Well - thank you everyone for so many very informative & interesting replies & ideas. A huge help to me...

I'm not too keen on using chemicals, so will probably start by letting nature takes its course with the finished sculpture, as an experiment! It will be under a tree, so it could get interesting!

The salt water idea appeals if I decide that green would be better than whatever occurs naturally - the thing should be moveable, so I could "douse" it away from any plants that may not be so keen on a salty soaking!

Again thank you all. It's so wonderful that people are so willing & generous with the sharing of knowledge. I don't know what life would be like now without the internet & forums such as this...

Kindest regards!

Chip