Welcome to Garden.org,
@Terrible !
I haven't watched the videos for the PVC greenhouse project -- the National Gardening Association website was recently merged with All Things Plants, and many of the NGA features are brand-new to a lot of us long-time ATP members -- but I did locate them in the "How-To Videos" section that can be found in the blue sidebar on the left-hand side of the page.
However, I did see that there is a materials list for the project (which can be downloaded here:
http://garden.org/howtovideos/... ) that calls for 12 pieces of 36-inch rebar.
The framing diagram (shown here:
http://garden.org/howtovideos/... ) indicates that the greenhouse is 14x20 feet in size. I believe you would first build the 14x20 wooden base frame, then pound the rebar into the ground just inside of that (so, approximately 14 feet apart for the width of the greenhouse).
I hope this was helpful for you; and I'm happy you asked this question, when I have a little more time I plan to take a closer look at these videos!
Sandy
