Standard practice uses pounded in posts. Commercial greenhouse catalogs often offer options for either base plates that are set in concrete, or pound in posts. The decision usually comes down to whether they have a concrete floor interior. Container based operations like concrete floors as it makes equipment easier to move, and everthing stays level. If you are growing in the ground, the pound in posts are cheap cheap and easy to set up. That said, most greenhouses are set up where there is at least some shelter from wind, if nothing else than to reduce the heating costs.
I have just finished taking apart a greenhouse and moved to to my site, which has some additional wind exposure. Pulling the posts took a front end loader.
Remember too that a wind is going to put a sideways as well as an upward force on the frame. The sideways component will increase the frictional force needed to uproot the post. (Don't believe me? Pull a fence post with a front end loader with the chain 30 degrees off vertical.)
Two easy ways to hedge your bet:
Drill a couple of half inch holes in each post. Put bolts through them extending a couple inches out each side. (Or sucker rod.) Repaint with rust resistant paint. Now you are dragging more junk through the soil.
Concrete for this application can be very low cement in content. Use a foot of washed gravel around the base of your post, mix up a slurry of 2 parts cement 1 part water, and as much gravel as it will wet (It will be about 8 parts gravel) Dump into hole. the surplus cement water, will coat some of the rock below.
More information on this in Kern's "The Owner Built Home" on 'no fines' concrete.
Finally, in your description, you omitted a base board on either side for anchoring your plastic cover.
Most accounts of greenhouse collapse in my area (Alberta) have been due to snow loads, rather than wind loading.
Note: I'm not an engineer. Your millage may vary. With my recycled green house I am going to pound the posts in with a sledge hammer.