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Light is probably the most essential factor for house plant growth. The growth of plants and the length of time they remain active depend on the amount of light they receive. Light is necessary for all plants because they use this energy source to photosynthesize. When examining light levels for tropicals, consider 3 aspects of light: (1) intensity, (2) duration and (3) quality.

Light intensity
influences the manufacture of plant food, stem length, leaf color, and flowering. A geranium grown in low light tends to be spindly and the leaves light green in color. A similar plant grown in very bright light would tend to be shorter, better branched, and have larger, dark green leaves. House plants can be classified according to their light needs, such as high, medium and low light requirements. The intensity of light a plant receives indoors depends upon the nearness of the light source to the plant (light intensity decreases rapidly as you move away from the source of light).

The direction
the windows in your home face will affect the intensity of natural sunlight that plants receive. Southern exposures have the most intense light, eastern and western exposures receive about 60% of the intensity of southern exposures, and northern exposures receive 20% of a southern exposure. A southern exposure is the warmest, eastern and western are less warm and a northern exposure is the coolest. Other factors which can influence the intensity of light penetrating a window are the presence of curtains, trees outside the window, weather, seasons of the year, geographic latitude, shade from other buildings and the cleanliness of the window. Reflective (light colored) surfaces inside the home/office will increase the intensity of light available to plants. Dark surfaces will decrease light intensity.

Day-length
or duration of light received by plants is also of some importance, but generally only to those house plants which are photosensitive. Poinsettia, kalanchoe, and Christmas cactus bud and flower only when day-length is short (11 hours of daylight or less). Most flowering house plants are indifferent to day-length. Low light intensity can be compensated for by increasing the time (duration) the plant is exposed to light, as long as the plant is not sensitive to day-length in its flowering response. Increased hours of lighting allow the plant to make sufficient food to survive and/or grow. However, plants require some period of darkness to develop properly and thus should be illuminated for no more than 16 hours. Excessive light is as harmful as too little light. When a plant gets too much direct light, the leaves become pale, sometimes sunburn, turn brown, and die. Therefore, during the summer months, protect plants from too much direct sunlight.

Additional lighting may be supplied by either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and are not very efficient users of electricity. If artificial lights are to be used as the only source of light for growing plants, the quality of light (wavelength) must be considered. For photosynthesis, plants require mostly blues and reds but for flowering, infrared light is also needed. Incandescent lights produce mostly red, and some infrared light, but are very low in blues. Fluorescent lights vary according to the phosphorus used by the manufacturer. Cool white lights produce mostly blue light and are low in red light. Foliage plants grow well under cool white fluorescent lights and these lights are cool enough to position quite close to plants. Blooming plants require extra infrared which can be supplied by incandescent lights, or special horticultural type fluorescent lights.

Questions & Answers
Q. Does the color of light make any difference in how plants grow?

A. Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors, producing a rainbow. Red and blue light have the greatest effect on plant growth. Green light is least effective to plants as they reflect green light and absorb none. It is this reflected light that makes them appear green to us. Blue light is primarily responsible for vegetative growth or leaf growth. Red light, when combined with blue light, encourages flowering in plants. Fluorescent light or cool white is high in the blue range of light quality and is used to encourage leafy growth. Such light would be excellent for starting seedlings. Incandescent light is high in the red or orange range, but generally produces too much heat to be a valuable light source. Fluorescent grow lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible, but they are costly and generally not of any greater value than regular fluorescent lights.

Q. What will 24 hours of light do to my indoor plants?

A. Many plants are grown in office buildings that are lighted for 24 hours. Different plants react differently to this condition. Most foliage plants originate along the equator, where plants get about 12 hours of high intensity light year-round. Many of these plants grow better under 24 hour light than under 12 hour light as the longer light periods enable plants to compensate for the low intensity of artificial light, by absorbing light over a long period. Certain annual bedding plants may receive enough light in order to bloom indoors under continuous lighting. Some plants that do particularly well under 24 hour lighting are Swiss cheese plant (Monstera deliciosa), coleus, cissus, schefflera and cacti. Continuous light also speeds up growth of many slow growing, flowering plants like orchids and cacti. In this way, they can be brought to maturity much quicker. Some plants do well either both 12 and 24 hour days, including philodendrons, Engligh ivy, pothos and dieffenbachia. Under continuous lighting, they tend to keep their lower leaves longer. Spider plant (Chlorophytum spp) grows best with 12 hours of light.

Q. How can I tell if my houseplants are getting the right amount of light?

A. One of the most common signs of inadequate light is yellow and dropping leaves. That is also one of the most common signs of too much light. To figure out if either of these situations is your problem, you will have to look more carefully at your plants. Flowering plants that don't flower, any plants with leaves angled toward the light and long, spindly stems, plants such as coleus whose red pigments have faded, or plants whose new leaves are smaller are all showing signs of inadequate light. Either move these plants to a brighter spot or supplement natural light with artificial light. Foliage plants require nothing more than an inexpensive, cool, white flourescent lamp; combine a cool white lamp with a warm red lamp to ensure the best bloom in flowering plants.

When your plant's leaves show signs of overall yellowing or have large spots of dry, dead tissue known as leaf scorch or leaf burn, the problem may be too much light. Use a diffusing curtain to reduce the amount of direct sunlight hitting the plant, or move the plant further away from the window. Even if the light level is correct for a given plant the same symptoms may develop if it was moved directly from a low light intensity to a much brighter spot. Make such moves gradually to allow the plant to adjust to the new light levels.