House Plants: Artificial Light

 
 
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 Light is needed by a plant for using nutrients and manufacturing food. It has a great influence on chlorophyll production, growth rate, leaf size, and flower and seed production. One of the common problems encountered by the indoor gardener is poor light availability, particularly in apartments with limited window space or during winter when light intensity is low. Artificial lights are useful when natural window light is insufficient. They make it possible to grow healthy plants in any spot in the home: bookshelves, a heated garage, an empty closet or even an attic. The use of lamps encourages compact, luxuriant growth and regular blooming.

As a supplementary light source, even a small portable reading lamp will improve plant health. It can be turned on for one or two hours at dawn and dusk to increase day length, especially in winter. Plants growing in a dim corner will visibly improve if they receive four to six hours of artificial illumination from an overhead light.

Only when a plant's entire light requirements are being met by artificial sources must special lighting provisions be made. Plants absorb red and blue light, both are used to control photosynthesis and various aspects of plant growth. Red light (6000 to 7000 angstroms) mainly controls maturation, and flower and seed production; it is particularly important to flowering plants. Used alone, red light will make plants grow tall and spindly. Blue light (4000 to 5000 angstroms) chiefly controls leaf development; plants grown under blue light alone tend to be short and stocky, with thick stems, dark green leaves and few flowers. Ultraviolet rays are used only in small quantities and can easily damage plants.

Incandescent lights (the round bulbs normally used in the home) can supplement natural daylight for foliage plants with low light requirements, but they do not provide enough light to meet and needs of flowering plants. They give off a large amount of red light and infrared radiation, most of which becomes heat. This causes cooling problems and burning of leaves. Because the light source is concentrated in a small area, light distribution is likewise restricted to a small area.

Fluorescent lights have a higher light efficiency than incandescent lights. emitting two and a half times as much light per watt. Fluorescent tubes vary in red-blue output. A combination of cool white and warm white is recommended for use on house plants.

Most house plants can be grown under fluorescent lamps alone, although it is a common practice to mix incandescent and fluorescent light in the ratio of two or three florescent lamp watts to one incandescent lamp watt. This provides the proper light quality without excessive heat. Daylight, warm white, and natural white tubes can also be combined in various ratios to provide a suitable quality for plant growth. Gro-Lux, Plant-Gro and other brands of agricultural fluorescent lamps are specially designed to provide a balance of red and blue light suited for plant growth. However, it is the quantity and brightness of light that is most important, not the quality of the light. A reflector, such as a mirror or white sheet of paper placed under the plants, will improve efficiency by reflecting light back onto plants.

As a rough guide, tubes should be arranged to provide approximately 175 to 225 watts of light per square metre of growing space. Lights are usually placed 30 to 45 cm above the tops of the plants. However since distance from a light source directly affects light intensity, this will vary according to individual plant requirements. If a large area is to be illuminated, it is better to use one long tube rather than two short tubes placed end to end; this avoids a dark spot in the centre, because intensity decreases near the ends of the tubes. Lights should be kept clean for maximum brightness, and discarded as they wear out (after about one year).

Commercial light systems are now available at varying cost. They usually include a reflector, a trolley with adjustable shelves, two 40-watt fluorescent lamps, and sometimes one 25-watt incandescent bulb. These can be connected to regular electrical outlets in most cases. These special units are not necessary. Lamps can be purchased alone and fitted into the design of your home, as long as they provide sufficient light intensity. A photometric light meter will help to determine correct placement of plants in relation to light sources. Automatic timers are also available; they can be connected them to the lights to switch them on and off at regular intervals.

The intensity and duration of illumination required vary from plant to plant. Older, mature plants need less light than young, active ones; flowering plants have high light requirements 6000 to 10,000 lux most foliage plants need from 1000 to 6000 lux; variegated and colored foliage plants need more; for root cuttings, 1000 to 2000 lux is sufficient; flowering bulbs need 500 to 1000 lux.

Experimentation is the only sure method to determine the best solution for each plant. If plants are not receiving enough light, they begin to grow tall and spindly as if stretching towards the light, and foliage becomes pale green. Such plants should be placed nearer the light source, or given a longer exposure period. If too much light is problem, bleaching of leaf and flower color occurs, and may lead to browning and shrivelling. Leaves are small and overly compact, and may curl under at the edges. Too much light inhibits flowering in species such as poinsettias, orchids and chrysanthemums. Most plants thrive with 14 to 18 hours of light per day when placed 30 to 40 cm below the lamps. Adjustments should be made according to individual requirements.

Growing habits of many plants are directly affected by day length; this phenomenon is know as photo-periodism. Some plants will not flower unless exposed to 12 to 14 hours of light each day for a certain number of days. These are known as long day plants and include calceolaris, many garden annual and begonias. Short day plants must have more than 12 hours of complete darkness each night to allow bud development; kalanchoe, poinsettia and chrysanthemum are examples. A third group is termed indifferent; they will flower as long as they receive sufficient light. This group includes African violets, tomatoes and impatiens. The light intensity required to trigger photoperiodic responses in plants is very low and in some cases less than 10 footcandles.

Care for plants grown under artificial light is the same as for regular house plants. Your success depends on the attention you give them. Proper growing media, temperature, humidity and ventilation must be provided. Watering and fertilizing schedules may require stepping up to keep pace with the steady, active growth encouraged by increased light. It is important to remember that plants don't receive natural rest kept under constant light. After flowering, most plants should be placed in a moderately-lit area, and given less water and no fertilizer to encourage dormancy. Apart from this, experience is your best guide.

Adapted from Agdex 285/20-8.

 
 
 
 

Other Documents in the Series

 
  General Care of House Plants
House Plants: Lighting
House Plants: Artificial Light - Current Document
House Plants: Watering
House Plants: Fertilizer
House Plants: Humidity
House Plants: Propagation
House Plants: Repotting
House Plants: Soil
 
 
 
 
For more information about the content of this document, contact Shelley Barkley.
This information published to the web on July 5, 2001.
Last Reviewed/Revised on October 28, 2013.