University of Maryland Extension

Light-Houseplants

Intensity

• Is dependent upon the distance of the light source from the plant;
• Decreases rapidly with increasing distance;
• Influences the manufacture of plant food, stem length, leaf color, and flowering;
• Is affected by the direction the windows face in your home.
    “Lowest light” places are north-facing windows or rooms that are artificially lit;
    “Average light” is available in east or west-facing windows; and
    “Bright light” is found when a south-facing window receives at least six hours of light. The brightest light is available in a sun room or bay window;    
• Is influenced by other factors like the presence of curtains, weather, seasons of the year, shade from trees and other buildings, and the cleanliness and type of window; and
• Is increased by reflective, light-colored surfaces inside the room.

                          The Effect of Deficient Light

A geranium appears spindly or “leggy” with light green leaves and diminished flowering when grown in low light. The same plant grown in bright light would be more compact, better branched, and have larger, dark green leaves. Many houseplants fail after a healthy start because of inadequate light. Other houseplants, however, are tolerant of a wide range of light intensity.

Duration

• Is the length of time the plant receives light. Most flowering houseplants are indifferent to duration;
• Is of some importance for houseplants that are photo-period sensitive: (PDF) Poinsettia, kalanchoe, and Christmas cactus will bud and flower only when day length is short (11 hours of daylight or less);
• Plants receiving low-light intensity respond to increased duration of light, as long as the plant is not sensitive to day length for flowering; and
• Most plants require a period of darkness to develop properly, so illuminate them for no more than 16 hours each day if using artificial light.

Quality

• Is improved by adding an artificial light source;
• Red, far-red, and blue wavelengths are most important for plant development. For healthy plants, supply all three wavelengths;
• White fluorescent tubes produce mostly blue light;
• Incandescent bulbs produce predominantly red or far-red light;
• Blooming plants require extra far-red light; and
• Incandescent plus fluorescent light sources, or horticultural “grow lights,” best mimic natural sunlight.

Remember that excessive light is just as harmful as too little. When a plant gets too much direct light, the leaves become pale, turn brown, and die. Protect houseplants from excessive direct sunlight during the summer months.

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