A cool down period is a night time temperature of 10 to 15 degrees cooler than what they receive during the day.
The length of the cool down is usually two to three weeks in the fall. If you are like me, you will give this to them without trying. I wait until the last minute to light those furnaces in the greenhouse.
If you are growing your orchids indoors, you can meet this requirement for most orchids by simply turning your heat down at to 55-60 Fahrenheit degrees at night (According to the power company, you should do this anyway to save money).
If you try and grow your orchid at 70 oF degrees day and night, chances are you will get beautiful foliage but no flowers.
Have you heard the terms "cool down period" or "sunburnt leaves" and would like to learn more about them? Temperatures play an important role in growing orchids.
Like most plants, orchids have their limit as to how hot or how cold they want to be. Each genus and each species have their own unique preferred temperature range. You need to learn what the proper temperature range is to successfully grow your orchids.
Many orchids like the Paphiopedilum, Phalaenopsis, and Dendrobium species and their hybrids will not flower unless they receive a "cool down period". The minimum length of the cool down period is usually two to three weeks in the fall. If you are like me, you will give this to them without trying.
I wait until the temperature in my greenhouse drops to the lowest safe level possible for my orchids before I start the heaters.
If you are growing your orchids indoors, you can meet this requirement for most orchids by simply turning your heat down to 55 °F (13 °C) to 60 °F (16 °C) degrees at night.
For those of you that live in an area where the temperature stays close to the same both night and day, you will have to try and cool down your growing areas by some artificial means such as an air conditioner.
The very best tool for checking your temperature is to use a maximum-minimum recording type thermometer. Mount the thermometer as close to your plants as you can. Keep the thermometer out of the direct sunlight for the best readings.
LOW TEMPERATURE
Orchids exposed to freezing temperatures and below will cause ice crystals to form in the plant tissue killing the effected area. Depending on the extent of the damage, the entire plant may be killed. Temperatures above freezing and below optimum temperatures may cause less damage, but damage will still occur.
How do I know if the temperature is too low?
If your orchid has been subjected to freezing temperatures, the plant, especially the leaves, will turn dark and mushy.
An orchid placed near a window exposed to freezing temperatures may receive damage to only the leaves that may have been touching the window.
Orchids grown in too low of temperature, but not freezing, will become very weak over time leaving the plant incapable of defending itself against attacks by fungi and bacteria.
What can I do?
If you are growing your orchid outdoors, watch the night-time temperature.
If the temperature is going to be below 50 °F degrees for more than two hours bring your plants indoors or protect them by covering them with newspaper, burlap, a light blanket, plastic bags, bubble wrap, etc.
Even the cool growing orchids do not like temperatures much below 50 °F degrees. Simply covering them will not protect them if the temperature drops below 40 °F degrees.
Keep in mind some orchids, like the Cymbidiums, if accustomed to living outdoors, can stand colder temperatures. I have left my Cymbidiums outside until it was just above freezing (It has even snowed on them). As long as the pseudobulbs don't freeze, the plant will be just fine.
I also grow Bletilla striata here in Utah outdoors and it comes up every year.
On the other hand, a night of temperatures below 55 °F degrees can kill a Phalaenopsis.
If you are growing indoors, be sure your plants leaves are not touching the window glass (see above). You can also add sheets of bubble wrap to the windows to help insulate them.
Turn up the temperature if you are growing them in a greenhouse or in a room that has it's own heat source.
HIGH TEMPERATURE
Temperatures that are higher than your orchid's preferred range can also do as much damage as too low of temperature. High temperature, in combination with intense light, is extremely dangerous.
How do I know if the temperature is to high?
If you notice yellowing or blackening of the leaves, they could be burning.
The leaf tips are brown and dieing back.
Leaves are dropping off, and the plant produces malformed growths and leaves.
Some orchids like Masdevallias will just fall apart.
Orchids that are kept too warm at night become weakened, resulting in weak and leggy growth.
What should I do to lower my temperature?
As you may have expected, reducing temperature is much harder than raising temperature.
If you are growing indoors and do not have air conditioning, move your plants outdoors under a covered/shaded patio.
The night temperature will be much cooler outdoors than indoors.
Increasing air movement with fans will help the plant increase evaporation from the leaves, which is how the plan cools itself.
Reducing light will also help. Sunlight is a great source of heat (infrared radiation) on your plants leaves.
Lower the amount of time your plants receive full sun. Add shading where necessary.
Try and grow orchids that will be happy in your conditions.
If you live where it is always warm don't try and grow the cool growers.
IDEAL TEMPERATURES FOR ORCHIDS
COOL
Max. Day 80 oF (27 oC)
Min. Night 50 oF (10 oC)
INTERMEDIATE
Max. Day 85 oF (30 oC)
Min. Night 55 oF (13 oC)
WARM
Max. Day 90 oF (32 oC)
Min. Night 60 oF (15 oC)
NOTE: These ranges are based on the genus as a whole, many species have different tolerances. Hybrids produced by crossing cool-growing genera with intermediate growing genera may grow well in either temperature range.
Linda's Orchids on CDs 
, Phalaenopsis