Definition: any of a large genus (Delphinium) of the buttercup family that comprises chiefly perennial erect branching herbs with palmately divided leaves and irregular flowers in showy spikes and includes several that are poisonous
The lovely delphinium grows in the soil but get its name from the sea: it comes from the Greek delphin- or delphis, meaning "dolphin," probably from the shape of its nectary—the gland that secretes nectar.
Likelihood of starring in a rhyming couplet: just a glimmer. It rhymes only with triclinium, but triclinium is a good word.
A. A. Milne featured delphiniums in his poem "The Dormouse and the Doctor," but used the delphinium descriptor "blue" for the rhyming bit. He likely understood that his audience might not appreciate just how good a word triclinium is.