I think your greenhouse would work well. What I do is close the heating vent and open the window in the orchid room-actually a bedroom, so my plants get the cool temps at night but are protected by not being outside. I let the room cool down to the mid-50's at night and then back up to the 70s in the day. 2 to 3 weeks of this is probably enough to get them to spike. If my plants were outside I would not want them to get below 50 degrees.