Lighting Indoor House Plants
by Ray R. Rothenberger
Department of Horticulture, University of Missouri-Columbia
Lighting Indoor House Plants
House plants are extremely popular indoor decorations. Attractive and constantly changing, they add a softness of line and provide a bit of nature indoors. However, the ideal location of a plant for decoration may not be the ideal spot for plant growth. Lack of adequate light is the most common factor limiting the growth of plants in many areas of the home. Supplementary electric lighting is usually the easiest and least expensive way to provide enough light for plants that do not receive adequate natural light.
Why do plants need light?
Light produces the energy plants need to make the food required for them to grow and flower. Plants are the only organisms able to use light to produce sugars, starches and other substances needed by them as well as by other living organisms.
Is light color important to plants?
Yes, certain colors in light rays are important for proper plant growth. Leaves reflect and derive little energy from many of the yellow and green rays of the visible spectrum. Yet the red and blue parts of the light spectrum are the most important energy sources for plants, and plants require more rays from the red range than from the blue.
What are the best sources of these colors?
Plants growing outdoors, in greenhouses or close to windows are exposed to a natural balance of the blue and red light rays that plants need. Where plants receive little or no natural light, you must provide additional light from artificial sources.
Which types of lights are best?
Most people are familiar with the incandescent light produced by ordinary light bulbs in our homes. As a single light source for plants, these bulbs are not particularly good. They are a good source of red rays but a poor source of blue. They produce too much heat for most plants and, if used, must be kept away from the plants, thus reducing the intensity of the light the plants receive. They are also about three times less efficient than fluorescent tubes in converting electrical energy to light. Further more, a standard incandescent bulb's life is often only about 1,000 hours, whereas a fluorescent tube's life is normally 10,000 hours or more.
Fluorescent tubes provide the best artificial light sources available for plants in the home. Other light sources such as sodium lamps may be used but are not normally available or adaptable for home use.
Fluorescent tubes are made in many sizes and shapes: circular, U-shaped, square or straight. Straight tubes in 2-, 4- or 8-foot lengths are used most frequently.
What is the best balance of artificial light?
Many indoor gardeners use cool white fluorescent tubes. Warm white fluorescent tubes also seem fairly effective, but fluorescent tubes listed as white or daylight are less desirable for indoor plant growth. Cool white tubes produce a small amount of red rays in addition to orange, yellow-green and blue rays. However, the red light produced usually is not enough for many plants unless windows or other artificial lights produce additional red rays.
A few incandescent bulbs in the growing area can furnish needed red rays. A general ratio of incandescent to fluorescent light is about 3 to 10, so for every 100 watts of fluorescent light, you should provide about 30 watts of incandescent light for a bet ter red to blue light balance.
Special fluorescent tubes also have been developed for growing plants. These have a higher output in the red range to balance the blue output. Many home gardeners have found that these tubes can be used in combination with cool white tubes. Use one special plant-growing tube to each one or two cool white tubes. This method is more economical than using all special tubes, since cool white tubes cost less than the special plant-growing tubes. Also, fluorescent plant-growing tubes use less electricity and produce less heat than incandescent bulbs, and you will not have to provide fixtures for both incandescent bulbs and fluorescent tubes.
May I use spotlights and other special bulbs?
Yes, although they are less effective than fluorescent lights and the combinations described previously. However, fluorescent fixtures may not be suitable for some locations. Reserve these special light sources for situations where supplementary light is essential.
How much light should plants receive?
The amount of light necessary varies with each plant. In general, the light fixtures available for home plant lighting make it practically impossible to produce too much light for most plants.
Plants usually are divided into three general categories: those suitable for low, medium and high light intensities. (These three groups are referred to in discussions of different plants in the last section of this publication.) The categories generally indicate the minimum light required but should be used only as a guide. Growth is often best in the higher rate of these suggested light ranges.
