Wow you did a good job with the pillars. I've seen some GH makers recommend mounting these on decks or even 3" concrete pads. One thing to remember these are cheaply made greenhouses [due to low price]. You could bolt these down to the earth's core but a 70-mph wind probably rips the GH away from the foundation. I have a 10x10 Lowe's plastic shed. I made a platform of 2x6's screwed the shed to that and used screw in ground anchors on the corners. It's been rock steady for over 5 years in storms with over 50mph winds. Most hobby GH's are considered temporary structures and don't require proper foundations. Your biggest issue will be heating a free-standing GH with twinwall poly to 65 degrees in your zone.
