Valerie, I looked up a few things about Dahlia coccinea var. palmeri. At a vendor near San Francisco Bay, it made mention that tubers could be safely shipped, through May.
Perhaps it originated from a rather cool, high elevation in a mountain area of MX. The tubers being of smaller size & yes, later to start. (I've no idea if it can be "forced", to begin growth any earlier, or if it just risks rot.)
Another site mentioned that is /was nearly indistinguishable from your 'Bishop of Llandaff', (Hybridized in 1927 ?) While I'd imagine, you are easily able to differentiate the differences between the two.
Although I quickly noticed that the Bishop is a semi-double flowered one, & award winning, too. While looking in Wiki, saw the D. coccinea has numerous synonyms, I couldn't find the locality type for any of them...
I would suspect there are a number of herbarium specimens kept somewhere, under at least some, if not nearly almost all of those synonym names.
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Apparently the "Bishop" is a good parent for dark leaved types.
I gather, it could be grown in a fairly good sized pot, if you've a good sized clump, with the pot being of a complimentary color, to the quite deeply dark foliage. Top dressing with compost was recommended, & I suspect, it actually prefers cooler summers. HTH
Scarlet, unfortunately my potted Dahlia is much farther along with growth, & due to leaving another pot above it, looks completely unnatural. I'll have to keep it shaded until it has started to get well on its' way with photosynthesis. (I really ought to have disturbed the tubers, prior to storing in the pot, & failing to check on it at the start of this month, would have been best. As I didn't notice a thing, around about mid- January.
Next, I may possibly try to carefully trim a few tips, or just attempt to eventually cover up, lower growth by lightly mulching it.
I can't believe I've nearly done the very same thing, going on the second year with this one, now.