[Cob] cob wall planter

[sly at riseup.net]

I have been asked to build a wall that has a planter at the top out of
cob.  Is this possible?  I was thinking if I put pond liner inside before
filling it with dirt maybe it could work but I was worried that watering
the plants might destabilize the wall.  Has anyone tried anything like
this?  They want the planter box to be at the top.

Thanks,
Sarah