The “Root over Rock” has been used for many years in Bonsai. This style consists of the plant’s roots being trained to cling to a rock and then spilling over it, eventually disappearing into the soil. The goal of this style is to mimic a tree that grows into a rock crevice, where the roots have to move about to find needed nutrients when the primary source is depleted. Once the roots reach the soil, they will become hard, thus growing back around the rock. The result is that the roots now become as effective as the actual tree trunk.
In nature, it is common to see a tree start growing onto another tree. In time, that tree will overpower the second tree, strangling it with the roots. To create this style, you will need to choose a suitable rock, one that is attractive, natural, and a good size, but not so large that it overpowers the tress. Then, you need to choose a healthy tree, one with an extensive root system.
Cut off any unsightly foliage and then wash the soil away from the root ball, as best you can, taking care that the roots are not damaged. Then with your rock and plant, you will place the plant over the rock. You might need to arrange it several times before you find a look that you prefer. It is important that you not place all of the roots to one side since you want your Bonsai to be viewed and enjoyed from any angle. If needed, the underdeveloped roots can be overlapped, helping them merge.
Now, the roots will need to be set into place. The easies and most effective method are to take grafting tape and while holding the roots in place, you will wrap the tape around the rock, making sure it is somewhat tight. The rock will need to be covered completely with the exception of the bottom so that the roots can protrude into the soil. Additionally, make sure the roots have no gaps where they could escape into the tape.
When done, ensure the roots all protrude from the bottom of the rock. Then, you will bury the rock along with the plant in a pot of clean sand. The goal is to plant it so the rock is not visible but the bottom portion of the trunk is. The Bonsai will need to be watered and then left in this pot for up to two years.
Over time, the roots will thicken and become strong. When the time comes to remove the pot, you will scrape the soil away, and wash it so the rock and root ball are exposed. You will need to be very careful with these new roots, as they will not tolerate too much stress. Next, using a pair of sharp scissors, remove the grafting tape, making sure you do not cut into the roots. Once that is complete, you will pot the tree in a plastic pot so that the rock is positioned above the surface, leaving it for about three months to help it rejuvenate. Finally, plant your Bonsai in a ceramic pot so it can be displayed and enjoyed.
Now keep in mind that when you pot your Bonsai in the ceramic pot, it will not look much like a Bonsai. Instead, you will see a busy tree that seems out of proportion. To encourage a tapered trunk and to refine the branches, you will need to prune the tree. Then in about six months, the tree will again be cut back to re-establish its shape. In all, you will be pruning and reshaping for about two years before you have the perfect Bonsai. With the “Root over Rock” style, you will have an appealing and aesthetically pleasing Bonsai. This style is advanced so just be patient and with a little time and practice you will achieve your goal.