When considering light, take intensity, quality, and day length into consideration. The types of bulbs used, if relying on artificial light, is of considerable importance as well.
Intensity | Quality | Day Length | Bulb Types
Light is commonly measured in foot candles, or the amount of light candle or candles will provide to a square foot of two dimensional surface. A light meter is the best way to determine if light intensitys proper for the plants you grow. Generally, light intensity can be guesstimated by holding your hand several inches above the leaves of a plant between it and the light source. If no shadow is cast, you likely do not have enough light to bloom most orchids. If a faint shadow with hazy outlines is cast, orchids that like lower light should grow and bloom well. This is about eight hundred to twelve hundred foot candles, and many types of Paphiopedilum and almost all moth orchid hybrids should be happy with this. If a distinct shadow is cast, but the edges are still not very clear, you probably have enough to grow multifloral Paphiopedilum, Phragmipedium, and quite a few miniature or compact corsage orchids if the latter are kept very close to the light source. The orchids we offer will likely get sunburn if provided with light strong enough to cast very distinct shadows with defined edges. Utilize some method of shading in this case. In a pinch, waxed paper works well to shade a window.
If leaves are a rich, forest green, the plants may not be receiving enough light. Mottled-leaf Paphiopedilum should have distinct patterns though. If leaves are a yellow-green or if they have red pigments on the upper surface, they are likely at the upper limit of their light tolerance. We recommend cutting back light in this case for slipper and moth orchids, but this is just fine for corsage orchids. The red pigments are expressed in plants as a sort of sun tan; additional light or insufficient cooling on a suntanned plant may cause burning, recognized by dark brown to black dry areas on leaves. However, many plants naturally have darker (Phalaenopsis and Doritaenopsis with red or pink flowers) or lighter leaves (summer-blooming Phalaenopsis) and even red spotting (Brachypetalum, Parvisepalum, and coloratum and vinicolor Maudiae Paphiopedilum).
Very accurate digital light meters are available for about one hundred to one hundred and fifty dollars. Economical light meters cost about twenty-five to thirty dollars, but check yours against an accurate light meter. We use one of the economical meters to give us an indication of light on a plant that is doing well then use that reading to gauge light available to other plants. We experienced that even some higher light loving plants will grow and bloom quite well under fluorescent lights. This is because they give roughly the same intensity of light from the time they turn on until the time they go off; whereas, even on clear days, the sun has different intensities during the day based on its position in the sky. Clouds, trees, seasons, etc. add additional variability to natural light intensity. If you engage in thirty-five millimeter photography as a hobby, you can use your camera's light meter to tell you the intensity of the light, but as we are not personally familiar with this procedure, we will allow you to explore this option elsewhere.
For simplicity, flowering plants require blue light and red light to grow and bloom. The sun provides enough red and blue light, but synthetic light can be deficient in red or blue wavelengths, and sometimes both. Manufacturers have spent countless dollars developing high tech, full spectrum tubes specifically for plants. We believe, as long as blue and red light are provided in sufficient intensity, plants will grow and bloom very well and possibly better than those grown in greenhouses. On light bulb packages, light color is usually represented as Kelvin or K. Full spectrum bulbs are about 5,000 K, warm white bulbs over-represent the red wavelengths and are about 3,000 K, and cool white bulbs burn more blue at about 4,100 K.
Photoperiodicity- Day Length
We usually vary day length slightly throughout the year with the aid of timers. However, not as much as some growers recommend. Most of the plants we grow are equatorial or nearly so, and do not experience a marked seasonal day length variation. Our main inspiration for this may be to reduce the irritation of the intense light emanating from the second story green room on our neighbors. We use the following as a rough guideline: fourteen hours from November to March and sixteen hours from April to October. Some orchids, certain Cattleya species in particular, use day length as a blooming cue. With these plants, it is imperative to provide the same amount of light per day as they would experience in nature unless you are purposely attempting to shift bloom period (we do not currently offer plants with this requirement).
Bulbs can be incandescent filament types like those commonly used in household lamps, fluorescent tubes or coils like those used in ceiling office fixtures, or high intensity discharge (HID) like those used in street lights and the new super bright car headlights. LED lights are new to the hobby, but could be promising for orchid growing.
