From the Super Mario Wiki
Bubble Berry is one of the berries found only in Flower Fields in the game Paper Mario. In the lower right path of the Flower Fields main hub, a trail will lead Mario to a dry pool. Two docks on either side are used for Mario to ride Sushie over to the Bubble Berry Tree in the middle of the pool (if there is any water in the pool). Unless the Water Stone is put back in the spring near Lily, the tree will be too high to reach. The Bubble Berry is used in a slightly different way than the other Berries. The Bubble Plant requires a Bubble Berry in order to blow a giant Bubble that will ferry Mario to the other side of a sea of thorns.
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