The onset of shorter days and longer, colder nights heralds winter’s arrival. Many factors play into keeping your orchids happy and healthy through this time — finding the right combination of variables can be tricky, especially for beginners. The phrase “proper preparation prevents pitiful performance” is appropriate here; doing so will help your orchids to reach their greatest potential and increase your knowledge and enjoyment of the hobby.
Generally, temperatures between 50° and 80° F (10° to 27° ) are ideal for orchids; but occasional brief periods of temperatures above 100 F (38 C) or drops even into the 30s (0 C) will not harm most orchids as long as no frost forms on the leaves. Cold hardiness, the measure of the orchid’s resistance to or ability to adjust to cold stress, should be considered when thinking about how to care for your orchids in winter. Some orchids like Miltonia or Masdevallia won’t mind if temperatures dip into the 40s (F) at night. Even more impressively, many Cymbidium species are naturally occurring at higher altitudes which frost at night, giving them the ability to take a nighttime dip around the 30s (F) without damage.
The degree of cold hardiness is influenced by environmental conditions, the overall health of the plant and what the native environment of the plant is. Some species are always killed by colder temperatures, while others can tolerate short periods of temperatures near freezing.
During the winter, flowering orchids brighten well-lit windowsills. On cold nights, it's a good idea to move plants away from the window or use bubble wrap as insulation between the pane and the plants.
Cold Damage
Cold damage symptoms usually become visible some hours to days after exposure to critically low temperatures, not during the cold exposure. Chill injury is that caused by low temperatures above freezing, and freeze/frost injury is damage from temperatures at or below freezing. Orchids that come from the more lowland or equatorial environments may take damage even if they do not freeze, whereas orchid species that hail from higher elevations may have more resistance. The rate at which symptoms develop depends on the severity of the exposure and the conditions in the growing environment after the exposure. Higher air movement and humidity at lower temperatures will exacerbate the amount and severity of the damage.
General symptoms of chilling injury include surface lesions, pitting, large, sunken areas and discoloration; water-soaking in tissues, usually followed by wilting and browning; internal discoloration (browning); increased susceptibility to attack by fungi and bacteria; slower than normal growth (this may be difficult to identify without undamaged plants for comparison or a knowledge of the orchid’s normal growth rate) and accelerated rate of natural death.
Frost damage takes place when the cells freeze and burst. Freeze damage looks like dark greyish purplish water-soaked areas that spread a bit over the course of a few hours to days after exposure, and then stop spreading. This damage will be found on the most exposed parts of the plant. There is no recovery from frost damage, as the cells are destroyed, though there may be recovery from chill injury, so long as the bulk of the plant was protected and there’s buds to start growing again.
If the worst happens and your orchids are damaged by the cold, don’t assume that they are goners (unless it’s freeze damage). Treat the plant as though it has just been repotted — avoid direct sunlight and high temperatures, warm the plant up, keep the medium moist, and give them a weak solution of liquid plant food with a full complement of minor elements. When you see the beginnings of new growth sprouting, apply a very light dose of fertilizer and continue as normal.
As a last resort, moving your collection or your most treasured and cold-sensitive plants indoors temporarily may be an option. While they do not provide optimal growing conditions, homes are much better equipped to keep out the cold than greenhouses. This step can make the critical difference between plant survival and plant loss, so optimal culture can be foregone for a few days until the danger of freezing passes. If your collection is too large to bring indoors in its entirety, grouping those that would be moved indoors together or marking them with a special, brightly colored, easily identifiable tag can save time and plants as temperatures drop. Plastic sheeting or tarps can protect indoor surfaces and furniture from damage from dirt and dampness that comes in with the orchids.
The positive side of the occasional winter chill is that, in many cases, cool periods help induce or enhance bud initiation and flowering.
