__ moss (garden mulch) (4)

I believe the answer is:

peat

(Other definitions for peat that I've seen before include "Vegetable matter in bogs, like turf" , "Vegetable deposit used as fuel and fertiliser" , "Natural solid fuel" , "Dried plant material used as fuel" , "Fuel from bogs" .)

I've seen this clue in the Newsday.com and the Newsday.

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