http://www.permies.com/t/12206...
according to this forum there are certain species that might cause problems in hugelkultur beds. My own experience is with black walnut which has a reputation for killing plants placed adjacent to it. Here in rural Alabama I found a pile of sawdust probably at least 100 years old. I thought I had a gold mine--but when I used it for mulch around some Asian lilies, my lilies died one by one. I later found out that the main constituent of the sawdust pile was black walnut. Later I bought a house with a black walnut tree in the yard (supporting a basketball hoop). I had the tree cut down. Since then the stump has been colonized with various plants--blackberry, and sand pear especially like that area. So some things don't mind growing in soil where a black walnut has been and others don't.
There are a few other trees that "poison the soil" similar to the black walnut. Cedar and black locust are mentioned in the above forum.
