Does anyone know anything about gardening?

Are the posts going to be attached to the wall too? If so, I think you'd get away with those.

As a slotted fence allows more wind through, it shouldn't be as flimsy as full panels, so personally I think you'd be fine.

If you know you're not going to run into any rock or large stones driving them in, and have a way of driving them in (sacrificial post and sledgehammer, or borrow/rent a post driver - a post level on your sacrificial post as you drive them in will help give you a more plumb post - you probably won't need a sacrificial post if you are buying long and cutting down, but driving in a post above your head is difficult), then have at it. If you have rocky ground, you'll end up digging anyway, in which case you'll find it hard to pack the earth back in around those firmly enough for them to last and will need to go with post crete any way. If you do end up digging, I used a manual auger and a post hole digger (the big tweezer like scoop things) and it was much easier than I feared. A 36" crowbar helped to loosen any rocks. Some wood preserver on at least the sunken/buried part (especially around ground level) will go a long way. Bonus points for post savers too.