The Swiss Federal Ethics Committee on Non-Human Biotechnology's report, The Dignity of Living Beings with Regard to Plants. A sample:
The Committee members unanimously consider an arbitrary harm caused to plants to be morally impermissible. This kind of treatment would include, e.g. decapitation of wild flowers at the roadside without rational reason.
Conclusion 3 expresses the different moral stances according to which it is unanimously held that plants may not be arbitrarily destroyed, in accordance with Conclusion 1. As 3 shows, the majority considers this morally impermissible because something bad is being done to the plant itself without rational reason and thus without justification. A minority considers this treatment to be impermissible as well, but for another reason: because this destructive treatment of a wild flower expresses a morally reprehensible stance.
There's much more. Thanks to Manny Klausner and to this Weekly Standard article for the pointer.