Caring for Houseplants: Light and Humidity

Master gardener Paul James answers viewers' questions.

Q: How do you determine whether a houseplant is receiving low, medium or bright light?

A: Generally speaking, the light coming through the windows on the south side of the house is brightest, the east side is next brightest, then the west and finally the north, where light levels are lowest. However, roof overhangs and structures such as covered patios can reduce the amount of light.

As the seasons change, so does the light level. In winter, light levels are lower than they are in summer due to the lower angle and reduced intensity of the sun. The only way to determine with any accuracy how much light your plants are getting is with a light meter.

These inexpensive gizmos will give you a pretty good idea of the light intensity in a given area of a room. Cheap meters under $20 will give you general readings such as low, medium or high. More expensive ones may actually measure foot candles, but you really don't need that degree of accuracy.

Q: Half the rooms in my house get too much light, while the other half get too little. What can I do?

A: If your plants are getting too much light, move them. As plants are moved away from windows, the amount of light they receive falls off dramatically. In the center of the room, the light intensity typically drops by 50 percent, while in the north corner of a south-facing room, the light will be only five percent of what it is next to the window.

If you can't move the plants, then consider installing adjustable shutters or curtains to cut the light levels. Even sheer curtains will reduce light more than you realize.

As for the rooms that get too little light, consider these options. First, try painting the walls white, which can provide as much as 30 percent more light as dark-hued walls. Consider hanging a large mirror, which can flood a room with several times more light than it would otherwise receive. If none of these ideas works in your situation, try growing your plants under lights.

Q: What are the symptoms of plants that receive too much or too little light?

A: Plants that receive too much light may react by curling their leaves downward and by losing their green color. Unfortunately, many gardeners mistake these signs as a lack of food or moisture, so they respond by fertilizing and watering, which may be the last things the plant actually needs. Too little light results in leggy or spindly plants with fewer leaves than usual.

In both cases, the solution is to move the plant to a location that receives more or less light. In most cases, the plant will respond within a week or two. Luckily, most houseplants, including nearly all tropicals, grow reasonably well under medium light.

Q: What tips do you have for gardening under fluorescent lights?

A: The biggest mistake people make has to do with how close the plants need to be to the lights. Too often people set the lights too far away from the plants, which results in leggy plants. Ideally most plants should be no more than four to six inches away from the lights. That's especially true of young seedlings.

Also, remember that fluorescent lights tend to give off less light toward their ends, so plants that need more light should be placed beneath the center section of the lights.

Fluorescent lights also dim over time and therefore give off less light. It's best to replace them when dark rings begin to appear at the tube ends. The discoloration indicates that the electrodes are beginning to deteriorate. Dust that accumulates on the tubes will also cut the intensity of the light, so every now and then you should clean the tubes with a damp cloth.

Q: Does it matter whether the source of the light comes from the sun or artificial lights?

A: No, plants don't seem to care whether the light they receive is natural or artificial, so long as the intensity is adequate. In fact, African violets grow better under fluorescent lights, perhaps because the filtered light they received on the jungle floor was closer in character to that provided by fluorescent lights.

Q: Do you have to use those expensive fluorescent light bulbs or are the regular ones ok?

A: The expensive ones definitely work better, because they emit more light in the spectrums plants prefer, namely the blue and red wavelengths. However after having personally tried them, I'm not convinced that the extra expense is really worth it, which is why I use regular 40-watt fluorescent tubes that you can buy just about anywhere.

Q: Does watering a houseplant more increase the humidity surrounding the plant?

A: No, in fact, watering a plant to increase the humidity is a sure-fire way to kill it, especially during the winter months, when the humidity in most homes is extremely low. During that time, many houseplants do indeed suffer from a lack of humidity. Symptoms include brown edges on the leaves, leaves curling downward and an overall lack of vigor.

However, if you add water to the soil, you won't actually increase the humidity. What you will do is cause the roots to rot from excess moisture. The solution, then, is to mist the plant regularly — once a day if possible.

Alternatively, you can place plants on pebble-filled trays to which you add water, you can group plants together because as plants lose moisture they raise the humidity in the area, and you can place plants near a container filled with water. You can also use a small humidifier or have a more elaborate version installed.

Q: Why do plants in office buildings always seem to look so healthy?

A: Building owners employ a plant contractor who routinely revolves plants of similar size and shape from the building to the greenhouse, where the plants recuperate.