University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article


INDOOR LIGHTING FOR PLANTS

Dr. Leonard Perry, Extension Professor
University of Vermont


Artificial lighting from light bulbs indoors can be used to start seedlings in spring, provide supplemental light for sunlight to many plants, and to provide the sole source of light for low to medium light plants. The main aspects to consider when providing light for plants indoors is the quality, quantity, and duration of the light.

Quality refers to the actual wavelengths the lights provide to the plants. Light may look white to us, but is in reality made of many different wavelengths as seen in rainbows or when light strikes a prism. It is the red and blue parts that the plant uses for energy and growth, so these need to be provided by indoor lights. Terms you may see for light bulbs that provide these are “natural”, “full spectrum”, or “balanced.” They cost more than the usual incandescent bulbs that mainly provide red light to plants, or the cool white tubes that mainly provide blue light.

A less expensive solution for a balanced light quality is to use both incandescent and cool white lights, or cool and warm (appearance, not temperature) white tubes in fluorescent fixtures. Keep in mind incandescent bulbs only may last 1,000 hours compared to 10,000 hours for fluorescent. If using both incandescent and fluorescent, a balance ratio of 3 to 10 is best. So for every 30 watts from an incandescent bulb, provide 100 watts from fluorescent.

Additional terms you will see when looking at fluorescent tubes are “T” numbers. These refer to the diameter, in eighths of an inch, of tubes. So a T8 tube is eight eighths, or one inch, in
diameter. Older tubes are T12, with most newer ones T8 or even T5. These newer ones tend to be much more energy efficient, and even with lower wattage can produce more light due to new
technology and materials in their production. There even are high-output tubes with longer life, so even though more expensive at first they are cheaper in the end.

Quantity of light is crucial and, in addition to type and wattage of bulb, is adjusted by distance of lights to plants. Fluorescent tubes give off little heat, so can be placed as close as four to six inches above seedlings and plants. Incandescent and similar bulbs give off more heat, so need to be kept a foot or more above plants. If you see browning or “burning” of leaves and leaf edges, this may be a sign your light is too close to plants.

Most plant growing fixtures you find in stores and catalogs utilize fluorescent tubes. These growing stands can be quite attractive, and expensive. If aesthetics aren’t crucial, such as in a heated basement area, you might consider making your own fixtures from lumber. If making your own, consider bolting them together in case they need to be taken apart to move or store. Use adjustable chains for hanging fixtures so they can be placed at the proper height, and moved up as plants grow. For seedlings, and the most light on other plants, use either a three or four-tube fluorescent fixture, or two two-tube ones.

When using any bulbs, the reflector can make a difference how much light the plant receives. Reflectors are advised for most indoor lights, especially tube fixtures. Light quantity can drop off dramatically, by two fold or more, near edges of reflectors. Seedlings and plants growing at an angle toward brighter light is a sign the quantity is not sufficient over them. I make sure, when starting seedlings, to have the fluorescent light reflectors out past the edges of the seedling flats. You can line older reflectors with shiny aluminum foil to provide more light, or even place such foil or white surfaces behind plant fixtures for reflection.

Light quantity also drops dramatically near the ends of fluorescent tubes, even new ones. If you have older tubes, and they are getting a bit dark near the ends of the bulbs, this is a sign they should be replaced. If you plan to grow many plants indoors under lights, you may want to look online, at a complete garden store, or at a camera shop for an inexpensive light meter. This can be revealing to see how much light is provided, and where, for your plants and even for reading lamps.

Light for growing is commonly measured in foot-candles—the amount of light given off by a candle one foot away. Two common T8 tubes placed six inches above plants will provide about 700 foot-candles. Reading lights, for comparison, often provide about 50 foot-candles.

Low light plants such as Chinese evergreen and Peace lily need between 50 and 250 foot-candles. Medium light plants such as African violets, begonias, dracaena, dumb cane, flame violet and seedlings need 250 to 1,000. High light plants such as most herbs and orchids need over 1,000 foot-candles.

Many plants may tolerate one level but prefer another. English ivy, peperomia, philodendron, and most ferns tolerate low light but prefer medium (in addition to high humidity for ferns). Many high light plants will tolerate medium light, but may be smaller, with smaller leaves, or may not flower and fruit if this is their attraction.

Light quantity and light duration are related, as a lower level of light often can be made up with a longer duration. If growing seedlings indoors, 16 to 18 hours of light at day is recommended, at 65 to 75 degrees F. This duration, or even 12 to 14 hours with brighter light, should be used if the sole light source for low to medium light plants. If using indoor lighting merely to supplement natural light from windows, you only may need a few hours additional at the end of the day. Don’t keep lights on continuously as plants need periods of darkness, with 55 to 65 degrees F ideal.

Natural and supplemental light is best for high light plants, as it is hard to get the quantity they need indoors in most homes solely from light bulbs. There are high-intensity fixtures for high light plants, as seen in sports arenas, that are quite expensive, require much power, and are best for serious hobbyists or commercial growers.

Various inexpensive timers to control light fixtures are available at most hardware stores. Some multi-plug power strips even come with timers. Other items to consider with artificial light include heating mats. These especially are important if starting seedlings. Trays to prevent water runoff are needed for potted plants. These can be lined with gravel and kept moist in order to increase humidity around plants to a minimum 50 percent or so.

Artificial lighting is useful for starting seedlings indoors, either to get a jump on the season or for those such as pansies that take many weeks to germinate and grow. Make sure before placing outdoors in late spring such seedlings, or even potted plants that have been solely or primarily under artificial lights, that you adjust them gradually over a couple of weeks or more to the “real” and higher light of sun. You may just want to leave potted plants in part to full shade. Put seedling flats here too, or cover with lightweight fabrics as used for frost protection.

If you haven’t tried growing plants under lights, join the roughly 2.7 million households that buy indoor plant lights each year, and the estimated 10 to 15 million households that grow plants under lights indoors.

Return to Perry's Perennial Pages, Articles