and found another inspiring photo from Aranya Gardens, also in the UK:
They're overturned wine bottles! I figure it would take over 700 bottles to do all the beds. I do like wine, but I'm afraid even the entire winter's drinking wouldn't generate this volume. So I'm just going to edge the herb circle (a small section that I'm planning in the middle of the beds). I figure about 100 wine bottles will do it. Between my wine drinking friends and I, we can get those together within a few months (and I'll be volunteering to take out the trash at all the holiday parties this year.)
Is it not the most beautful newspaper-covered balloon you've ever seen? Soon to be a pumpkin head to be worn in Saturday's Halloween parade.