Embarked in an area with mountain biome and ocean biome. Dug out the soil layers in the ocean biome, then breached the caves. No plants grew. So I breached all 3 caves, still no plants. Thought maybe that the red sand layer was bugged, so I dug deeper into the clay; didn't help. Saving and re-loading didn't help. Then tried to abandon the fort and reclaim, and all the dug out soil was flooded with the ocean (the ocean was not breached, and it was frozen even if there was a breach) I know there is vegetation in the caves, it just isn't spreading.