The Basics – Buying Plants and Building Your Greenhouse to Withstand Winter
Preparation for weathering winter’s chills begins at the point of purchasing your orchids. Choose carefully for the growing conditions you are able to provide your plants. If you keep your home or greenhouse cool in the winter to save on heating costs, avoid warmer-growing genera such as Phalaenopsis and Vanda. Instead, stick to intermediate- (most Brassavolas, Cattleyas, Dendrobiums, Epidendrums, Laelias or mottled-leaved Paphiopedilums) or cooler-growing (Cymbidiums, Masdevallias, Miltonias, Odontoglossums, Oncidiums, plain-leaved Paphiopedilums or Sophronitis) orchids. White or yellow Vandas, as well as some Dendrobiums (phalaenopsis-type and antelope-type), are especially cold sensitive, do not like temperature drops below 60 F (16 C), and can be particularly prone to losing leaves when exposed to cooler temperatures. Seedlings and immature plants, particularly those in flasks or compots, are also much more sensitive to chills than their mature counterparts. While selecting cooler-growing plants may work for winter, do keep in mind that summer gets hot, so be sure to plan for that. When adding to an orchid collection, choosing carefully to match the conditions you are able to provide as a grower will go a long way toward keeping your orchids in optimal health through winter’s gloom and chills.
Properly planned, a greenhouse may be constructed and oriented to minimize the effects of winter’s chill on your orchids. The use of twin-walled instead of single-layered glazing materials adds insulation and reduces heat loss through the greenhouse exterior. Double-layered materials are also better able to withstand the weight of accumulated snow in the wintertime (but never allow the snow to remain on the greenhouse or it may cause damage). Attached greenhouses (those with one or more walls attached to a building), partially excavated and earth sheltered greenhouses retain heat better than freestanding models, and so cause growers less difficulty and expense to heat. Because one or more sides are opaque, attached and earth-sheltered greenhouses do not generally receive as much sunlight as a freestanding greenhouse.
The direction of the greenhouse roof relative to the sun’s movement allows growers to take best advantage of available light. During the cold months, to optimize winter growing conditions, an east-west orientation will allow the greatest amount of sunlight to reach your orchids through the shortest days of the year. This must be balanced with the needs of your orchids, the layout of your property, trees that might shade the greenhouse, etc. For maximum year-round exposure, a greenhouse with a north-south orientation is a better choice.
The next step is critical. Provide “backup, backup, backup and alarms,” says Jan Szyren, horticulturist and greenhouse coordinator for Michigan State University. As Szyren notes, an alarm system is critical for notifying growers if the greenhouse temperature should fall below a certain level, as is a backup heating system. Supplemental heating need not maintain optimal temperatures, but protect the orchids from cold damage during a power outage or unusually cold temperatures. Should disaster strike and temperatures drop, try to keep your growing area above 45° F (7° ) at minimum. Gas- or diesel-powered generators, generator fuel and long, heavy duty extension cords for heating appliances and fans to distribute the heat are available at most home-improvement centers to heat your greenhouse during an emergency. Automated systems are available; manual systems are less expensive, but require that someone be on hand to operate them.
Kerosene or propane heaters are another option for supplemental heat if the power goes off, but their fumes can damage flowers quickly, and to a lesser extent affect plants as well. In addition, these should never be used in attached greenhouses, as the fumes are toxic to people and animals. Greenhouses in which this equipment has been used should be ventilated before being reentered.
Ann Jesup of Bristol, Connecticut adds to and emphasizes the importance of seasonal preparation — “going over the greenhouse to check for and seal any air leaks, have the alarms, furnace and heating system tested, and,” she adds, “pray that the temperatures are not going to drop (in the greenhouse, anyway ...).”
The Basics – Day Length
Another consideration for growers in northern latitudes is the shorter day length winter brings. Many orchids need 12-14 hours of light every day to flower successfully. Relying on daylight alone during winter months will not provide sufficient light to induce budding, especially for the higher-light-demanding orchids like Cattleya or Vanda. Genera with high light requirements may produce healthy green foliage but without sufficient illumination might never reward their growers with the desired flowers. For indoor and greenhouse growers in the north, supplemental lighting may be necessary for optimal plant health and flowering. Depending on the type of light source chosen, the heat generated by the lamps may help counteract cold temperatures.
There are some orchids that are short-day plants, and will only bloom if given less than 12 hours of daylight. Do your research on what you have to know for sure, and give those conditions as needed.
The Basics – Dormancy and Water, Fertilizer, Air Movement, and the Rest
Cultural factors to consider in keeping orchids happy through the winter months include less frequent watering and fertilization. It is worth the extra effort to read up on the seasonal needs of the orchids in your collection. This is especially important for the species, and if you can find the ancestry of your hybrid, will benefit them as well. Some orchids enter a period of partial or full dormancy; they will require a rest period at this time of year and could be adversely affected if they do not receive one. As temperatures drop and daylight is reduced, their growth slows down or even stops, depending on the species or hybrid. The quantities of water and fertilizer that were required for spring and summer growth are no longer needed and could negatively affect the plant unless reduced or eliminated accordingly. For example, some Maxillaria may refuse to bloom if they are fertilized in the winter, as they will think that summer has never ended.
