Dealing with Cold Temperatures

from Martin Motes' book Florida Orchid Growing: Month by Month.


Forecasts of temperatures below 40º F should stimulate us to action.

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 50s (12-13º C). A few exposures to temperatures below 60º F (15º C) 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 80s (27-32º C) by day. Vandas will tolerate brief excursions into the upper 40s 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. 

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 40s 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 40s (8-9º C). They must, however be protected from both frost and freeze. Be extra cautious on those clear still nights when the temperature drops to the 30s (3-4º C).


Water is the only feasible source of heat available to plants grown in the open, under trees, in shade houses or on patios in South Florida. Ground water here (and in most of the rest of the world is about 63º F (16º C). Water out of municipal systems is not far different. On truly cold nights turning on the water can be of great benefit to our plants, provided that they have not been over-watered in the days and weeks preceding, thus inviting the ever present fungi to do more damage than the cold. For this reason as well, in general, orchids are better off dry until temperatures approach frost or freezing. The logic for maintaining plants dry is not only to minimize fungal problems but also because cold air is typically very dry air. If plants are wet in very dry and rapidly moving air say 10 or more mph, evaporative cooling can take place, chilling our orchids further and faster than they would if dry. When the water goes on it needs to be in heavy volume and it needs to stay on to keep the plants thoroughly bathed in its warmth. Very still air on the other hand, presents a different danger as frost is possible at temperatures higher than is commonly realized. In calm air frost can form at higher elevations and settle in on plants while the surface temperature is only in the upper 30s (4º C). The best forecast for nights when the temperature will hover near 40 is a light wind of 2-5 miles per hour. This light wind mixes the warm air near the surface and draws warmth from the earth. Clear, cloudless, still nights with bright shining stars elevate the spirit but harbingers frost.

If it is not practical to bring all the Phalaenopsis, vandas and hard cane dendrobiums into the house or garage, think of using water to help protect them. Shade cloth or even patio screen like a lacy Mantilla holds in a surprising amount of heat. Under screen, a fine mist head (½ gallon per minute) attached to a hose and left running beneath the bench or plant rack will provide several degrees of additional warmth that will often sufficiently temper the chill and ward off any light frost settling in. Growers with swimming pools frequently turn on the re-circulating pump to keep a supply of warm water near the pool's surface where it can add heat to the ambient environment. A few degrees of warmth frequently make all the difference to our sensitive orchids. In more open areas not protected by a permanent irrigation system, an oscillating sprinkler at the end of a garden hose is very effective. These are readily available at Home Depot and garden shops for a few dollars. On frosty nights, start the water at bedtime and let it run until the sun is up. The extra water once or twice in a month will do no harm to orchids that have been properly and judiciously watered the remainder of the month. In fact, these occasions present the opportunity to be sure that excess fertilizer salts have been leached from the pots and medium. A good work can be born of necessity! 

Remember that Himalayan dendrobiums and "'warm growing'' Cymbidium hybrids will positively relish temperatures down to 32º F and a light frost is just the ticket for great bloom. Keep the water off these!