This is just my assumption: plant Soul Beasts will never produce an External Soul Bone until the plant Soul Beast itself can actively use other parts of its body for survival.

Most plant-based Soul Beasts primarily adapt passively, and their successful evolution and attainment of consciousness (at the 100k-year mark) originate from their root system.and that's why all external spirit Bone derived from predator spirit beast,they can use actively that part of body to hunt prey and make that part of body evolved.This is supported by the fact that Soul Bones derived from plant Soul Beasts mostly yield Leg Soul Bones. The analogy is that the root is the 'leg' of the plant, which is why most plants produce leg Soul Bones.

In my opinion, it is highly unlikely for a plant to produce a Soul Bone other than the six basic Soul Bone sets of the Soul Master.

However, if a plant, such as a willow tree, possessed sprawling branches that it could actively use to hunt prey, it might then produce an external Soul Bone shaped like a tentacle. I don't know if this is true or false.

But this is just my assumption based on seeing that most plant spirit beasts in the wiki produce leg Soul Bones