A  There are two important factors I believe will influence whether your cattleyas will readily withstand the temperatures described. 1) The nutritional status of the plants — you should reduce tissue nutrient levels fairly drastically and harden the plants off. Fertilizer should not be applied at every watering; once a month would be plenty, and make sure the formula is low in nitrogen, which should be nitrate nitrogen and not urea or ammonium nitrogen. 2) Watering should be reduced, with an extended interval between waterings. Water only when conditions are sunny but preferably not frosty. This means that if you have damp, cloudy weather it is not detrimental to wait three to seven days past the normal watering date. Cattleyas will be little affected over winter by being on the dry side. Also, schedule any watering as early in the day as possible so the plants are well dry be fore nightfall.

I would add that if your greenhouses are vented, close the vents quite early in the afternoon and let the heat build; even temperatures in the 80s are not likely to damage the plants over winter. That gives you a reservoir of heat to carry into the night. This should not be done on the day that you actually water. — Andy Easton.