In the order to get Winterblossom you will have to use Ice Flower Seeds on the Empty Flower Pot to grow the plant. Once it is fully bloomed it will remain at the final stage- not like other plants.
As the growing Winterblossom does not involve NPC like other plants, it can be seeded whole year.
References:
https://tibia.fandom.com/wiki/Flower_Pot
https://tibia.fandom.com/wiki/Ice_Flower_Seeds
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