Until spring, low humidity is the curse of houseplants

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 afternoon showers.

Bonsai, African violets and orchids are among the plants subject to the dangers of low humidity. This bonsai is from Bonsai West, one of the exhibitors in the 2004 Rhode Island Spring Flower & Garden Show.

Mine currently huddle in the corner of the dining room, as if waiting for a bus that never arrives. Here, in the brightest part of the house, the schefflera, citrus, orchids, clivia and philodendron sit out the winter — yellowing, wilting, attracting mites and generally longing to be with the plants on the other side of the windows. If we can get to April still standing, we consider it a triumph. The lemon tree may have dropped half its foliage by then, but it refoliates with abandon once back outdoors on the patio.

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 percent to 60 percent, however, can be difficult.

A home with forced-air heating may have an automatic humidifier integrated into the ductwork. It is typically fed by a water line, which tops up the reservoir as needed, and moistens the hot air from the furnace or heat pump. But homes without this device soon find themselves parched: The already low humidity of the dry winter air is pushed down by the heated air in a home, especially if the occupants like it toasty. The humidity level drops to 20 percent or lower — desertlike. Static electricity bites our fingers. The long-haired cat becomes a fright wig with legs.

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’s 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.

These are favored partic- ularly by fanciers of bonsai, African violets and orchids. When you have large plants in big pots, as I do, the trays become less practical. Baker said I could simply fill the existing pot saucers with gravel and top them up with water.

Fortunately, we also have portable room humidifiers. Years ago, I had one that worked up some steam and sent visible vapor into the air. You could see the plants clearing their nasal passages. Most satisfying. I went in search of a modern version a couple of winters ago and found something called a cool-mist humidifier. This has two thick pads that wick cold water. An impeller blows air over them and humidifies the room. The device works well: I can see the needle on my humidity gauge climbing to 50 percent. And I hear the machine taking deep, filling drafts of eau de Alexandria, Va. The water in its tanks glugs down into the reservoir.

Still, I’d rather see the vapor and know that water is pouring into the air. Happily, warm-mist humidifiers are still around. 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.

THE OTHER THING I like about warm-mist versions is that there are no pads to clean or replace, burdens accompanying the cool-mist types, though you do have to clean the heating plate occasionally. A third type of humidifier uses ultrasonic sound waves to send microscopic droplets into the air. The ultrasonics have fallen from favor because they also generate a white dust, Conklin said. All these are different from vaporizers, which saturate the air for people with respiratory ailments.

When it comes to humidifiers, it seems, cool mist rules. Only 17 percent of humidifiers are warm-mist versions, Conklin said. Most of the rest are cool-mist, either room versions or larger console models that will humidify a whole floor.

One of the effects of low humidity on houseplants is the worsening of such pest problems as infestations of mites. Scientists know that mite populations build when hardy plants are subjected to hot, dry conditions in summer. The reasons for mite problems indoors in dry conditions are not as well understood, but may have to do with the fact that dry plants are not pushing new growth and so the mite damage doesn’t get repaired, said Michael Raupp, an entomologist with the University of Maryland.

And the pest problems worsen when you overfeed plants during this period of rest and stress. Mites, mealybugs and scale “all enjoy a nitrogen fix,” Raupp said. “It’s what sucking insects adore. They convert it right into protein to make eggs.”

So, keep your plants humid but half-starved, and turn down the thermometer a few degrees. In just a few weeks, your bus will come in.

Keeping a room’s relative humidity at an optimum 50 percent to 60 percent can be difficult.

Keeping a room’s relative humidity at an optimum 50 percent to 60 percent can be difficult.

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