I had a 8x 8 twinwall greenhouse, and it blew over backward within the first month it was up. My husband had his brother make 24" stainless steel spikes that we bolted to the frame and drove into the ground. We put Quikrete in on top of the stakes for good measure.
Even so, the greenhouse only lasted three years. There is just too much weather and wind where I am. This past Spring, I went to a local Amish shed maker and had them modify a shed for me as a greenhouse. Picture attached.
