I have this rather invasive buttercup in my garden. Very shiny bright
yellow flowers that blooms in both sun and shade for almost a month in the
spring. It takes a fair amount of work to control it. I always thought
it was an introduced species - I've seen English garden pictures where it
always seems to be paired with Herb Robert.

It does grow in a tumble with other plants in the garden. Is this plant
also a marginal? Is it considered a wetland weed or what? How invasive
is it in a wet situation?

It is one of my favorite spring flowers, but it does take extra work to
control.