Pen wrote:
A picture's worth a thousand words. There's too many possibilities...

Tell me about it!

I can spend all day guessing but I was just thinking of bird's foot
violets (Viola pedata). It loves gravely soils and it's almost
impossible to transplant once established. Here's some images:

http://www.ct-botanical-society.org/...violapeda.html
http://www.ohiodnr.com/dnap/heritage...a%20pedata.jpg

That second one is uncannily similar. Not quite, though.

With the help of an elderly lady on the block, we seem to have a winner,
though -- an obscure variety of hibiscus. I always thought the leaves
were fatter, more like strawberry leaves, or the Girl Scout logo. (Okay,
the 'laevis' Hibiscus comes closest to what I know.)

The one we have is somewhere in-between the 'wild hibiscus' and the
ha-ha-only-serious 'hibiscus cannabinensis':
http://plantsdatabase.com/showimage/547/
http://plantsdatabase.com/showimage/46271/

The lobes we have are much more like -- oh, try this image of a "true"
Maltese Cross, where the fingers radiate outward and there are
indentations at the ends of the fingers:
http://www.netpages.free-online.co.uk/gms/sinister.htm

Anyway ... it looks more like a garden escapee, now, than a native
wildflower. But boy, is it plucky (judging by where it's decided to grow).