Cool and wet conditions can accelerate the breakdown of organic growing media (such as pine bark and sphagnum moss) and lead to root rots and plant rots. For most cases, simply reduce the frequency or the volume of water until you start to see the new growth or initiation of buds in spring. For orchids that have a water-based dormancy such as Catasetums, in greenhouses, some growers simply stop watering. The humidity in those greenhouses is enough for the plants to stay alive and harden off until spring. However, (and this is where a lot of newbies get it wrong) indoors, you MUST keep watering, just either with lesser volume or frequency of water. Indoors, if you are getting a water-based dormancy right, the pseudobulbs will wrinkle a bit, but the plant won’t shrivel to death. If the plant shrivels a lot, just add a little more water to keep it wrinkled until spring, but not so much that it fully plumps back up.
It's a general best practice to stop fertilizing in winter unless an orchid is winter-blooming. A surplus of fertilizer can accumulate in the growing media until the fertilizer salts burn the orchid’s root and leaf tips, and actually inhibit rather than enhance growth. Additionally, for fertilizer-based dormancy, like some Maxillaria and some Cattleya, simply stop fertilizing.
Although winter requires preparation by the orchid grower, the shorter days are exactly what initiates bud development for seasonal species such as Cattleya trianae.
The Basics – Indoor Winter Preparations
Because they have less control over the quality and amount of lighting reaching their plants than under-lights growers, windowsill growers face special considerations during the winter months. The day length is shorter in the northern latitudes, so supplemental lighting may be needed for part of the day to maintain a comfortable day-night balance for the plants. At this time of year, the sun’s angle is lower on the horizon and may no longer be shaded by trees that have lost their leaves for the winter. A fresh snowfall can also reflect a great deal more light than normal, which may help some plants with blooming.
It's important to note that while ambient chill by a window is tolerated by most orchids, a cold draft is not. Cold moving air is death to most orchids except maybe Cymbidium or certain high-altitude Pleurothallids. If your home is heated to about 65F-75F, then no preparation is needed. However, for super chilly nights or drafty windows, a heavy curtain between the window and your orchids can act as an insulating barrier to help keep the nighttime temperatures from damaging sensitive tropical plants. Thermopane windows will also help — their two panes of glass separated by an air pocket are good for both orchids and heating bills in the wintertime. Even sheets of bubble wrap covering the glass will help keep warmth in and winter chills out. Never allow the foliage to touch the glass; condensation can freeze on the windowpane and kill your orchid’s leaves. If buds start falling off of a Phalaenopsis inflorescence the first thing to suspect is a drafty window (second thing is ethylene gas from a nearby heater).
The loss of humidity as a result of forced air heating can take its toll on your orchids. Remember, most orchids grow optimally with 60%+ humidity. If your home is especially dry, and you will know by the way your own skin reacts to winter, provide extra humidity for your orchids. A tried and true method is to set orchid pots on trays of pebbles that are filled with water; just make sure your plant is not sitting in the water. Misting a couple times a day or getting a humidifier pointed at the plants can also help. If your apartment is small, you can run a hot shower with the bathroom door open to raise the humidity of the entire apartment. While it may not be the most efficient use of water, it most certainly will also help flush out your sewage pipes!
Additional Reading
Batchelor, Stephen R. 2001. Your First Orchid. American Orchid Society, Delray Beach.
Giacomelli, Dr. Gene A. “Greenhouse Structures.” Agricultural and Biosystems Engineering Department, Controlled Environment Agricultural Center, University of Arizona.
Jones, Susan. Cold Damage. Orchids 72(3): 180–181.
Light, Marilyn H.S. “Conservation Part 25: Orchids on a Windowsill.” OrchidSafari Archives, July 1999.
Slump, Kenneth. 2001. Protecting Orchids from Cold. Orchids 70(8):753–757.
Wright, Jane. 1994. Orchid Pests and Diseases. Web article from book Growing Orchids in Canberra. P. 56.
Susan Jones was the editor of Awards Quarterly and assistant editor of Orchids. American Orchid Society, 16700 AOS Lane, Delray Beach, Florida 33446
