I do not recommend a free standing greenhouse.

We had a 60 mph wind gust come along one afternoon and it shifted our glass
greenhouse 6 inches. It also blew out about 10 panels completely. If it
were not for the raingutter catching the panels we would have lost every
panel on one side of the roof. We immediately sold the greenhouse and we
are replacing it with a Turner Greenhouse on a real foundation.

http://happymoosegardens.com/stuff/greenhouse.htm
Here is the glass greenhouse that we got rid of after the wind blew.

Bobby Baxter
Happy Moose Gardens
http://happymoosegardens.com