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Growing Requirements
by David Horak
Light
Light is undoubtedly the most important factor in determining whether or not an orchid will flower. Not only is the intensity of available light important, but its quality and duration need to be considered as well. Lower light levels available for longer periods of time can be nearly as effective as brighter light for a shorter time. There are limits, of course, but this principle is the basis for the successful culture of orchids under artificial light. Growing orchids under lights is more challenging but more controllable once the basics are understood.
How much light do orchids need? The rule of thumb is to provide as much light as the orchid's leaves can take without burning. Put simply, orchids should have bright light green leaves, and the growths should be strong and compact. With too little light the leaves are elongated and narrow, and the stems and pseudobulbs do not support themselves well. Plants with luxurious dark green leaves due to low light conditions may look healthy and beautiful but may rarely, if ever, bloom.
In the home, place orchids such as this mini Cymbidium close to the window and use curtains or light shading to regulate the amount of sunlight they receive.
There are tremendous variations in the amount of light different orchids need to bloom. Phalaenopsis are among the least demanding, requiring relative low light levels to succeed. Paphiopedilums need slightly brighter light. Orchids in the Cattleya, Dendrobium, and Oncidium alliances require fairly bright, lightly shaded conditions. A good way to test whether there is enough light on your windowsill or under your plant lights for an orchid to bloom is to hold your hand approximately one foot over the top of the plant at the brightest time of day, to see how strong a shadow is cast. A very slight shadow indicates that there is sufficient light for a Phalaenopsis to bloom. A strong clearly defined shadow means there should be enough light for a Cattleya to bloom.
Vandas and their relatives need the brightest conditions possible short of full sun. Occasionally, growers succeed in bringing a vandaceous orchid to bloom on a windowsill or under artificial lights (to the accolades and admiration of their peers), but in general these orchids tend to be better suited to growing in a greenhouse, or at least summering outdoors.
Light intensity is usually measured in foot-candles. Full midday summer sun, for example, can reach intensities as high as 10,000 foot-candles. The majority of orchids require roughly between 1,000 and 4,000 foot-candles of light for at least four hours a day in order to grow well and bloom. In a greenhouse, this level of light is relatively easy to attain. In fact, most greenhouses need approximately 50 percent shading if the orchids are to do well. This is usually achieved either with a specialized open-weave shade cloth stretched over the greenhouse for most of the year or a light coating of whitewash on the exterior of the glass. In the home or apartment, 1,000 foot-candles approximates the brightest light available within one foot of an unobstructed window with an eastern exposure; it is often referred to as bright shade. The light available in a window with an unshaded south or west exposure, where a distinct, clearly defined shadow can be cast, as described above, is approximately 3,000 to 4,000 foot-candles. Farther away from a window the light intensity falls off dramatically. While there are exceptions and mitigating circumstances, an orchid that's more than two feet away from the average window may grow well but will probably not flower. In the home it is best to place orchids as close to the windows as possible and use curtains or light shading to control the amount of sun coming in. An eastern exposure is probably the most accommodating, as it is bright enough to bring many plants to flower but unlikely to cause plants to get sunburned. A lightly shaded southern exposure offers a longer day length and may make it possible to grow the greatest variety of orchids with differing needs. No matter where you grow your orchids, remember that the aspect of the sun changes from low on the horizon in winter, with short days, to high in the sky in summer, with long days. This change can result in quite dramatic seasonal differences in the amount of light the plants receive and may necessitate moving them. Be especially vigilant in early spring and early fall—the stronger light may burn delicate foliage in less than an hour.
Cattleya Irene Holguin with a sunburned leaf. If they get too much sun, delicate orchid leaves can burn in an hour or less, especially in early spring and fall, when the sun is low on the horizon.
If you have a porch or backyard, you can move your orchids outside during the warmer seasons. The balmy temperatures, constant air movement, and natural light are beneficial for the plants, which may profit tremendously from a summer spent outdoors. However, proceed with care: Leaves can burn very easily outdoors. As little as ten minutes of direct sun can cause severe lesions that appear as pale yellow to white patches. The damage may impair the look of the plant for several years; if the burns are severe, they may lead to the plant's demise. Orchids usually do fine in unshaded morning and late-afternoon sun but must be protected from full midday sun. Set them under a tree or shrub, by a fence, or anywhere there is some dappled shade. When first moving a plant outside for the season, let it slowly get used to the brighter light by increasing the level of exposure over several days.
In a bright unshaded window or greenhouse, orchid leaves may also burn because the glass can magnify the effects of the sun's rays. This most often happens in the spring when the days become longer and the sun climbs higher in the sky. Feel the temperature of the leaves. If the leaves feel hot, move the plant to a shadier location or provide supplemental shading.
Some orchids are photoperiod sensitive, which means they bloom seasonally in response to a short or long day length. Regardless of where you grow orchids, avoid having other lights go on and off at irregular intervals or exposing the plants to a 24-hour light source. In the home or office and in many urban areas, this may be nearly impossible, and as result, some plants may bloom irregularly or not at all.
