Alright, my bad for not reporting this earlier, I thought it was known.
The Bulwark is not plantable, not for druids, not for normal plants. The first mushrooom to appear in the wall a month or so ago was here:
(https://i.imgur.com/zAlciqN.png)
Obviously this area cannot actually be reached, even if you could plant here.
At about the same time this nest of wasps appeared, it was still there as of today:
(https://i.imgur.com/6Ty1Etn.png)
Finally, I planted a lovely little grove of plants here a few weeks back:
(https://i.imgur.com/s1Eq1XB.png)
But they disappeared. I thought, zounds, those reckless plant snatchers are at it again. But when I entered the Bulwark, I found the same garden floating in the ceiling, no doubt scaring the nearby dwarves. That is, I think it was the same garden, I didn't memorise the one I planted outside, but this seemed to have the same plants and size/pattern as the outside garden that had gone. When I took the screenshots today, there was still a single spooky Silkroot up there, which I couldnt get on camera, but I have highlighted the location:
(https://i.imgur.com/yd3JzJe.png)