What is under the sand.....just raw soil? If so I would mix the sand with bark fines and perhaps a small bit of compost and just plant things that require very good drainage - bulbs, lavender, succulents, ornamental grasses, etc.

I dislike very geometrically defined planting areas (unless a need for raised beds) so I'd be inclined to remove the wood siding, add enough soil to bring up to grade and treat as you would any other area of the yard.

btw, even older pressure treated wood (off the market for more than 15 years) has minimal impact with leaching to enclosed or adjacent plantings so I'd dismiss that concern. Newer treated wood is a non-issue.

What gal said. If you have heavy clay and the wood is still in good shape I ‘d use it as a base to build a raised bed on top of it and bring in a good garden loam to fill it up.

tj

Thank you!! And thank you especially for the insight on wood treatment. Truth is, I haven't even dug under; I was just assuming it's wood on the bottom, too. But time to find out!

The bed looks like it could use a few inches of soil so mix in some topsoil. This bed has so many possibilities. You could plant an herb garden. Or you could put in plants that require a permanent location such as a strawberry patch, or asparagus or blueberries.