Watering
Watering frequency depends on the growing medium, time of year, state of plant activity, and environmental conditions, and it should not be done on a fixed schedule. Different growing media vary in their degree of water retention, but a good rule of thumb is to water the medium as it approaches dryness. Generally, watering needs to be more frequent, perhaps two or three times a week, during the growing season when the plants are active and the days are long. In the fall and winter when growths have matured and the days get shorter, the plants may only need to be watered once every seven to ten days. Sometimes a light sprinkling may be all that is required.
Lack of water and low humidity has caused the leaves of this Miltonia to emerge wrinkled. To avoid such problems, adjust watering and growing conditions according to the plant's needs.
If you are not sure whether an orchid needs to be watered, lift the pot and judge its weight; a dry pot weighs less than a wet one. When in doubt, wait another day or two before you water. Always keep in mind that more orchids are killed by overwatering than underwatering.
When it's time to water, do so thoroughly, making sure to use cool to tepid water. Both hot and cold water can seriously damage roots and leaves. Water until the medium is completely wet and water runs out of the bottom of the pot. Water from the top rather than the bottom, since most potting media float. Never dunk several plants in a shared bucket in an attempt to save water or fertilizer. It's an ideal way to spread disease from plant to plant, especially viruses. Occasionally wet or wash all plant parts to remove dust and sticky plant secretions. This will help reduce the chances of insect problems. But don't allow water to get on the leaves when the plants are in very bright light or direct sun—the water droplets can magnify the light intensity, and the leaves can get burned.
Ideally, water early in the day so that the leaves are dry by nightfall. Water left on the leaves and in the leaf axils (where the leaf meets the stem) promotes infection with bacterial or fungal diseases that can disfigure or kill a plant or cause an emerging inflorescence or new growth to rot.
Watering needs also change with the weather. Try not to water on cloudy, cool, wet, or humid days. Minimize watering during periods of prolonged cloudy and rainy weather; increase the humidity in the growing area instead. When it's sunny, hot, and dry, you will most certainly have to water your orchids more. However, watering during hot, humid weather, especially when the nights are hot, can promote root rot in orchids that prefer cooler conditions. Water lightly and rely on light sprinklings rather than heavy watering.
Most orchids can survive, not thrive, if kept too dry, but overwatering can kill an orchid within a couple of days. The leaves and pseudobulbs become lax and start to shrivel because the root system is rotting and the plant can no longer take in moisture. At this time an inexperienced grower might assume that the plant is not getting enough water and respond accordingly, thus compounding the problem. The right response is to remove the plant from its pot and inspect the roots. If they are soggy, brown, and dead, trim them back to live tissue and repot the plant in new medium and let it dry. Keep the humidity high and light levels low and hope for new root growth. If you don't intervene in time, opportunistic bacterial rots will soon turn the plant to mush.
If the roots appear whitish but shriveled and the medium is dry, the plant has received too little water. Replace it in its pot and move it to a humid location. Water lightly and gradually increase the frequency of watering until the stems, leaves, and roots begin to plump.
Growing Ascocenda Sandy Schultz in a basket allows for good air circulation around the epiphytic plant's root system and assures excellent drainage.
Water quality is very important for plant health. Clean rainwater is ideal for orchids. It normally has a slightly acidic to neutral pH of about 6.9 to 7.0, with virtually no dissolved minerals. Water from most municipal water systems and well water are generally adequate but vary considerably in quality. Water with a very low pH (acidic) or a very high pH (alkaline or basic) that contains high levels of dissolved minerals such as calcium and magnesium can damage plants and impede nutrient uptake, literally starving plants. While both calcium and magnesium are important plant nutrients, they can occur in forms that are not usable to plants. In many places, especially those relying on well water, the water is hard, with over 200 ppm of dissolved solids and a high or low pH. (A reading over 120 ppm is considered hard water, whereas soft water will have 60 ppm or under.) Very hard water leaves a whitish film or powder on the leaves and growing medium when the water evaporates. The film is usually considered unsightly but only a cosmetic issue. However, if enough minerals accumulate, they can block light absorption and impede photosynthesis.
Orchids can tolerate a pH from about 5.0 to about 8.5. Municipal systems are regularly monitored and treated, so you can request an analysis from your local water department to find out how your tap water rates. Most cities add chlorine to the water to kill pathogens. Chlorine is usually not a significant problem, but it may cause some root damage or leaf-tip dieback in more sensitive plants. If it proves to be a problem, try running the water through an activated charcoal filter system or let the water stand in a bucket for 24 hours to allow the chlorine to evaporate.
You can also use a filtration system such as reverse osmosis (RO). The result is excellent water with virtually no dissolved solids. In fact, it's important to add some fertilizer or unfiltered tap water to both RO and distilled water to prevent the water from robbing nutrients from the plants through osmosis.
David Horak has been growing orchids for more than 25 years. He is the curator of Orchids and the Robert W. Wilson Aquatic House at Brooklyn Botanic Garden. He is the current president of teh Greater New York Orchid Society and the chairman of the New York International Orchid Show.
Photographs: Charles Marden Fitch