my first clue as to whether it was a weed .. may have been that it is within inches of the wall.. and probably straight down from the eave/gutter.. where the bird deposited it for you ... or the wind stopped and it fell from the sky ... ken Here is a link that might be useful: ah.. yes.. windborne ...

Maybe Senecio vulgaris, common groundsel??? If so, pretty early for bloom although it is a spring bloomer. Can be tough to control as it has developed resistance to many products. Contact your local extension service for the right treatment. hortster Here is a link that might be useful: common groundsel

Top is Chelidonium majus - Greater Celandine Below is a Symphytum species - Comfrey.

Thanks, floral_uk. That seems like a strong possibility, though none of the flowers I'm looking at seem to open as much as Cardamine's flowers. Maybe I need to give them a few days. The plants are about 9" tall which is about the same for Cardamine. I see this Cardamine is sometimes called "Sand bittercress" which would accord with these plants growing in virtual gravel. Here are several photos I just took (some of which are virtual duplicates but they look beautiful!) I'll get a good photo of the basal leaves later today.