i think it's personal preference. there isn't enough pressure on the sides to make it necessary for re-enforcement, but some folks think it looks better to have the posts. i don't have posts on any of my beds. i wouldn't use nails though...i think lag screws or decking screws are much better.

I used deck screws on a couple and some brackets I had on hand for others. I haven't used any 4x4 posts yet, that does cut into the planting space.

HTH,
Gumby_CT

We used 4x4 posts on the corners of our beds for a couple of specific reasons: (1) we had to terrace our beds and dig them into the slope in our backyard, and (2) we made two of our beds 2x10 and we had concerns about the pressure on the sides. I'm not sure if it really would have made a difference. As a caveat, we don't really know what we are doing either - just making it up as we go along.

Also per flatlander above, we used lag screws.

Pictures are in the link below.

Here is a link that might be useful: Our SFG gallery - so far.

If you will look at the posts I have on here concerning building the "different" sfg,you will see that I put my posts on the outside of the "box"- and I installed galvanized corner brackets on the inside of each corner for better stability.Predrill all of your holes for screws,and use deck screws that are intended for the type of wood that you are using.That way,you do not waste any of the internal space.

I went three 2x6s high without posts, though I did "connect" the three boards together with 2x4s every so often. I guess that works the same as posts.

Here is a link that might be useful: Sinfonian's garden adventure!