Saikei is a derivative of Japanese Bonsai but has its history in the Chinese art form of Pen-Jing. When we look back at the scrolls depicting bonsai in Japan in the early periods we see that they were planted with rocks as you can see when you look at the picture below.
Gradually over a period of time the rocks used as additional elements were removed from Bonsai however 2 styles remained where rocks are a material part of the planting:
The practice of planting more than one tree in a pot was also refined to provide bonsai with the forest or multiple tree style.
Saikei differs from bonsai in the way that it is approached. In bonsai the artist will approach the material as a tree or trees and so the point of concentration is the tree, its shape and the pot in which it grows. In Saikei the artist will create a landscape with rocks, trees, soil, moss and aggregates; the focus in Saikei is on the combination of these elements in a harmonious way to create an image of a landscape.
There are in principle 5 elements that a Saikei must contain: