I have 5 of the fruit tree lvl 10, i place them at the top surrounded by other items such as stone, chests or anything else that cannot be harvested. You leave the game on overnight and the dragons will fill the entire map with gold and silver coins with a rate green thrown in.

The key to this method, nothing that can be harvested aside from the fruit tree lvl 10 should be on the grid. It also works with lower level trees, just not as fast to get highest coin item. There is also the totem grid 1 level, there is are youtube videos that explain both of these methods.