Tridents through rock

[PWC]

I had a rock that I collected on a beach in South Australia when I lived there some 5-6 years ago. It is solid and had 2 holes that went right through from the bottom and out the sides. I was playing around with some Trident whips at the time so thought why not put them in from the base and see what I end up with. Not sure if I should leave then sitting at soil level or have the roots exposed on the side of the rock.

I root pruned and repotted today with the trunks at soil level. I have not done much regarding the tree development so far. I would like to keep smaller to keep the rock a significant part of the composition. From a scale perspective I would envision them around Shohin size.

I do not expect the rock to fracture now as the base of the trunks appear to be flaring on the out side of the rock

Any thoughts on the roots being exposed or not are welcome, Maybe have the roots travel out along the base then into the mix?

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[Topiarius]

Fascinating idea, will be interested to see how this develops.

[shibui]

The new roots are a classic response to restricted sap flow as when we layer plants.
Is this a single trunk threaded through the hole or 2 trees? Where are the original roots?
Not clear what sort of rock you have and how hard it is. I once threaded Ficus rubiginosa through holes in coastal sandstone. They slowly popped the rock as the trunks thickened. The power of trees growing is not to be underestimated.

I really like root over rock bonsai and tridents are great to produce ROR but I can see some possible issues with your planting.
No problem exposing the roots provided some root tips are in the soil. Exposed roots will still thicken provided sap is still flowing from root tips to the trunk. If you are concerned that root tips are not far enough advanced you can plant a little deeper for another year and expose fully in 12 months.
The problem I see is that the new roots are all on one side of the rock. The original trunk inside the hole will gradually die from reduced sap flow. When that happens there's nothing holding the tree onto the rock and thickening roots will slowly push the trunk away from the rock. I've seen this happen many times with developing Root Over rock tridents.
If it is possible to move some of the new roots so that they wrap around the rock that could help long term.

[PWC]

Is this a single trunk threaded through the hole or 2 trees? Where are the original roots?

There were 2 individual whips, the original roots are still alive under the rock base. I have reduced them as they were quiet heavy, hopefully this should promote finer feeder roots.

Not clear what sort of rock you have and how hard it is.

Definitely not sedimentary more likely igneous.

The problem I see is that the new roots are all on one side of the rock. The original trunk inside the hole will gradually die from reduced sap flow. When that happens there's nothing holding the tree onto the rock and thickening roots will slowly push the trunk away from the rock.

Good point I had not considered that, I was concerned the lack of roots on the top side of the trunk could cause problems with reduced sap flow to that side of the tree.

If it is possible to move some of the new roots so that they wrap around the rock that could help long term.

I am hoping the original roots build a mass of finer roots that may act as an anchor, I may have an opportunity next repot to wrap roots to assist as well.

Thanks for your suggestion and thoughts.