How can I prevent my hoop house from being uprooted by strong winds?
I'm not sure if you have already built this or not, but ideally you should build footings just like you would for a deck or a fence. How should I build the concrete footings for my deck? is a particularly good example.

Essentially, dig post holes down past the frost line or whatever is suggested in your area. The holes should be bigger (mushroom out) on the bottom. Set your posts and fill the hole with concrete. Now as long as you have a solid way to attach everything to the posts, it's probably about as strong as you can get it.