Magical Seed

From the Super Mario Wiki

The Magical Seed seed which can grow flowers wherever they are planted. In Paper Mario, Mario must find four of these, which are usually held by Bub-ulbs, so that he can gain access to Flower Fields. The doorway is the archway by Minh T., in front of the post office in Toad Town. Each time Mario gets a Magical seed from a Bub-ulb, one of the seeds will have a different color.

These are the seeds locations:

• The last area before Buzzar on Mt. Rugged
• The center of one of the Forever Forest circle areas
• Across from Tayce T.'s house in Toad Town
• Kolorado after the player gives him the Volcano Vase on Lavalava Island