Select a suitable rock.
I find scoria easy to work with. It can be shaped easily, it generally has interesting pockets and fissures and it holds moisture for quite a long period once completely saturated. A small amount of shaping was necessary to make the rock sit flat and in the direction that was desired.
Select a suitable plant.
For this rock I have selected a three year old Chinese Elm. The tree has a nice lean to the trunk which will follow the contour of the rock. Elms are great as root over rock style plants their roots grow quickly and are quite thick and fleshy.
You need to remove all soil from the roots, the closer to the rock they can be attached the better. Remove small roots and roots that cross over each other if possible. Remember the more roots you can retain the quicker your tree will recover.
Placing tree on rock.
Attach the tree to the rock it is best to avoid perching the tree on the top of the rock. This Elm is placed on a natural ledge half way up the rock. Try to follow a natural course with the roots. Roots like water, tend to follow the path of least resistance. I have tied the roots down with plumbing thread tape. This tape is flexible and will stretch as the roots grow it is also water and rot resistant.
Repot the tree in fresh bonsai soil, covering all the roots. I like to use pb bags for this, this bag is a pb 40 size.
The plastic bag allows you to roll down or remove the top portion of the bag as you expose the roots. Remember the roots will grow in size quicker in the soil than they will exposed, so don't be in too much of a hurry to uncover your creation.
Trim up the top growth to compensate for the root you removed earlier. This Elm was made just as it was coming out of its winter dormancy this is the best time for repotting Chinese Elms.
This tree was created last spring and is now well on its way.I will post progress pictures of the elm we made today as it matures.