While there are many factors which can make blooming happen very quickly in orchids; a fall in night temperature, increase or decrease in day length and even suddenly restriction in water availability, these factors will not be successful unless your orchids have been grown with fufficient light. For most hobbyists, this factor is nearly all the time at the heart of the question “Why won’t my orchids rebloom?” You can’t work magic or miracles and without sufficient light all through the growing season, no amount of cajoling with decreased nitrogen, extremely cold nights, dehydration or similar abuse will make flowering happen. If you have noticed a general decline in the number of your plants that flower, it’s highly possible that the amount of light you have getting to your growing area has decreased. Don’t forget that trees and landscaping grow as well and can shade windows after a long time. I was once asked at a basic orchid culture class why one of the attendee’s vandas no longer flowered. This prompted her to re-ask the question and insist there was a larger quantity of light as the shelter had been especially constructed for her vandas. The second time she refused to accept my answer I asked if she’d trimmed the vines growing on the arbor. When she asked me how I knew she had grew vines, I said “you don’t have sufficient light.”
So what is sufficient light? While the actual levels vary with the genus, usually orchid leaves should be a light yellow-green rather than a lush sort of grass green. In addition, the leaves of your plants should be firm and stand upright. Even Phalaenopsis which need fairly low light will, depending on the genetic background of the plant, keep their leaves horizontally or somewhat upright and plants that produce very long, floppy leaves are being grown under insufficient light. Think of the leaves being the plant-equivalent of solar collectors. The lower the light, the bigger the leaves need to be to collect the same amount of light; the heavier the leaves the harder it will be to keep them firm and upright.
There are several ways to measure the light your plants are receiving and you can’t really trust your eyes. Our eyes are designed to effectively adjust to the light they receive. While the most correct way to measure light levels is purchasing a light meter which you can get a fairly good estimate using just your hand. On a clear day, position yourself so your hand is between your plants and the light source and around 12 inches from the leaves and observe the shadow, if any, your hand casts. You probably do not have sufficient light for any orchids with the exception of the jewel orchids which are grown for their leaves more than flowers, if you can see no shadow. If the shadow is unclear and faint (below left), moderating your light level should be sufficient for Phalaenopsis and Paphiopedilum which require light less than most orchids. If the shadow you see is sharp (below right), you most probably have sufficient light for all except the highest light-needing orchids like cymbidiums, vandas and ascocendas.
Don’t let anyone tell you that you can’t grow a mixed collection of orchids. Hobbyists do it all the times. They just take advantage of the difference in conditions throughout their growing areas. As far as light levels are involved, direct sunlight entering an unshaded window can be as high as 4,000 to 8,000 footcandles measured directly at the glass while it may only be as low as 500 footcandles just inside the shade away from the side of the same window. Lower light plants like Paphiopedilum and Phalaenopsis can be organized in these shady areas while higher light plants like oncidiums and cattleyas can be put closer to the center and nearer the glass. It just takes a little experimentation.
Quality versus Quantity
Plants need red and blue light and they reflect green. Light from the red end of the spectrum is crucial for flowering while light from the blue end is crucial for growth. This is generally not a problem when plants are grown outdoors or in a greenhouse unless the greenhouse skin is very old and has yellowed to the point that the quality of a spectrum is poor. Nevertheless, for plants grown under lights, this can be an important factor and is the reason that many books recommend a mixture of 50:50 cool-white and warm-white fluorescent tubes in the light rack. Quality of a spectrum can also be an issue if you are growing your plants in a sunroom covered with bronze or other colored glass since these covering materials may remove much of the spectrum used by plants.
Duration
Longer is not always better. First, plants need a night just as we do. They are adapted to the daily cycle of the sun and different parts of their metabolic cycle are achieved during light and dark periods. In addition, many orchids, particularly species, are adapted to changes in day length. This is called photoperiodicity. Fall-blooming cattleyas flower when day length shortens while spring-blooming ones when day length increases. The old cattleya cut-flower growers used this knowledge to arrange that flowering should happen for important holidays and it’s used today by Poinsettia growers for the Christmas market. Why is this important to the hobby grower? It’s really quite easy. If your only choice for a growing area is one that is lit late into the night, it would be best to focus on those plants like Phalaenopsis that flower not concerning day length.
Too Much Light
Unlike too little light whose effects are frequently insidious, the opposite can be true of too much light. Too much light, particularly if it’s a sudden change generally causes dramatic damage (sunburn) in very little time. The first indication of too much light is frequently yellow leaves. If left alone, this yellow leaves will finally turn white and then dark brown and dry because the sunburned area dries out. Plants exposed to too much light continually for a long time but not enough to cause sunburn will not be developing properly with yellow, hard leave. If the problem is caught before the chlorophyll has been completely destroyed it is frequently possible to reverse the damage. If white spots or sunken areas appears, the damage will be irreversible and the best thing one can do will be to stop further progression with more shade.
This is one area of orchid culture where you want to make changes SLOWLY. Orchids are easily sunburned if light levels increase too quickly. When moving plants around, particularly when bringing them outdoors after winter, to be more careful or safe than is necessary of excess shade. Make changes when you are going to be home and can watch the plants. Feel the leaves. The palm of your hand is around 93F. If they are hot to the touch, the leaf temperature is around 95F and extremly damage can happen in very little time.