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As the term implies,cool-growing orchids originate in temperate climates and at high elevations, where they’re frequently cooled by cloud cover, but they also do well in warmer conditions.
One such group of orchids is that of the genus Disa, which grow naturally in the mountainous regions of South Africa’s Western Cape province, in particular on Table Mountain in Cape Town. Here, the plants experience cool winds year-round, with cold rain and occasional snowfalls in winter. Summer temperatures are high, but the plants’ roots are kept cool by fresh mountain streams. Provided they’re protected from frost, they can survive in temperatures of 43 to 46 degrees Fahrenheit without risk of damage.
Relatively new to the cultivated orchid scene, Disas, until recently, were noted for being difficult to grow. They’re very distinctive, with vibrant colors, long-lasting flowers, and a triangular shape that makes them look nothing like a typical orchid.
Disa uniflora (syn. Disa grandiflora) is the most widely grown Disa species, and is only found on Cape Town’s Table Mountain and surrounding areas. Although the name uniflora suggests that the plant should only produce one flower, the species doesn’t have a single-flowered stem. Disas are generally small in stature, while the flowering stems can reach more than 3 feet in height. Many hybrids are available that easily and quickly mature from seed. Colors vary from simple pinks to vibrant reds, golden-yellows, and sunny-orange hues.
In the wild, the plants grow along small rivers and cold streams, making their home in the roots of reeds, which provide thick mats of tangled fibers in which the Disas embed their roots.
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In summer, Disas can become quite dry, in contrast to winter, when the plants can survive submerged in cold running water for short periods of time. In some areas, Disas are covered in snow for short periods without ill effect. Unlike most orchids, Disas thrive in damp, cool conditions, and flower in summer, when the weather is normally dry. The Western Cape enjoys a Mediterranean climate that’s characterized by cool, wet winters, and hot, dry summers.
Over the years, many mistruths have circulated about the cultivation of Disas, all of which have been dispelled. As long as a few simple guidelines are followed, the plants can be successfully cultivated.
Disas make good companions to Cymbidium and Odontoglossum orchids, and are best suited to greenhouse cultivation, but will grow in shade houses and outdoors. They can tolerate a low winter temperature of 43 to 46 degrees, and a maximum of 86 degrees in summer. Exposure at either end of this range will cause the plants to suffer. High humidity and very hot summer temperatures are disastrous unless the plants are protected by air conditioning or wet-wall cooling systems.
Light, Water, and Humidity
Although Disas grow naturally in strong light, they’ll be happier if they’re protected with some shading in the summer, such as 40 to 50 percent shade cloth suspended as high as possible above the plants. Air movement is of prime importance for healthy Disas growing in an enclosed environment or controlled greenhouse. Ventilation will keep bacterial rot at bay and dry any excess water from the foliage. In summer, greenhouse air vents should be opened fully to take advantage of fresh air. Indoors, particularly if the plants are kept in a small, poorly ventilated room, an oscillating fan can provide air movement.
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If there’s any secret to growing Disas successfully, it’s constant replenishment of fresh, cold water. Disas are best grown standing in trays of water. A good watering regime is to fill the tray at the beginning of the week, add fresh water midweek, and add fertilizer in the latter part of the week, before draining the tanks at the end of the week and refilling with fresh water. (Stagnant water can become smelly, and algae can form easily, especially in summer.)
Fertilizer can happily stay in the tank for a couple of days before being drained away. Avoid getting water on the leaves, and maintain a water level halfway up the pots. Tap water can be used without any negative effect, but rainwater is preferable, as it contains fewer chemicals. Deionized water can also be used, but this isn’t necessary unless the quality of the available water is very poor.
Plastic pots are best, and trays can be simply constructed by lining a solid container with plastic or polyethylene. Clean the trays when the plants are repotted after flowering, and rinse with fresh water to remove any slime or algae.
The feeding regime that should be followed for Disas is the same as the one for feeding Odontoglossums. Odontoglossums can’t tolerate strong fertilizers, which can burn their fine roots and damage the developing bulbs. They thrive on a low concentration or weak fertilizer. Like Cymbidiums, they prefer a high nitrogenous fertilizer in spring to support and encourage new growth, followed by a general feed in summer and a high potash-based fertilizer in autumn. In winter, a weak general feed may be offered to sustain the flowering stems.
Maintenance and Repotting
After flowering, or if the stems have been cut, the rosette from which the flowering stem emerged will begin to yellow, and the plant will appear to die off. This isn’t uncommon, so don’t be alarmed. Allow the stem to rot at the base, but don’t remove
it. You can cut it down with a clean, sterilized blade, but leaving the stem to rot allows natural bacteria to form, which encourages the growth of plantlets at the base of
Photo by Shutterstock/Uta Scholl
Removing the stem will result in bacterial infections, and the plant is likely to perish. When the young plantlets have hardened, remove and separate them from the original pot. The plantlets are vegetative forms identical to the mother plant.
Large established plants produce stolons and tubers from which new plants will grow. Allow the new plants to emerge from the compost before splitting them off from the main plant. The tubers will shrivel once the plant is established, as they’re merely a source of sustenance for the developing plant. As long as the potting medium shows no signs of rot or decay, the healthy root ball can simply be wrapped with fresh sphagnum moss and placed in a slightly larger pot.
Sphagnum moss is the best medium in which to cultivate Disas. Additions of perlite and peat have been met with some success, and sharp sand was used prior to moss, but none of the alternatives has proved as successful as sphagnum moss.
Michael Tibbs is a distinguished orchid grower and breeder, and the author of several books on orchids. He lectures widely and is an accredited judge in South Africa and a senior judge with the American Orchid Society. This article is excerpted from his book Orchids Handbook (Fox Chapel Publishing).
Disa Uniflora and Its Hybrids
Origin: Tropical and South Africa
Min/Max Temperatures: 43 to 86 degrees Fahrenheit
Flowers: During summer; blooms last up to 10 weeks
Light: Prefer diffused light, with 50 to 70 percent shading
Feeding: Replenish trays with very cold, fresh tap water