EUGENE, Ore. – Light is essential for houseplant growth, but the meager light wrested from gray winter days in the Pacific Northwest may be only 30 percent of bright summer sun. What's a gardener to do when indoor light wanes?
When thinking about light levels for houseplants, consider light intensity, duration and quality, recommends Ross Penhallegon, horticulturist with the Oregon State University Extension Service.
"Light intensity influences the manufacture of plant food, stem length, leaf color and flowering," Penhallegon said. "For instance, a geranium grown in low light tends to be spindly with light green leaves. A similar plant grown in bright light would tend to be shorter, better branched and have longer, darker green leaves."
Distance from the light source and the direction windows face determine light intensity. Southern exposures have the most intense light. Western and eastern exposures receive about 60 percent of the intensity of southern exposures. Northern exposures get only 20 percent of the light of southern exposures.
Other factors influencing light intensity include the presence and type of curtains, weather, shade from buildings or vegetation, the cleanliness of the window and the surroundings that reflect light.
To compensate for low-light intensity, increase the time a plant is exposed to the light it needs to survive and grow. Incandescent lights such as the common household light bulb, fluorescent lights or special horticultural fluorescent lights will work, as will as any light that burns a metal element.
"Incandescent lights produce a great deal of heat (red spectrum), but they do not use electricity efficiently,” Penhallegon said. Florescent lights are cool or have blue light. Combine incandescent and fluorescent for the best houseplant light. The effect of compact fluorescent light bulbs (CFL bulbs) on plant growth is still unknown but research is under way.
"Fluorescent tubes provide the best light for the money," Penhallegon said. "Special plant-growing fluorescent tubes make plant foliage and flowers appear more attractive, but they are more expensive than the cool white tubes, and in general, plant growth may be no better."
Suspend fluorescent fixtures about 24 inches above the pots, but no closer to avoid heat injury. Check the plants three times a day for two days to make sure the lights are not too close.
Day length or duration of light is important only to plants that are sensitive to day length, including poinsettia, kalanchoe and Christmas cactus. But all houseplants need some period of darkness and should be illuminated for no more than 16 hours.
"When a plant gets too much direct light, the leaves become pale and can sunburn, turn brown and die," Penhallegon said. "During the summer, houseplants need to be protected from too much direct sunlight."
Ivy plants, philodendrons, foliage begonias and peperomias, grown mainly for their foliage, do well in indirect bright light. Tender plants such as African violets and gloxinias also should receive indirect light during the summer when the sun's rays are intense.
On the other hand, most flowering plants require high light levels to develop good flowers. These plants grow best in direct sunshine for at least half a day. Windows with an eastern exposure usually suit them best. Cacti and succulents, grown for their unique forms, require a sunny location. Coleus and crotons must be grown in high light to maintain their decorative foliage colors.