Low light plants. Plants referred to as low light intensity plants generally should receive between 50 and 250 footcandles. (A footcandle is the measurement of the light received by a flat surface one foot from the point of emission of one international candle.) Under artificial light, a few plants in this group can be maintained at as little as 10 footcandles.
One way you can estimate the amount of light available is to calculate the number of watts available per square foot of plant area. Low light plants should receive between 10 and 15 watts of artificial light per square foot of growing space. A single fluorescent tube, such as a 2-foot 20-watt tube or a 4-foot 40-watt tube without any other light provides only enough light for plants in this category.
Medium light plants. These plants prefer 250 to 3,000 footcandles. Best growth occurs above 1,000footcandles unless plants also receive extended periods of direct sunlight. Give them artificial light in the 500 to 1,000 footcandle range or 15 or more watts per square foot of growing area.
While plants in this group can be held in the 250 to 500 footcandle range, growth is best with more light. A fixture containing two fluorescent tubes is sufficient for plants in the low to medium light range. Adjustments in the number of tubes used may be made if you regulate the distance between the tubes and plants.
High light plants. These plants generally are not satisfactory for growing under artificial lights in the home. However, if you want to try, use special high-intensity lamps. These plants need at least 2,000 footcandles or 20 watts per square foot of growing area, but should have higher intensities for best growth and flowering. Fixtures containing three to four fluorescent tubes are necessary for plants requiring high light.
How far from the light should I place plants? Most plants should be located with the tips of the plants 6 to 12 inches from the light source. The intensity of light drops rapidly as the distance from the light bulbs or tubes increases. Figure 1 sho ws this reduction of light intensity with distance below and to the side of tubes. Fluorescent tubes also do not produce as much light at the ends as they do in the center. Therefore, the brightest spot under a fluorescent fixture is directly beneath the center of the tubes.
Figure 1. This graph shows the light output in footcandles from two standard 40-watt fluorescent lamps measured at distances from 6 to 36 inches. (From USDA bulletin DNG 187, Indoor Gardens with Controlled Lighting.)
The light fixture's position should be adjustable so you can keep the distance between the light and the plant fairly constant. Fluorescent shop or workroom fixtures often are hung on chains with S-hooks for easy adjustment. These fixtures are easily raised or lowered from link to link. If the fixture is not movable, you may make some adjustment by raising plants on stands, shelves or boxes.
How long should I use lights? In most cases, plants receiving no outdoor light should be lit from 16 to 18 hours each day. If some additional light is received, 12 to 14 hours each day may be adequate. Lights should be used at the same time that pl ants receive window light. Using lights at the beginning or end of the day will not usually be as effective as using lights during daylight unless natural daylight is quite bright.
How can I get the most from artificial light?
Reflectors and reflective surfaces can maximize the available light. Bulbs with self-contained reflectors are helpful.
Porcelain-coated reflectors are excellent and require little maintenance. Keep reflectors clean and free of rust or any coating that reduces their effectiveness. White paint or aluminum foil beneath or around the growing area helps reflect light and makes it more efficient.
Space plants far enough apart to allow light between them. Arrange plants so they do not shade each other. Keep tubes clean and replace old tubes promptly.
How should I light the bottom of a tall plant?
You may want to supplement light placed above the plant with spotlights around the base of the plant and directed on the lower leaves. Also, you can use fluorescent tubes in a vertical position to provide side lighting from the top to bottom of the plant. See Figure 2. This vertical position also can be used for smaller plants arranged on shelves.
Figure 2. Tall garden starts and grows small pot plants.
Do I really need to measure light?
The eye is a poor judge of light intensity since it automatically adjusts to different light intensities. Light measurements are helpful in setting up a plant growing area but should be used only as a guide rather than as a rigid rate.
Light meters that measure footcandles may be used if available. Photographic light meters normally do not read in footcandles, but some manufacturers supply a conversion table. When available, these may also be used.
