Mad Cow has brought this to us :
Groundsel is an unassuming little weed that grows wherever it can
find a space. Nowadays it's thinly scattered over every continent
including Antarctica. The secret of its success is that the small
plants grow and seed very quickly: it can easily have three
generations in one summer.

Groundsel (Baccharis halimifolia) is a declared class 3 noxious weed,
where I live.

--
Barbara

http://www.flickr.com/photos/barbara-h/