The white and purple forms of Honesty (Lunaria annua) self-seed freely around the garden. I introduced a few of each in years gone by and now they pop up wherever takes their fancy.

 L. annua is (despite the name) a biennial in my experience; perhaps it grows quicker if conditions are favourable? Introduced from Europe it has naturalised in the UK and I'm very happy to let it fill in any bare spots of soil. The rosette of leaves adds greenery in the first year of growth and the flowers are very pretty the following spring. After flowering the coin-like seed pods start off green then dry out to a papery pale white in late summer/early autumn. 

 Hence the Americans call it Silver Dollar Plant. Some say money is the root of all evil so I'll stick with Honesty.