Incandescent bulbs can give considerably intense output, but are typically rich in red light. They are also very hot which can burn plants. This means that of the energy used to make the bulb create light, a considerable portion is wasted as heat output instead of light. These bulbs usually require frequent burnout replacement. For these reasons, incandescent bulbs are rarely used for plants.
Fluorescent bulbs are available in many different colors and even full spectrum, and the compromise between intensity and energy use is acceptable as they burn cool- less energy is given as heat at the expense of light. Intensity from these bulbs is limited by the volume and surface area of the tube, so bulbs commonly need to be fairly close to the plants. Alternatively, several, high wattage tubes can be used. High output (HO) and very high output (VHO) fluorescent tubes are available, but require special ballasts and fixtures. Straight tubes are the classic type of fluorescent bulb, but newer compact fluorescent bulbs and spiral tubes that screw into common incandescent light fixtures are becoming useful for agricultural applications.
High Intensity Discharge bulbs come in two basic types Metal Halide and High Pressure Sodium. The basic difference is in their output wavelengths. Metal Halide lamps provide light rich in blue wavelengths and High Pressure Sodium lamps are higher in red/yellow output. They can be used together or switched to match the growth and flowering of your plants, but it should be noted that each bulb type usually requires a unique ballast. HID bulbs give a considerable amount of light per watt and they are commonly used for light-loving plants or for a large, horizontal growing space. Light movers help expand the grow area by slowly, automatically moving the fixture over the plants. Startup costs of these units is much higher than for fluorescent light gardens, but bulbs are long lasting, and light output is more intense.
In our opinion, fluorescent lights are useful for growing plants on vertical shelving racks, whereas HID lights are conducive to growing in a single layer horizontally. We grow our plants under four, four foot, forty watt fluorescent lights per two foot by four foot shelf. Most of the two foot by four foot shelves have two, two bulb shop light fixtures suspended by chains from the shelf above for a total of four bulbs above each shelf. Bulb height is adjusted with chain links that hold each light fixture, and bulbs are between one quarter of an inch to twelve inches above the highest leaves based on the requirements and size of the plants on each shelf. We used to use a varied mixture of bulb types such as GE's Plant and Aquarium and Chroma 50 bulbs, Philips Agro bulbs, and Vita Lites and VitaLite Power Twists, but expense has led us to settle on simple warm and cool white forty watt T12 tubes. Specialty bulbs cost about $5 to $25 each and many have argued for and against their use. We are completely satisfied with using warm and cool white bulbs (about $2 and $1 each, respectively) alternated in a one to one ratio.
Warm white (left) and cool white (right) forty watt, four foot long fluorescent bulbs in a shop light...
Note that there a two shop light fixtures, side by side, very close to the upper leaves in the collection canopy. Bulbs are alternated warm white, cool white, warm white, cool white...
Fluorescent bulbs will continue to illuminate for many years after installation, but their intensity significantly diminishes after about twelve months of twelve to sixteen hour days. Bulbs are replaced about once every twelve months over corsage orchids and sometimes we let them go a little longer over slipper and moth orchids. Bulb replacement is rotated to maintain consistent light intensity throughout the year instead of replacing all bulbs at once.
We have discovered that, at times (e.g. when there is a small distance between the tubes and the leaves), this can be too much light for slipper and moth orchids, especially younger plants. In such cases, one bulb is removed from one of the above fixtures. We monitor the leaves and blooming habits of the plants to determine proper light intensity. Generally, under fluorescent lights, we recommend 15 to 20 watts of fluorescent light per square foot of growing space. That is three to four forty watt fluorescent bulbs suspended above a two foot by four foot (eight square feet) shelf. For young seedlings, two or three forty watt bulbs within 6 inches give us good results.
If you grow on a windowsill or in a greenhouse, east, south, and west windows will give good results and may even require various amounts of shading during all or part of the year. Consider adding supplemental light if you grow in a north-facing window.
We have recently experimented with using compact fluorescent bulbs (usually spiral tubes with a built-in adapter to fit standard incandescent sockets) as supplemental light in various situations, and we are pleased with the results. Choose compact fluorescents that are the output equivalents of 150 to 200 watt incandescent bulbs. These higher wattage compact fluorescent bulbs will give a couple square feet of illumination sufficient enough to bloom Phals and lower light slipper orchids. Simple fixtures complete with reflectors with strong clamps are available inexpensively at most hardware stores.