Actually, we have a small greenhouse whose frame is made of PVC. It's an octagonal dome we call "The Igloo". (My wife dreams up weird projects and then makes me figure out how to build them.)

We also have a much larger greenhouse made from the tubular steel frame of an old 10' X 20' portable garage.

Both are currently covered with screen cloth for shade and insect control. The clear plastic will go on around Halloween.