I used these products link below...call email Frank from that link..he is one of the most helpful people I have met in the industry

Here is a link that might be useful: shade cloth

Hello
Just use any of the shade cloths they have at the nursery. Various degrees of shade; they will have from 10% up to as high as 70% or more. Always shade on the outside. For you people that have the smaller greenhouses ... under 100x20, have lots of exchange air or the roof open up. Small greenhouses change temperature very, very quickly.
Below 50F most things don't grow really and over 85-90F most plants don't like that either and won't thrive that well at all.
Hope this helps.
sunny_canuck

Here is a link that might be useful: esoteric

"Below 50F most things don't grow really and over 85-90F most plants don't like that either and won't thrive that well at all."

Usualy I see it in referance to soil temps, but I think it is used interchangeably, although I have heard that the canopy can get colder as long as the rootzone is kept warm...

Ok I have seen this or similar statements numerous times, but I never seem to be able to get an answer to my follow up question, perhaps I have always just missed the answer...

Just how bad is it for the roots to get below 50 (or 55, or 55.4, or whatever the critical temp is)... so the plant stops growing if the roots get too cold, but for how long? is it only for the couple of hours that the roots are that cold? in which case, I do not really mind if my plants are not growing from 4 am to 6 am.

Or is this going to cause my plants to go Dormant, and they are not going to grow again for 3 more months?

Jimmydo2,
The basic answer to optimum temperatures obviously depends on the plant in question. For many of the plants we grow, they are from temperate zones and do best within the range of temps you quote. The 55.4 F idea is from research showing how to get the absolutely highest yields from say a tomato plant. Going to 50 F will probably reduce yield somewhat, but the plant will survive. Another very important factor is how long the plant is subjected to lower than optimum temps. I think you are correct, in general, to not be concerned if your plants are not growing from 4 am to 6 am, provide the temps are not so low as to cause other problems. This assumes you are willing to accept a 0.67% yield loss on your tomatoes....

Well it is not tomatoes, it is Citrus, I guess I can have even a 1 fruit yield loss per plant ;). So, I guess I could go back to just worrying about keeping the plants from freezing, rather than trying to hold 55 Degrees :)

since even when we have a "Hard Freeze", the really cold only lasts about 3 hours

Hi dbwc -

As you can see, I live in Michigan, and I grow orchids, too. I finished my greenhouse 2 years ago, and bought some shade cloth at that time.

I have to tell you that I found out how to shade my greenhouse by talking to the professional growers in my area. Shadecloth not only cuts back on light, but also works to keep the greenhouse cooler. Here in Michigan, the time to roll the shadecloth up is when the sun is out and the vents don't open, sometime in November, usually. And it usually goes back down in March.

The professional orchid growers will clue you in to what % shade you need, and how to best use it in your area.

My shade cloth is installed under the ridge vents so it does not interfere with their operation.

I hope I was able to help you a little bit. Good luck!