NB: I didn't realise this at first but, this is only available if you own the DLC! No DLC, no fern for you :(

Once the fern has spawned from its world event (I forget exactly what it's called), it will (at least to my knowledge), remain in one place until found. It will only be obtainable at night and is guarded by a couple of caster enemies, so be prepared for a fight. Below are a couple of screenshots of where you want to be. The green arrow marks it:



The fast travel point you want to start at is Greenfields if you have it. Otherwise, you're going North-East of Bor. Hope this helps someone! Let me know if I've gotten anything wrong. Three of my characters died looking for this thing, so be grateful for their sacrifice.