A thick layer of cardboard works too, tape removed, and covered in a thick layer of mulch. Blackberry crowns should first be dug and the soil sifted for root remnants. Besides that, it works for pretty much all herbaceous weeds except quick grass (stoloniferous/rhizomatous jerk). Cardboard will breakdown so should be replaced annually, but this will eventually improve the soil. Do a search: "Sheet mulching" Travis Owen Rogue River, OR amateuranthecologist.blogspot.com