Question
QUESTION: Wayne - I have been growing orchids under fluorescent lights and 40-50% humidity for a number of years with very mixed results.
1. I have phals,phrags,dendrobriums and a variety of cattleya hybrids about 12" under 4 T12 plant lights. Two phals are about to bloom and 2 are quite red speckled on top of leaves and reddish below.
2. I have a variety of cattleya hybrids about 2' or more below 4 standard T5 lights. I suspect they are still too close to the lights because the foliage is either light green or speckled red.
Having said this, they bloom more than not but I sense they get too much light and have never been able to find any guides for the light intensity in ft-candles that should be appropriate. Can you suggest any guidance? I do have some phals that I grow in a window that bloom and look much healthier. Thanks

ANSWER: Jim, thanks for your question.  Phalaenopsis should have 1,000-1,500 foot candles of light.  The leaves should be glossy and should increase in size with each new leaf.  Cattleyas should have 1,500- 2,000 foot candles.  Leaves should be a light green in color, stiff, and leathery in texture. Having said that, there are a complex array of factors that enter into determining light level at the leaves.  One factor is the light spectrum which depends upon the type of florescent bulbs you are using.  Another factor is that by simply moving the plant between the center of the light bank and the ends can make a big difference in the amount of light a plant gets.  Another factor is daylength which determines how much total light the plant is expposed to each day.

The only time I have seen orchid leaves burn under florescent lights in when they are actually touching the leaf. When orchid leaves are exposed to a high light level, this stimulates the production of anthocyanins-- hence the red pigment.  Much like melanin in humans, this serves a protective function to prevent the cholorplasts from bleaching out with too much light. You should get your best flowering in cattleyas at this level.  However, other factors such as feeding, humidity, and air movement play a role in allowing these plants to fully use the light they are getting and serve to cool and protect the plant surfaces. The challenge with orchids is to obtain a balance for each type of orchid between light level, air movement, feeding, and humidity.

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QUESTION: Wayne - Thanks for your prompt reply. I believe the light intensity figures you quote are for sunlight. Fluorescent figures are much less, more of the order of 300 ft-candles.
Can you recommend a source for suggested fluorescent light intensity levels?

Jim

Answer
Jim, the "Yearbook of Agriculture", U.S. Department of Agriculture, 1973 shows the following for 2 lamps mounted 2 inches apart:  At a distance of one inch, they recorded 1,100 FtC after 200 hours of use.  For 4 lamps mounted 2 inches apart, after 200 hours of use, measure 1,600 FtC one inch from the light. In the latter instance, new bulbs produced 1,800 FtC.  When this distance is increased to 4 inches from the bulb, these figures become 570, 1,100, and 1,300 FtC respectively. It is generally recommended that the four bulb fixture of 40 watts each will provide a full spectrum of light for orchids in the range of 2000 FtC over an area 2 ft x 3 ft.

Sylvania Engineering Bulletin 0-286 titled "Orchid Growth with the Standard Gro-Lux Florescent Lamp" indicates 15 lamp watts per square foot at a distance of 10-12 inches above the plant tops for low energy plants such as phalaenopsis.  For 160 watts of light this translates into an area of about 10 square feet of growing area or an area of 4 x 2.5 feet.  For high energy plants such as oncidiums, cattleyas, etc, they recommend 20 lamp watts per square foot, which would require 200 lamp watts (ie 5 bulbs) for the same amount of area.  The general rule of thumb proposed in this bulletin is a "replacement perid for all lamps of 5100 hours"  This is an annual replacement for operating at a 14 hour day.

There are many considerations in designing a lighting setup.  These are set forth in a couple of well-written articles.  One is a website presentation titled "Florescent Plant Lighting" at www.waynesthisandthat.com/fluorescent.html.  Another is an article published in the December 2007 issue of Orchids magazine (published by the American Orchid Society).  I recommend the artcile entitled "The Enlightened Way" by Ernest Gemeinheit (page 906).  Ernest is a probationary AOS judge and describes his approach to growing orchids under florescent lights.

Hope this is helpful. Just for your information, I have been growing under florescent light for three decades and, most recently, have branced out to first a High Pressure Sodium (HPS)light with ballast and this year, for the first time, an HPS light with a remote ballast with a light mover.