no plant volunteered for the role. They’d much rather be back in the subtropical rain forest, luxuriating in the dappled shade and paddling in the afternoon showers.

As many other houseplant minders know, the key to survival is not so much the moisture in the soil, though that’s important, as the moisture in the air. With sufficient humidity, most houseplants achieve a measure of health; without it, they grow sullen.

Keeping the relative humidity at an optimum 50 to 60 percent, however, can be difficult.

One impulse is to take a spray bottle and mist the plants and the air around them. In a greenhouse with continual spraying, misting keeps humidity levels high. But the odd squirt from a mister is not going to do much except promote carpal tunnel syndrome. “For most people,” said Baker, “it’s something that would occur so infrequently as to add little benefit.”

Water trays are an effective method of raising the humidity. Set underneath plant pots, they allow the water to evaporate in the air around the foliage. You can fill the trays with gravel, which increases evaporation and, most critically, raises the pots above the level of the water. If you set the pots directly in the trays, the water wicks into the soil, keeps it wet and rots the plant roots.

You can also buy humidity trays that have a plastic grid insert that keeps the pot dry, available from garden supply catalogs or garden centers. American Plant Food sells three versions, including one that is 10 inches by 26 inches and 1 1/2 inches deep, priced at $22.99.

Fortunately, we also have portable room humidifiers. A cool-mist humidifier has two thick pads that wick cold water. An impeller blows air over them and humidifies the room.

Warm-mist humidifiers are still around, too. They are not as popular as cool-mist models because consumers fear that a warm mist conveys mold and bacteria, said Laura Conklin, of Kaz, a Southboro, Mass., manufacturer of Honeywell and other brands of humidifiers. In fact, the heating kills virtually all pathogens.