Mariken

http://www.waysidegardens.com/gardening/PD/48591/

Extremely dwarf Ginkgo tree.

Alternatives to Ox-Eye Daisy and Dame's Rocket

http://www.coopext.colostate.edu/4dmg/Weed/non.htm

Some good suggestions like black-eyed susan, blanket flower, and woodland phlox.

Land Banks

Thanks to Nancy for this link.

http://www.nytimes.com/2011/08/04/garden/finding-the-potential-in-vacant-lots-in-the-garden.html

Vacant lots are ecological assets.

Weeds

http://www.nytimes.com/2011/07/31/books/review/weeds-by-richard-mabey-book-review.html

Sounds like an interesting book.

Permaculture

Thanks to Nancy for this link.

http://www.nytimes.com/2011/07/28/garden/permaculture-emerges-from-the-underground.html

Very interesting, but it is unlikely that the medieval village government would allow it.