Ferroseed begins to evolve at Lvl 40 so just train it up that level and it should evolve unless it's holding an Everstone.
Ferroseed will evolve into Ferrothorn.
Through leveling up; Ferroseed will evolve into Ferrothorn at level 40.
Ferroseed does evolve in Ferrothorn at level 40.
Ferroseed evolves into Ferrothorn at level 40.
Ferroseed evolves at level 40
Ferroseed evolves into Ferrothorn at Level 40 Ferrothorn
ferrothron
Ferroseed evolves at level 45
It evolves by leveling up. Ferroseed evolves into Ferrothorn at level 40.
Level 40
Lv 45
Level 40 ;-)