It depends on the orchid and what happens during the day that follows the cold night.

Coldest night, winter just past was1C, day following was sunny and got to about 15C. Autumn flowering Cattleyas resting in the shadehouse appear fine. But i sort the cattleyas so that the ones that flower in winter are in a slightly heated glasshouse.

Continuous cold and damp with not much sun even though the temperatures are higher is much worse for orchids than a few low temperatures at night.

Outside down on the ground next to Den. speciosum were
Cattleya Bactia
Cattleya Chocolate Drop
Cattleya x Schomburgkia cross.

They are just starting to grow (spring southern hemisphere) so i'm moving them back to better growing quarters at the northern end of the shadehouse now that the sun is moving south and the northern end is no longer shaded by the boundary fence.

Everyone has a different take on this issue.
Freezing is dicey. You are running a risk the closer you get to the freezing point. An orchid is like a human, mainly comprised of water. You wouldn't like to be out a night either :-) I would be extremely careful and run them very very dry. If you are new, I wouldn't risk it if you don't have a good sense of what is happening in your zone, some experience and another choice. May I suggest as an alternative for learning, if you are the daring type, to try 1 orchid that you don't care about as the control plant this year, see how it goes. Then next year you can have a better sense of what happens to the plants?

Under cover may also be a double edge sword. Right now I have 'chids outside in full sun and running bone dry. We are experienceing 40-44º nights and I am stressing a bit. They are not. Rather have them in light, especially if there is no rain in sight. Shade, water and cool temps produce rot conditions fast.

Grew outdoors in your zone, watch the weather and be very very careful. You should have good enough ambient humidity (RH) to run dry. Expect them to be trashed and see how you feel about that too. Then make the call. Trucking orchids around stresses them.
Hopefully Olya_Grove will appear... she is a stellar outdoor grower in your zone!

Good Luck
Clara

Hi
Have found over the years that while thwey will survive
temps down to frost it causes all kinds of problems .
If you have a choice I'd wouldn't allow them to go below 50 degrees Definitely don't allow frost to settle on the leaves. Cover them with cloth or frost cloth during cold spells. I use Christmas lights around the trees but be careful they don't touch the plants. Don't use plastic as a cover as the sun the next day will cook them.
It is possible to keep them outdoors but the very least that willl happen is they stress. Definitely nothing below 32 degrees ever.
Your best bet is to keep them mobile and move them into the house at temps below 45.
gary

Catts can endure temps in the lower 50s as long as the roots ARE NOT WET and the day temperature rises. On the other hand, Laelia Anceps and Purpurata will flower with better color if allowed to cool into the 40s. As Claritamaria says, keep them very dry.

Thanks, Clara! :-)

I have nothing to add to what has been said, really...I grow without a greenhouse...but last year, had to create a temporary shadehouse for orchids, and insulated it with blankets/covers...

I start worrying about 50s. Phals, esp. species, phal.typ dends, some of my warm growing species and vandaceous - even though some a more cold tolerant that others - start going inside the garage/house/shadehouse when temps are in 45-50 range.

Catts, laelias, lots of different hybrids - 45-48....

When it getting closer to 40, I am starting to worry about L.anceps, Purpuratas, various epidendrums, certain dendrobiums...

And as it gets below 40, I do not risk it and haul inside cymbidiums, epi. radicans, dend. kingianiums and speciosums,and nobile type dends... at 32, I do not have a single orchid out. Not a one.

What a headache. Florida is a paradise for orchids, but in winter, I really wish I had a greenhouse. I am starting to work on a permanent shadehouse ...a structure that I can insulate for the winter, and have only shade cloth on during the summer...

Good luck. Orchids are fun!
Olya
p.s. every winter, I have at least once a dream (while sleepling) where I realize it is freezing/snowing outside and I did not cover any orchids...hehehe

Thank you for all the advice.

Last night it got down to 43 degrees - way too cold, I'm thinking. Afternoon temp was about 66.

I placed an order for more orchids and asked about minimum temps and the gentleman was kind enough not to scold me, but he did impress me with the need to pay more attention to the night time temps.

So, I spent the afternoon drilling holes, placing hooks and hanging chains so I could move all the orchids to a small enclosed area that I can keep more temperate. The weather is very unseasonable this year - too cold for this early in the fall. We have had 90+ degree days in September in years past, so I am not ready for winter weather.

I just hope my orchids will forgive a couple of nights being in the mid-upper forties - they are dry because it was windy and the pots dried out quickly before the lower temps hit.

So, thank you for all the advice. I will try to err on the side of keeping them above 50 - and happy.

We are scheduled for 45 degrees tonight. Today the greenhouse was closed all day and it got up to 78 degrees with a high humidity all day. The heater will be on tonight set for my standard winter setting, 58 degrees in the top of the greenhouse.

Sunday the heater goes off again and the gh will be open since we are forecast for a return of summer weather.

This transition season takes some paying attention; none of the devil-may-care days of summer or the buttoned-up winter.

I have started moving the dry winter orchids onto one bench so that they will not get watered with the others. Watering will start to slow down all over the greenhouse. I will take the plants out in groups for inspection and staking on days of warm weather.

We might have as much as 6 more weeks of good weather, but when it turns cold it happens fast.

When i started all those 35 years ago in the Glasshouse purchased by my wife. i studiously maintained a minimum of about 12C. Didn't have all that much success because the glasshouse was not positioned in the optimum spot to get maximum all day filtered sun. Anyway eventually ran out of room, as you do, so put surplus cats in the shadehouse where my son grew his collection of Australian native orchids. No heater, but to my surprise many of the cattleyas grew and flowered better there.

Of course you need to do a lot of sorting out and if you get frosts there is not much that will get through winter in an unprotected situation.

As for worrying about Den kingianum and speciosum just because the temperature dipped to 40F. Don't. They will take temps down to 2C, no worries, as will some of the "Cattleyas" such as Laelia anceps.

I think the minimum of about 12C is a good starting point for a general collection but you need sunlight and a daily rise in temperature for optimum results.

I currently have about six sets of growing conditions and cannot grow lots of stuff and i think you would need about ten sets to grow everything.

I cannot help much with the "When to bring them debate" but i suppose at higher latitudes its a combination of Temperature and when the sun has moved south so far that it has lost its punch.

But nothing much will grow in the fully shaded North end of my shadehouse in winter so i to have an annual ritual of moving orchids around.

As for the dry winter orchids, in my collection, Den bigibbum and canaliculatum, i just group those high up in my warmed glasshouse so they get warm day temps and very little watering till spring arrives. Den. kingianum and speciosum are out in the weather and they survive rainy spells without any problem but i think winter sunshine hours average about 6 per day.

My warmed glasshouse is not all that warm, I just run the heater for an hour or so before sunrise so that the day temp.will lift. So most days the min. in there is between 5 and 10C. Had some cold damage the day the outside min was 1C. a few damaged leaves here and there when the min fell to 3C before the heater came on.

I hope these ramblings are of value.

I try not to let the temp get below 36F, if it does I move the plants around to a warmer spot or even inside my home in trays and then put them back out the next day.

I have to say some of the chinese paphs do really well near freezing, but in some place in nature they are exposed to very cold environments. Depends on the plants of course.