The use of wattage per square foot of growing area is a useful and easy way to estimate light required. With this approach, light measurements are unnecessary unless problems develop.
How can I tell whether there is enough light?
The growth pattern of the plant can be a good indication. No growth may indicate poor light but could be a sign of other problems as well. A healthy plant under poor light may develop long internodes (length of stem between leaves). Leaves may develop smaller than normal on some plants. Color of many plants in poor light may be pale green, and lower leaves may yellow and drop.
What window produces the best natural light?
Consider size, direction, overhang and shade from trees or buildings. Large windows provide the best growing conditions and allow plants to be placed fairly far back into a room. But even low light plants usually do not receive enough light at distances g reater than 10 feet from an average window. The best windows for plants are those not shaded by a large overhang, trees or structures outside them.
Windows facing south provide the brightest light conditions for the longest duration. In winter, any house plant benefits from the light of a south window. However, plants that do not need bright light may be sunburned by the bright light at south windows in late spring, summer or early fall. Place plants requiring less light, such as African violets, at a north window or to the side or interior of a large south window during these times. South windows are most appropriate for plants requiring bright light and some direct sunlight.
East and west windows are well suited to many plants in the medium light range, while north windows are satisfactory only for plants requiring the lower light levels. These plants should not receive direct sunlight.
Can lights keep plants from blooming?
Some plants, generally known as short day plants, can be kept from flowering under the light durations normally used for artificial lighting. Best known in this category are the poinsettia and chrysanthemum. To induce flowering indoors, give these plants only about 10 hours of light each day until flowers become visible and color shows.
Will artificial lights start seeds?
Vegetable, annual flower and some perennial flower seeds may be started successfully indoors under lights for later planting into the garden. For stocky growth, keep seedlings within a few inches of the tubes as soon as germination begins.
Proper growing techniques are essential. Details for starting seeds indoors are available in MU publication G06570, Starting Plants From Seeds.
Is a light timer necessary?
A timer is a valuable asset since lights should be turned on and off regularly and consistently. Twenty-four hour timers available from electrical supply houses are adequate. The electrical cord from the timer should be the three-prong type, or you should use a grounded adapter. The use of water around plants makes grounding electrical fixtures important.
Plants for indoor lighting
African violet, Saintpaulia species. This is one of the most satisfactory flowering plants for growing under lights; it grows and flowers well between 500 and 1,000 footcandles. Light 16 to 18 hours each day.
Aluminum plant, Pilea cadierei. This small plant with colorful leaves tolerates a low light level but needs high humidity. Other suitable related plants include artillery fern and moon-valley pilea.
Arrowhead vine, Syngonium podophyllum. These small plants are bushy and later, creeping or climbing. They may be kept at a low light level, but they are denser at a medium level.
Asparagus ferns, Asparagus densiflorus 'Sprengeri' and Aparagus setaceus (formerly plumosus). These durable potted plants with fleshy roots produce fern-like leaves. Although tolerant of low light, the plants grow more abundantly and densely in a medium light range.
Aucuba-Leaf croton, Codiaeum 'Aucubaefolium.' Crotons are not appropriate for the light garden as they require high light for good color development and growth. However, this smaller type with yellow spots can be grown at medium light intensities.
Begonias: Angelwing, B. coccinea; Beefsteak, B. xerythrophylla; Iron cross, B. masoniana; Flowering, B.semperflorens; and Painted B. rex varieties. Most plants of this family are suitable for growing in the medium light range. Begonias like high humidity and constantly moist soil. Give them a long light duration, from 14 to 18 hours each day.
Cacti and succulents. Many plants may be grouped in this general category. These plants need highlight intensities; most are not satisfactory for growing under artificial lights. However, you can use lights to maintain them for limited periods whe n they cannot be exposed to bright light or direct sunlight. When artificial lights are used, provide high intensities, and keep plants close to the source.
