University of Vermont Extension
Department of Plant and Soil Science
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
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
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
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
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
Return to Perry's Perennial