Grass Clipping Mulch is great for potatoes (prevents scab) - and it means you don't need to earth them up - put on in no more than 2 inch thick layers at a time. Here's one I did earlier: potato grass mulch on Flickr - Photo Sharing!
It's apparently also good on carrots to prevent Carrot Root Fly (haven't tried this myself though).

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~ What do I think of Western civilisation? I think it would be a very good idea ~ Gandhi