I really love root over rock style bonsai, especially trident maples. Well established trees have roots that flow over and flatten out to clasp the rock as if the tree has been there for ages. I have been experimenting with ways to develop root over rock bonsai for more than 20 years in an endeavour to produce ever better root over rock plantings.
The old Japanese method was to paste ‘muck’ (a mix of clay and cow manure) onto the surface of the rock where the roots are to grow then press the roots of a seedling into it and plant the result into a pot or the field. When I tried this I found many of the roots with muck rotted and new ones sprouted which often were not in contact with the rock, spoiling the arrangement. Even if the roots in muck survived, new roots often sprouted and grew off into the surrounding soil. These new roots often grew better than the carefully arranged roots on the rock and left large scars when removed. Panty hose was suggested as a way of holding roots onto the rocks but roots grow through and the nylon takes a very long time to degrade and is almost impossible to remove once the roots have grown through it. Cling wrap and electrical tape were other methods trialled. These work really well but only on convex surfaces. If the rock has a hollow the wrap spans straight across, leaving the roots underneath free to wander away from the surface of the rock. Eventually I tried another material from the kitchen drawer which has worked best and produces excellent results more often than any other I have tried.
Here’s what you need:
Trident maple seedlings:
Some of the seedlings will have a single strong ‘taproot’. Put them aside for other projects. Select the ones with several strong roots that will go over your rock. Wide spread roots will usually fit better than strong vertical ones ( the trunk usually splits or roots break off when you try to force the base of the trunk tight against the rock)
Now plant the whole thing up to the base of the trunk. the pressure of the soil will push the foil and roots within close against the rock surface. Do not let the soil cover the top of the foil because tridents will send out new surface roots at the expense of your chosen root hugging ones.
After a year you can unpot the tree and unwrap it to check progress. The foil will have started to decay and will probably need replacing anyway.
At this stage you may still be able to make modifications to the roots if they are not in the ideal positions. If lots of new roots have sprouted you can remove some to leave discrete roots running down the rock. Where roots cross, the top root will be pushed away from the rock as the one under it thickens so try to avoid crossing roots.
Lots of roots close together will merge into a single plate of root over time.