Light is another critical factor in growing and inducing orchid plants to bloom. In fact, lack of adequate light is the chief reason for an orchid’s failure to bloom. On the other hand, too much of a good thing is a problem, as well – for instance, when it results in sunburn. And although some species require full sunlight, others need restricted light to perform well. Keep in mind that in nature, orchids seldom grow either in extremes of light or where the air is stagnant. Fortunately, you can supply your indoor orchids with appropriate light, by both natural and artificial means, to keep them healthy and induce them to bloom.
In your home, the amount of light an orchid receives can be controlled by its placement in a window. Large east or south windows are preferred, but west-facing windows will do very well if strong afternoon light is softened with thin curtains. Bay windows can be excellent locations, and skylights and solarium greatly expand the indoor area that is well lit enough for growing orchids. Experiment: sometimes moving your orchid just an inch or two-into more or less sun than it had been receiving-can mean the difference between thriving and simply existing.
On very cold winter days, move orchids away from the glass somewhat. At the slightest hint of sunburn interpose a thin curtain, or move the plant back from the window or to another location where the light is not so strong. Orchids that develop dark green foliage and fail to bloom may need to be moved to a brighter window.
Modern windowsills are seldom wide enough to hold any but the very smallest flowers; however, wider ledges can be added to the sills or narrow tables drawn up under windows to provide “foot room” for the orchids. Orchids should not be so tall as to look out of scale with the window; 30 inches is sometimes cited as the maximum height for a window plant. You will have no difficulty finding many this size or smaller. In fact, many so-called miniatures are less than a foot tall; two or three shelves suspended in a window may hold a dozen or more.
Greenhouse windows available as add-on structures to the house are a tempting way to gain additional space for orchids, but keep in mind that the light in such windows is stronger than in conventional ones. Extra-careful attention to shading and ventilation will be needed for your orchids to prosper there.
What’s Enough Light?
Plants that flower need more light than plants grown simply for their foliage, because of the extra energy and materials needed to create a bloom. This is why it’s quite possible to grow orchids that look fine but don’t flower. They’re getting enough light to survive, but not to bloom.
Orchids give the best clues as to proper light, for no light meter will be a better guide than actually looking at the leaves. Orchids in the proper amount of light usually have leaves that appear moderate to light grassy green in color. If the level of light is too low, the leaves and growths appear deeper and greener. They may look healthy, but they’re not getting enough light to bloom. In lower light, the leaves also get less shiny, so less light will be reflected. A very low light will make the orchid softer, with stunted foliage, elongated stems, and greater distances between the stems.
Too Much Light
Orchids generally do not receive too much light when grown indoors, but they can be subjected to excesses when put outdoors for the summer. If heat builds up too much, particularly in direct sunlight, the leaves can burn and blacken in a form of “sunburn.” Even if the plant doesn’t actually sunburn, high light can deplete food reserves so that the flower actually starves, which shows up as yellowing in the leaves. Orchids at their very highest light-level tolerance will often become red-tinged.
Measuring Horticultural Light
Light for growing orchids is often expressed in terms of foot-candles of light. Foot-candles (abbreviated fc) simply measure the intensity of light, the amount that actually falls on a surface, or its illumination. A foot-candle is the density of an amount of light. Orchids are classified into three basic light groups: high (3,000+ fc), medium (2,000 to 3,000 fc), and low (1,200 to 2,000 fc). Foot-candles do not, however, measure the quality of light or its duration, and both are horticulturally important. But foot-candles do give a guide to the relative needs of different orchids, to help gauge certain categories of light requirements. Measuring light intensity by foot-candles is useful when assessing actual sunlight, which consists of the full range of the light spectrum, perfect for orchids. If you are growing orchids in sunlight, the quality of light does not have to be of much concern. The intensity of that sunlight is of interest, and it can be measured in foot-candles, using a photometer.
Another way to gauge relative intensity is with a hand/eye test. Put your hand between the light source and the plant, with the hand 6 inches above the leaves, and look at the shadow cast. A sharp-edged shadow means the light is high, a soft-edged shadow is probably good for medium- to low- light orchids, and no shadow cast means the light is not strong enough for an orchid to flower. Outside, in full sun on a bright summer day, the amount of foot-candles illuminating a surface is about 10,000. An overcast day yields about 1,000 foot-candles of light. The indoor light near a window can be as low as 100 foot-candles, or as much as 5,000 if the orchid is right up against the window on a clear day at noon at the standard time on June 21, which is when the amount of light falling on the Northern Hemisphere is greatest. Measure light intensity on that date at noon to determine the maximum foot-candles that reach the orchids growing there.
Orchids need a certain duration of light intensity. Those with highest light needs bloom best with a full day of light of at least six hours. Even the lowest-light orchids need their light for as long as possible during the day.
