Blooms purple the morning after rain or sprinklers running, all around the corner of the solarium. Blooms drop after a day.
Plant stands 1-3ft tall. Single stem with opposite leaves, flowers from cluster of new leaves at top of stalk. Usually only one 5-petaled flower per plant at a time. It can handle a light frost but dies back to the ground in a hard freeze. It’s trying very hard to spread; new stalks are popping up around the stepping stones.
My best guess is some sort of wild petunia – Ruellia brittoniana?