How to create a Bonsai rock planting
First of all, a suitable rock needs to be found; it should have many small cracks and holes in it providing the tree with space to grow roots. Sometimes rocks can be found in nature, but increasingly the rocks are handmade and sold in Bonsai stores.
Next, select the tree or trees you will use to plant the rock with; Junipers and Pine trees are often preferred but also deciduous trees can be used. The selected trees should be healthy and have long roots, therefore plant them in a large container at least a year before using them to create a rock planting.
Finally, we prepare the soil mix, and we have some options. You can mix akadama and garden mould in a ratio of ½ to ½, or we can use a stickier mixture like Akadama, Sphagnum moss and Keto (a sticky river mud from Japan). Make sure to read the Bonsai substrate article for more detailed information on soil mixtures for different species of trees, climates, etc. Sometimes it will help to apply Keto underneath, or around the root mass to prevent soil from being washed out when watering the planting.
After having selected the rock and the tree, follow the step-by-step plan below to create the rock planting. We also created a video of the process, planting Juniper trees on a rock, see above.
Planting trees on a rock should be done during the early spring season, around the time the trees start showing signs of new spring growth.
After having created the rock planting you can finish the composition by placing the rock on a flat platter. Prune the tree immediately after having planted it on the rock to lower the demand of nutrients from its root system. Start fertilizing a month later, as the limited amount of soil will easily lose its nutritional value. Also water regularly, as the soil will dry out quickly.