Thank you WillieSea
I noticed that the soil in the BYOH cell was linked to an NPC too, and that it had a parent enabled, however after looking at it again this morning, I think the enabled parent is to do with rendering the objects at the appropriate time in the BYOH. This might be obvious to people who know what they are looking at
When the soil is added to a different cell, there is no linked ref in the edit details and the BYOHPlanterNPC is not a visible option in the list when adding a new one. So I got the NPC out to have a good look at him. I don't think he was ever meant to run around Skyrim judging by his state of deshabille! I cannot find him located in the BYOH cell or Tamriel. He's an invulnerable, essential, level one Nord, with a script. The script properties seem to indicate that this is where the core element of the growing process is located;
flPlanterPlantableItem (Form List)
flPlanterPlantedFlora (Form List)
GameDays Passed (Global Variable)
growDays (Float)
PlanterItemPlanted/NotPlantable/OnlyOne/PreviousItem (Message)
resetDays (Float)
So I created and renamed a duplicate, dragged him into the render screen. In link ref I used the "Select Reference in Render Window" to link the soil to the NPC. I then also checked initially disabled on the NPC. I clicked initially disabled because the NPC doesn't show in the render screen of the BYOH cell (or the game), a random half naked guy standing in the corner is not in the plan and this is the only disable I know. This would probably stop the scripts working all on it's own and when tested in game, yep, nothing happened. I don't know much about scripting at all, but it occurred to me too that the NPC may be called by a script without ever rendering it.
Next, I wondered if the script was dependant upon the exact ID of the NPC, so my rename would obviously not work, so I went back to the original and dragged him into the render screen, and I noticed when I did, that his ID had a [D] after it, which, I believe means, deleted? Does this imply that the NPC is rendered to create the link then deleted? Is he deleted when the game first loads that cell? This thought has only just occurred to me so I haven't yet tested it, what I have tried is deleting the NPC in the CK. First I link referenced him to the soil, and then I deleted him. A kind of marker appeared in hs place, simular to the BYOHGardenMarker(Xmarker) in the exterior cell but without the red X. (The garden marker is the parent of the soil for the exterior.) It lead from the spot where the NPC was placed to the soil. All interesting but none of it made it work when I tested it in game. And that's where I'm up to now this morning, I might try leaving him in the CK, with this [D] after his ID and see if it deletes him in the game.
I've spent some time tinkering with this, but I'm quite new to it all, and my tinkering is by no means conclusive and under scientific conditions, i.e. I probably did lots of silly things and for example, I've only looked at BYOH in Falkreath, assuming (yes I know dangerous) that it will be simular if not identical in each hold location, so I'll put this out here and see what other people think