Crazy that they call that a poison garden. When I read the title I was thinking they'd be growing poison ivy, manchineel trees, sandbox trees, and other things like that. Plants that you would want to keep people from touching because they might actually cause physical harm.
I guess if I was legally growing a bunch of weed, opium, coca, and some khat I wouldn't want tourists putting their dirty hands all over them. Wonder how often they "prune" their plants.