Weed/Flower id please...
 

I have a pest in my garden that I just can't seem to get rid of. I think
it's a weed but I'm really not sure. It has small yellow flowers, and when
you break off a stem, it bleeds this bright orange, sticky sap that stains
everything it touches. When I moved into my house last year, it was growing
in the very badly neglected garden. I pulled it all, thinking it was a
flower that I wasn't to crazy about and proceeded to plant my garden. Well
the darn thing came back with a vengeance. I can't seem to get it all and am
a bit leery of using herbicides. Does this pesky fellow sound familiar to
anyone? I'm in Oshawa, Ontario ( zone 5) if that helps at all.

thanx,
Shnarf