He means the Super Mushroom. If you eat it, you grow higher than the trees for a few seconds.
All mushrooms (including the Super Mushroom, though it's very rare) grow around trees and tree stumps, all year round. Normally, they have no function other than to be sold for Bells, but in autumn, you can sometimes find a piece of mushroom furniture when you pick them.
"If no mistake have you made, yet losing you are...
a different game should you play." - Yoda
"He means the Super Mushroom. If you eat it, you grow higher than the trees for a few seconds."
Jokes aside, all of the past mushrooms are in the game. Tall mushrooms, short mushrooms, round mushrooms and elegant mushrooms. All of the mushrooms that previously were used to collect the mushroom set are in the game.
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