One of my neighbours has been growing spuds just in grass clippings for the last couple of years!
We have quite a lot of communal grassy areas and she's been blagging all the clippings from that. She just lays the seed potatoes on sheets of cardboard, covers with clippings and adds a load more once a week or so as the grass gets cut. She's had some amazing spuds from it and the ground underneath has had weed suppressant on it for a year - so she's improving some horrendously weedy land as well.
Seems like a really nifty idea - although I have some reservations about nutrient deficiencies. I find it hard to believe that grass can provide all that's needed (unless it comes via a cow of course

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