Cast iron plant, Aspidistra elatior. This plant tolerates cool locations and poor light; it is one of the best plants where only low light conditions exist. A variegated form is also available.
Chinese evergreen, Aglaonema modestum. Aglaonemas are relatively small tropical plants but can tolerate light as low as 10 footcandles. Other species with different leaf form and variegation are also available.
Cissus species: Grape ivy, Cissus rhombifolia, and Kangaroo vine, Cissus antarctica. Cissus are climbing plants that prefer medium light conditions for best growth, although kangaroo vine tolerates lower light. Although better for greenhouse culture, another beautiful relative is the rex begonia vine, Cissus discolor.
Coleus, Coleus blumei. These extremely colorful plants are popular for garden use. Under lights they tend to have less color than outdoors unless they get very bright light.
Dieffenbachia, dumbcane. Many satisfactory species and varieties are available. These plants have large, showy leaves with spotting and variegation. Medium light range is best and will prevent loss of lower leaves. Plants may become too tall but can be cut back.
Dracaena: Corn plant, Dracaena fragrans massangeana; Dragon tree, Dracaena margina; and Ti plant, Cordyline terminalis 'Ti.' Most dracaenas become large plants. They are well adapted to house conditions and tolerate low light although they also do well in medium light. They need to be kept warm and constantly moist for best growth. Ti plant develops best leaf color in bright light and therefore is less suitable for locations where it receives only artificial light.
Flame violet, Episcia cupreata varieties. These close relatives of the African violet have beautiful foliage in many color patterns. Flowers are red, pink, purple and yellow. Light needs are the same as for African violets. The plants need h igh humidity and temperatures that do not drop below 65 degrees F.
Ferns: Birdsnest fern, Asplenium nidus; Boston fern, Nephrolepsis exalta bostoniensis; Holly fern, Cyrtomium falcatum; and Maidenhair fern, Adiantum species. The word fern brings to mind the Boston, or sword, fern with its many varieties. These ferns are durable houseplants that tolerate low light but prefer a medium light level. Keep them cool and moist. Holly fern likes cooler home temperatures; however, birdsnest fern prefers warmer temperatures. Ferns may develop brown leaves or leaflets at low humidity. Maidenhair ferns should never be allowed to become dry.
Ficus species: Rubber plant, Ficuss elastica varieties and Weeping fig, Ficus benjamina. These large plants are well suited to most home conditions and are best grown in a medium light range. Low light may result in a drop of foliage. Keep soil evenly moist. They tolerate low humidity.
Gloxinia, Sinningia speciosa fifyana varieties. This colorful African violet relative needs a medium light range to prevent elongation of the stems as well as to promote abundant flowering. Buds sometimes appear but fail to develop. This may indicate insufficient light, too low humidity or mite damage.
Hoya: Hindu rope plant, Hoya carnosa 'Hummels compacta' and Wax plant, Hoya carnosa. Hoyas are vining plants with thick waxy leaves. They tolerate low light but need medium light to grow and high light to flower. Plants should not be moved; allow them to remain undisturbed. Keep slightly cooler and drier in winter.
Impatiens, sultana; Impatiens walleriana holstii. This popular garden plant can also be grown indoors. It needs medium to high light indoors for compact growth and good flowering. It is very susceptible to spider mite damage.
Ivy, Hedera helix varieties. These vining plants with leathery leaves grow best in medium to bright light areas. They need constant moisture to prevent leaf drop, and they tend to grow better when kept slightly cool indoors.
Jade plant, Crassula argentea. This succulent plant is well adapted to indoor conditions. Although most crassulas prefer sun, jade plant will tolerate lighting as low as 25 footcandles; however, growth will be thin and leaves small, Avoid overwatering and tight soils.
Herbs. Some people are interested in growing culinary herbs under lights. For best growth, most of these require fairly bright light. If you attempt them under artificial light, use as much light as possible with a long duration.