Manipulating Natural Light
Light varies considerably throughout the year everywhere except at the equator. It is high in summer, low in winter, often differing by a factor of five to ten times as much. Light also varies a great deal during the day. Many windows are perfectly suitable or can be adapted nicely for growing orchids, especially if the flowers are kept close to the glass. Windowsill orientation makes a difference in the intensity and duration of light received. A south-facing window generally works best for orchids, then east (which gets mostly morning sun), then west (though westerly windows can get excessively hot since they get much afternoon sun), then north. North windows are touted as being no good for growing orchids, but that may not be true if they face reflective walls, or can be made more reflective, or can be more illuminated by the addition of artificial light.
Keep sunlight-transmitting glass clean, push orchids closer to windows, buy plants that best suit the light available, keep plant leaves clean so they can utilize light absorption to capacity, and space plants so that no leaves are blocked by leaves of other plants. When necessary, add artificial light, and increase the amount of time the artificial lights are on to compensate for what they lack in sunlight power. Make the environment around the plants as light-reflective as possible. Use matte-white walls, aluminium foil, mirror tiles, or, best of all, Mylar lining, which can reflect up to 98 percent of light available.
The Trick of Duplicating Light
Artificial light varies greatly in light quality, and there is a large difference in the spectrum of light that various types of light bulbs emit. If you are simply adding more light to an environment that is already receiving natural sunlight, light quality doesn’t matter significantly. But if you grow orchids completely under lights, quality of light becomes critical. The best growth in completely artificial light is through a combination of light sources that complement one another to produce a spectrum closer to sunlight than any type does separately. To help make up for what artificial lights lack in solar power, keep them on for 14 to 16 hours per day. One advantage to growing completely under lights is that there are never any cloudy days, and so watering and culture techniques remain more constant.
Regular incandescent bulbs are not good for growing orchids because they give off enormous amounts of heat and because they must be too close to the plant to be useful, burning it. Fluorescent tubes work much better. They cover a wide area, provide even illumination, give off relatively small amounts of heat compared with the amount of light they produce and are inexpensive and efficient. They are best for low-light orchids. Those that require more than about 1,800 fc probably won’t bloom well under fluorescent lighting.
All fluorescence are deficient in some portion of the visible spectrum. The standard “cool white” bulbs are much higher in blue and green wavelengths but are deficient in the photo-synthetically important orange-red spectrum. “Warm white” fluorescence are deficient in the blue portion of visible light, leaning more toward the yellow region. A good, inexpensive solution is to mix standard cool white and warm white bulbs together for growing orchids, which seems to give enough of all the wavelengths to satisfy the plants.
Fluorescent tubes will last a long time before burning out. Cool whites are rated at 20,000 hours if left on continuously. But for horticultural purposes, their useful lifespan is considerably less than burnout time. Replace fluorescent tubes in orchid use every year, certainly not more than every two years, even though they still light. Light is highest in the middle of a fluorescent tube, where higher-light plants should be placed. Light drops off dramatically at the ends of the tubes, and short tubes, such as 2-foot ones, are virtually useless for growing orchids. “Power-twist” fluorescence, which has more surface area, emit 15 percent more light than standard ones.
For orchids, you need a minimum of four 40-watt tubes, spaced 6 inches apart. The closer the tops of the orchids are to the tubes, the higher the light. Place plants within 8 inches of the tubes, or closer to get more light, especially at the ends where the light is less intense.
High-intensity-discharge (HID) lamps are the best artificial lights for orchid growing, although the most expensive. These include high-pressure sodium lamps and metal halide ones, quartz tubes filled with sodium or mercury vapors under pressure and surrounded by an ultraviolet-absorbent envelope coated with phosphors. HID lamps are very efficient and give excellent-quality spectral light, but their special fixtures drive up cost. Over long-term use, however, the low cost of running can offset the initial outlay. Virtually any type of orchid will bloom under HID light, since, for example, a 1,000-watt lamp can emit as much as 12,000 foot-candles. A 400-watt lamp typically illuminates a 6’x 8′ area with 1,000 to 2,000 fc.
Metal halide fixtures cost less initially than sodium ones, and they can be converted easily to hold sodium lamps if desired later. But halide lamps deteriorate much more rapidly than sodium. At burnout, they have lost half their power, whereas sodium has lost only 20 percent. Sodium lamps should be replaced every two years; metal halide needs yearly replacement. High-pressure sodium lamps are higher in the red-orange-yellow spectrum, the preferred choice in commercial greenhouses. But sodium lamps give off a yellowish, very unattractive light that makes the plants appear an awful pink grey while under them. Metal halide light is more balanced and natural looking, higher in blue than standard sodium. “Agro”-designated sodium vapor lamps have been specially designed for horticultural use, adding 30 percent bluer as well as more overall light. Metal halide emits roughly 125 lumens/watt (lumens are the measure of brightness available per watt), while sodium vapor can be as high as 140 lumens/watt.
Be careful when using water near HID lamps, for droplets on hot bulbs can sometimes cause breakage. Use a reflector to keep very intense HID light from shining directly in the eyes. A rotatory can continuously move the lamp over the orchids, ensuring even lighting and avoiding hot spots.