I heard about this on the radio recently - (Saturday Gardening show with Simon Marnie) in Sydney. They said it was the caffine content which did the trick, not the grounds so much - and apparently there is more caffine in plunger coffee than drip. I wished I had written down the dilution rate as I always have just a bit left over:-)
This was also mentioned on Gardening Australia tonight. Spray it on ground and plants. Is there a website for this programme which may give the 'recipe'?
Thanks Sooze - I just love top tips like that, I think how handy they might be one day, I think that I must remember them, then I promptly forget them ...