I've never used hay or straw in a proper hugelbeet, but I have grown plants in straw before - I have not grown in hay, specifically, so I don't know how that might differ.
The method I use is:
- Arrange your straw bales into a keyhole garden, or whatever other pattern you like, (personally, I like to stack two bales on top of each other - the first time I did this, I only used one, and the raised bed effects declined midway through the season)
- Throw some soil and/or
compost on top of the straw
-
Water it daily - I collect my
urine and distribute that over the straw as well
- Plant your items of choice, (I've tried both seed and transplants - seeds do quite well, I find)
- Mulch your plants with fresh straw throughout the season, and if you want to continue, apply new bales over the top to decompose through the winter and deepend the resulting soil. (You can start your straw planting beds the year before you plan to plant them, this way, and get ahead of the game, but if you do that, definitely stack at least two layers of bales. I've never done more than two layers myself, so I can't comment on what it would be like using more.)
I take for granted that a hugelbeet would perform in roughly the same way.