The main problem I have with the Green party is that being environmentalist and left wing, it plays into the hands of the anti-environmentalists of the right who try to portray environmental issues as "lefty" issues. Though I suppose that's not really the Green party's fault.
As for zero growth, infinite economic growth on a finite planet is not sustainable or possible long term in its current form. Maybe Green proposals aren't brilliant, but they have correctly identified quite a big problem that should not be brushed aside. Why should the environment always come in second to "economic growth", which is how it is at the moment?
I actually find that the Green party's policy to give prisoners the vote and its frankly idiotic foreign and military policy (Lucas voted against air strikes on ISIS in Iraq) are far bigger problems for me.