All fingers and just thumbs

In the hostel back in Nelson there was a sign saying 'Do you have a green thumb? Then help us out in the garden..'. And in other places I've heard mention of this green thumb. It appears that the Canadians and the Americans refer to having gardening prowess as having a green thumb. What use though is a green thumb? A thumb is pretty useless in the garden on its own. The inventors of this language have it right again, for there's a lot more you can do with green fingers!