A miniature greenhouse can be as simple as a clear plastic dome that you can move around the garden as needed. It can be constructed from one-inch flexible landscape tubing and a few stock PVC fittings. Keep several domes on hand for quick shelter for any vulnerable plants if your area is subject to unseasonal temperature variations.
Cut the flexible landscape tubing into five-foot-long sections.
Apply PVC cement inside one hole at a time in the 4-way connector. Apply PVC cement to the end of one of the five-foot sections of flexible landscape tubing. Insert the tube into the hole, wiping away any excess PVC cement with a clean rag. Repeat for all four pieces of tubing. This is the frame of your mini greenhouse.
Apply PVC cement inside each of the PVC end caps, one at a time. Apply cement to the final half inch of the ends of each leg of the mini greenhouse frame, and insert them into the PVC end caps one at a time. Wipe away any excess PVC cement.
Position the 8- by 8-foot square of clear 3mm plastic sheeting over the frame so that the four corners are even with the four legs of the frame. Adjust the sheeting until any overlap along the legs is even at each end. For example, if there is a two-inch overlap at one end, the opposite leg should also have a two-inch overlap.
Beginning at the top of the dome, working down one leg at a time, staple the plastic sheeting to each leg every two inches. Wrap any overlap at the end of each leg around the leg, and staple it in place.