Norfolk Island pine, Araucaria betempbylla (formerly excelsa). This is a durable plant for home conditions. Although tolerant of poor light, it develops better growth at medium light conditions. With poor care and low light, it may lose symmetry.
Orange: Dwarf or Calamondi; Citrus mitis. Potted citrus plants prefer bright light for best growth and fruiting. You may keep them at medium light intensities during midwinter. The calamondin orange is the most popular potted citrus, although Meyer lemon and Ponderosa lemon are also satisfactory for home use.
Orchids. Few orchid species are well suited to the average home's artificial light conditions. Seedling plants do well under artificial lights; however, mature flowering size plants need bright light for best growth and flowering. Some genera better adapted to the light garden include Epidendrum, Paphiopedilum, Brassavola and Phalaenopsis. All orchids need excellent drainage and air movement.
Peperomias: Baby rubber plant, Peperomia obtusifolia; Emerald Ripple, Peperomia caperata; and many other suitable species. Most peperonlias have fleshy leaves and stems. They are small plants when adapted to home conditions. Provide a medium light level, although peperomias can tolerate poor light for limited periods. Avoid overwatering, which may promote stem rots.
Philodendrons: Heartleaf philodendron, Philodendron oxycardium; Splitleaf philodendron, Philodendron pertusum; and other suitable species. The heartleaf philodendron tolerates low light; most prefer a medium light range for best growth indoors. Most are well adapted to home growing. They should be maintained at fairly uniform moisture.
Pleomeles: Lance dracaena, Pleomele reflexa and Malaysian dracaena, Pleomele thaliodes. These plants are gaining popularity because of their durability as potted plants and tolerance to low light conditions. They grow best in a medium light range with uniform soil moisture.
Palms: Areca palm, Crysalidocarpus lutenscens; Dwarf date palm, Phoenix roebelenii; Neanthe bella palm, Chamaedorea elegans 'bella'; and Kentia palm, Howeia forsteriana. Neanthe bella and kentia palms are most tolerant to low light conditions. Other palms are best suited to medium light areas. Bright light may cause fading in some species. Palms prefer fairly uniform moisture, but occasional slight drying can be beneficial. Palms grow relatively slowly indoors. Keep close watch for spider mite damage.
Pothos, devil's ivy; Scindapus aureus. This popular variegated climbing vine is similar to heartleaf philodendron and satisfactory for low and medium light conditions. It requires less uniform moisture than philodendron.
Sansevieria, snake plant, mother-in-law-tongue; Sanseveria trifasciata. This very popular and durable plant grows well indoors. It tolerates poor light but makes best growth in medium to bright light conditions. Dwarf and variegated forms are available. Avoid overwatering.
Schefflera, umbrella tree; Brassaia actinophylla. This plant prefers bright light but tolerates medium light or even low light for limited periods. It will drop foliage in extended periods of poor light or if kept too wet or too cool. Spider mites are a common problem.
Spathiphyllum, 'Clevelandii' or 'Mauna Loa.' These plants adapt to low light conditions but need medium light to flower. Their white, anthurium-like flowers are long lasting. Do not allow plants to dry out.
Spider plant, airplane plant; Chlorophytum comosum 'Vittatum.' Long drooping leaves and runners producing small plants make this a popular hanging plant. Solid green and variegated types are available. They tolerate low light but grow best in the medium range. Keep well watered.
Swedish ivy; Plectranthus australis. This creeping plant is used for hanging containers. Provide a rich sod for it. Although it tolerates low light, more dense growth and branching occurs at medium light intensities.
Wandering jew, Tradescantia fluminensis and Zebrina pendula. These popular hanging plants tolerate home conditions and may be grown in low light. However, growth is more compact, and color development is better in medium light. They prefer uniform moisture.
Copyright (c) 1995-1996 University of Missouri. Published by University Extension, University of Missouri-Columbia. Used by permission.