A play on the term "grassroots"; referring to activists, usually Leftists, whose protests are assumed to represent mainstream public opinion but that in fact represent those of lunatics. (See Frank Chu).
The term differs from "econuts" in that the letter is presumed to concentrate on environmental activism, whereas the "nutroots" are more broad-based in their ideological interests. There is some crossover however - econuts are normally assumed to be a subset of the nutroots.
In the UK the term is usually translated as the "loony Left".
That the WTC was destroyed by controlled demolitions planted by Bush administration employees and/or agents of the Mossad is an example of a belief common at the nutroots level.