I would like to convert this to a bog garden by adding a couple of inches of pea gravel, then filling the rest with triple mix (1/3 each of soil, peat and compost). I could also add sand to the mix if advisable.

The location is best described as having some sun in the morning, and dappled shade the rest of the day.

I am planning on doing this in the Spring. Plants on my list so far include some tropical punch from alocasia and colosia, maybe a canna, plus native cardinal flowers and a few sedges (Ice Dance and Bowles Gold). Maybe some horsetail, Jack-in-the-Pulpit and a Ligularia (Desdemona).

Any thoughts, suggestions, advice?

That said, you could do a nice wetland garden. You might plants that like somewhat shady and "mucky" conditions like skunk cabbage, or false helebore. I'd also recommend using the blue cardinal flower instead of the red since the Red prefers more sun. Also try marsh marigold which can take shade.

Good luck!

Lonnie

Bogs are very specific ecosystems which are sunny and acidic, with very little disolved minerals. In fact, even most people's tap water is too dirty to use for a bog garden. Lining it wouldn't be for the sake of making it water tight, but rather to keep the concrete from leaching minerals.

That said, given that you have a shady location anyway, I wouldn't go for a true bog garden anyway. Instead you might go with something approaching your original plan of a richer soil mix and swamp/wetland plants.

Also, I'd add wild calla to that mix of plants I suggested as well.