Light Intensity

Human activities do not require as much light as plants require for growth. No matter how many windows and skylights are designed into a home or office building, the light intensity inside will never equal that outside. Even unshaded greenhouses filter the sun’s light and reduce it by at least 15 percent.

To understand light intensity requires knowing how light is mea­sured. Light intensity is expressed in the units of lux or footcandle. A lux is the illumination received on a surface that is 1 meter from a standard light source known as unity. A lux is an international measurement comparable in use to the metric system. In the United States, the foot – candle unit is more commonly used and understood. One footcandle (fc) is equal to the amount of light produced by a standard candle at a distance of 1 foot. Direct-reading meters are manufactured that mea­sure light intensity in footcandles up to 10,000 (the illumination on a typical clear, sunny, summer day). A light meter is the only way to measure light intensity accurately and should be the first piece of equip­ment purchased by a beginning plantscape designer.

The challenge of bringing plants accustomed to outdoor light inten­sities into a home or shopping mall is best appreciated through several examples. The average residential living room has a light intensity of 10 to 1,000 fc by day and as few as 5 fc by night. A good reading light provides 20 to 30 fc. A typist may have 40 to 50 fc of illumination on the keyboard surface. The average shopping mall provides 20 to 30 fc of light in pedestrian circulation areas and up to 100 fc in sales areas. How is a plant grown in 10,000 fc of light to survive?

The keys to a plant’s survival are acclimatization and maintenance of the minimum light intensity required for its survival in an attractive and healthy condition. Note that survival and attractive appearance are the maintenance objectives, not growth. Acclimatization is the adjustment of an outdoor plant to interior conditions. It involves both morphologi­cal and physiological changes in the plant and takes time to occur. The minimum light intensity is the level of illumination necessary to allow the acclimatized plant to produce new leaves at a rate equal to or slightly greater than the rate at which old leaves senesce (age) and abscise (fall off). Both the time required for acclimatization and the minimum light intensity will vary depending on the plant species.

There is a great lack of carefully researched and comprehen­sive data on which an interior plantscaper can rely. To the credit of forward-looking industry professionals, the Professional Landscape Network (PLANET) and the Florida Foliage Association (FFA) con­tinue to promote research and publication of research data needed by members of the interior plantscaping industry. Although much is still not known about the needs of plants indoors, more is being learned every day.

The procedures necessary for successfully transplanting a tropical foliage species to an interior locale must begin during production of the plant. The growers can start reducing the light of selected plants once contracts for their purchase are signed. Note that acclimatization also involves the plant’s adjustment to reduced water in the soil and in the surrounding air. Based on current knowledge and technology, plants are usually acclimatized in the following way.

Light-intensity acclimatization Light intensity is reduced gradu­ally over a period of several weeks or months. Each change reduces the light by 50 percent until the desired intensity (usually 100 to 200 fc) is reached. The acclimatization process cannot be rushed without a severe reaction by the plant; that is, defoliation or death. Once indoors, the reduced light provided to the plants must be of sufficient duration to permit the plants’ slowed photosynthetic processes to manufacture adequate food. Most plants require at least twelve hours of continuous light every day, including weekends. A reliable timed-lighting system is essential.

Nutrient acclimatization The high fertility level of soil necessary for maximum plant growth during production is unnecessary and even life-threatening to the indoor plant whose use of soil nutrients is greatly diminished. Thorough soil leaching at the beginning of the acclima­tization period and occasionally afterwards will prevent a buildup of soluble salts.

Moisture acclimatization The frequency of watering is reduced dur­ing acclimatization to prepare plants for their more stressful interior locale. The high humidity levels of the production area are also gradually reduced to ready plants for the drier air of home and building interiors.

Temperature acclimatization Production area temperatures are usually higher than human comfort levels in order to promote more rapid plant growth. During acclimatization, temperatures are gradually reduced to the range common to most interior areas (65° to 75°F).

Updated: September 29, 2015 — 10:44 pm