I have decided to plant one this year at the far edge of a pond. I am near the coast and the soil there is very hard packed sand, rocks and what looks like clay.

I need to dig out a large enough area and fill with good dirt for it to be happy. I realize that I have to dig deep enough so the water will wick into the bed but I am wondering how wide or what circumference it needs to be. Also does anyone know what the ideal depth is?

I want to start getting the bed ready in advance. Thanks