Recently the weather was nice. The sun was shining and the buds on my bonsai are starting to open. Time to work on a project that has been 2 years in the making. A root over rock planting with a trident maple!
As long as I have been growing bonsai have I longed for a home-grown root over rock bonsai. I have started several projects without a lot of thinking about the plants used nor the stones. Some of them I dissolved. Some I still have around. None of them is particularly interesting. So when a few years ago I found a seller of a large range of Seiryu rocks I decided to get a box full of them.
Root over rock bonsai style
This bonsai style (In Japanese Sekijoju) emulates a tree growing in a rocky environment where over the years the topsoil has eroded away. It has roots clinging to the rockface, draping down into the soil below it. This style is different from rock plantings in that the growing tips of the roots that take up water and nutrients grow in the substrate in the pot, and not on the rock.
A well-done root over rock bonsai does not have clear spaces between the roots and the rock. The connection between the roots and rock is such that the rock is completely immobile. The roots should not cross each other, and follow a natural path down along the rock. The rock itself can have a range of sizes. The rock however is an integral part of the composition and should as such not be too small and might be of considerable interest.
In order to get the roots to properly connect with the rock it is best to start with young flexible roots. On the one hand this could be achieved with an airlayered tree. However, for the roots to develop a lot of growth is needed. As such, good root over rock plantings are often started using seedlings.
About the rocks
Two years ago I purchased a box of Seiryu rocks. These rocks are highly fissured and have enclosures of brighter rock in an overall blue-greyish hard rock. They are often used in aquascaping, but are also great for bonsai. Over winter I have been sorting through them, gluing some together and selecting some for individual use.
About the seedling
In 2019 I was lucky to find a trident maple tree full of seed and brought some seeds home. After planting them and waiting for spring, I ended up with a tray of trident maple seedlings. This is one of them. All of 2020 this was allowed to grow unrestrained. I clipped the main root in early spring in order to encourage side roots.
Putting the trident root over rock bonsai together
Planting root over rock bonsai 
Planting root over rock bonsai
Clean the rock. The first thing to do when you get rocks like this is clean them out. In this case, the cracks in its surface were filled with roots and soil, all elements which will later create spaces between the root and the rock.
Wire the tree. Depending on how you wish to grow the tree, you might want to wire the trunk and shape it before planting the rock. I put a medium sized wire along the trunk of the sapling so I could add some movement to the otherwise boring young trident maple.
Bare-root the tree. As the seedling was grown in coarse substrate bare-rooting was a matter of gently tugging and shaking of the root ball, and the substrate fell out. Make sure you keep the roots moist from here onwards.
Positioning the tree. The position of the tree is very important with a root over rock planting. Often one sees root of rock trees where the tree sits on the side of a smooth rock, and the roots are “tied” around the back of the trunk. There is no natural way one could imagine a full-grown tree growing like this, and I try to avoid this. As such, I positioned the tree in a natural lower point in the rock, in this case, high up on the rock. The roots were subsequently placed in the cracks along the rock surface following the natural shallows and waterlines in the rock.
Securing the roots. Getting the roots to cling to the rocks mean that they should follow the contours of the rock as much as possible. But once you place the roots where you want to, you are short of hands to keep them in place until you make the final wrap. I like to take a handful of potting soil, soaked in water and use this as temporary glue. I put the roots in the crevices and slather some potting soil muck over them.
Wrap the lot up. In order to ensure the roots stay close to the rock, and to stop them from growing into the substrate in all directions, the roots and rock are wrapped. This could be clingfilm, tape of, as in this case, aluminium foil. The benefit of the latter is that it can be shaped to follow the rock contours, rather then just follow the overall high point of the rock.
Trident maple sapling root over rock bonsai 
Trident maple sapling root over rock bonsai 
Trident maple sapling root over rock bonsai
Planting and forget about it. With that the main work is done. The rock was placed in a larger container and back-filled with substrate. Key here is that there is plenty of substrate below the rock AND that the top of the plant is not buried. You do not want new roots to grow from the nebari, and pop outside the foil covering the rock: All roots should grow as close to the rock as possible. Now it is off to a forgotten corned in the garden. Let the tree grow. For some fast-growing species such as trident maple, once may consider taking a peek after one year. For most species at least 2 years are needed for the roots to develop enough to take hold onto the rock. Is it important to check after 1-3 years and correct the roots where needed.
