One major obstacle for neophyte growers is in understanding the diversity of cultural requirements of various genera of orchids. Orchids are such a vast group of plants which have succeeded in nearly every conceivable habitat on earth, that knowledge of a specific genus’s cultural requirements, rather than a general knowledge of what “orchids” like, is necessary to successfully cultivate the various types. Most cultivated orchids come from tropical regions but differences in elevation and other geographic features of their native habits can mean dramatic differences in the response of orchids to various external conditions. Most emphatically these differences can be seen in different genera’s toleration of cold. While some orchids are native to regions where frost is more the norm than the exception, others are hyper-tropical plants for whom 50 F (10C) is far too cool. Knowing which is which is essential in a mixed collection of orchids. A great irony for beginners is to discover that their extra nurturing efforts to protect certain orchids have in fact done more harm than good.
Dendrobiums are among the most confusing for new orchid growers. This huge genus, well over a thousand species divided into 15 sections, ranges over nearly a quarter of the planet. Found from western Indian all the way to Micronesia, dendrobiums inhabit an incredible variety of ecological niches. Ironically, the two sections most common in horticulture are diametrically opposite in cold tolerance. Section Dendrobium, the soft bulb or “nobile types” whether in their pendulous forms like D. anosum and aphyllum or in the upright types like D. nobile and its hybrids, positively relish the cold. Temperatures right down to frost are the best culture to produce the most prolific blooming of these plants. Without cold and drought stress in winter these plants will retain their leaves and produce an abundance of vegetative growths but few if any flowers. Stressed by cold and dried out properly these plants lose all their leaves and in spring the bare bulbs are covered in flowers. The opposite is true for the “hard cane” dendrobiums of sections Spathulata and Phalaenanthe. Loss of leaf on D. phalaenopsis types is usually indicative that they have suffered from too much cold. Temperatures below 60F (15C) can produce this undesirable effect. D. phalaenopsis and evergreen types should receive the maximum cold protection.
Other sections of the genus have slightly different tolerances. Section Callista, D. farmerii, D. lindleyii (aggregatum) and their relatives can take temperatures nearly as low as the nobile types and will bloom all the better for exposure to temperatures in the 30's (3-5C). Section Formosae, D. formosum, D. infundibulum and the new hybrids prefer slightly warmer conditions but are quite happy with temperatures in the 40's (6-9C). Other sections of Dendrobium in cultivation such as Pedilonium, Latouria, and the Australian hybrids of section Dendrocorne have slightly different requirements and those growing these more “exotic’ will succeed best in researching them. Try B. Lavarack et al. Dendrobium and its Relatives, Timber Press.
After the cold sensitive “hard cane” dendrobiums, Phalaenopsis are the most tender of commonly grown orchids. Phalaenopsis will be strongly induced to bloom by temperatures in the mid 50's (12-13C). A few exposures to temperatures below 60F (15C) will produce the desired spikes and thereafter the plants will be happiest if they are kept above 60. One or two nights down to 50 or slightly below will do little harm but are to be avoided in the best kept collection.
Vandas come next on the scale of sensitivity. Like Phalaenopsis they are stimulated to bloom with sharp drops of temperature into the 50s at night, especially when the temperature can be induced to climb into the 80's (27-32C) by day. Vandas will tolerate brief excursions into the upper 40's but are best keep above 50 degrees. Temperatures below 50 for very long or very often will produce the tinkling sound of falling Vanda leaves, turning the plants into palm trees. 52.
Oncidiums of the “mule ear” type with thick fleshy leaves (O. luridum, lanceanum etc.) have warmth requirements similar to vandas. The thinner leaved Oncidinae will usually take temperatures into the 40's with aplomb. Many of the hybrids in this group have been bred to Miltoniopsis and to Odontoglossum to increase their cold tolerance. A caution with this group is the ability of wind to strip heat rapidly from their thin leaves. The cold tolerance of these will be much greater in still air.
With the exception of some species of Amazonian origin like Cattleya violacea, most cattleyas can take quite cool temperatures. Most growers have few concerns for them even in temperatures down to the upper 40's (8-9C). They must, however be protected from both frost and freeze. Be extra cautious on those clear still nights when the temperature drops to the 30's (3-4C).
In addition to the cold loving nobile dendrobiums, certain other genera from the high Himalayas such as deciduous Calanthe and Cymbidium species and hybrids, actual require quite cold temperatures to stimulate them to their best bloom. Even “warm growing, temperature tolerant” hybrid cymbidiums flower best when chilled repeatedly into the low 30's.
All orchids tolerate cold best when they have proper nutrition. Avoid too much nitrogen which might stimulate too soft of growth and increase the dosage and frequency of application of both magnesium and potassium in colder weather.