Yes OO, 2x2 should be physically strong enough for what you describe, but they'll need to be driven about 2ft into the ground. If you concrete, you'll need lots of it to have any effect. The biggest problem is, the part that's in the ground will go rotten after about three years, and not much longer if you treat them with preservative. But if you have loads, it won't be a problem to replace them when the time comes. ;D
Geoff.