There is a pair of horse chestnuts about a block away. The local squirrels scoop up many of the conkers and some wind up planted in my landscape to sprout the following spring. I threaded 6 spring sprouts found in my landscape one spring through the drain hole of the bottom of an old terracotta pot. I forget exactly when I did this, but by 2017 (2+ years later) they looked like this:
View attachment 426568
They were left in an out-of-the-way corner of the garden and cut back to keep them from interfering with anything I actually cared about (I did use them to explore the somewhat unusual budding of horse chestnuts = some excessive pruning that further detracted from the underlying point of fusing trunks).
Yada, Yada, Yada, ...
I just unpotted this mess to find that these trunks had nicely fused (as expected
).
View attachment 426579
I also found that the fused trunks had broken the terracotta into several pieces and that even some of the terracotta that had defined the drainage hole remained quite tightly embedded in the 'trunk'.
View attachment 426574
Even though it is just a damn horse chestnut, the points I get from this are that it will take something like 5 years to produce a nicely fused trunk, regardless of the species and that it doesn't occur until well after growing roots above the tourniquet' (i.e., ground layering occurs) when done in this fashion. Methods with less rigid constraint of the trunks will undoubtedly take longer to fuse to the same degree.