There Is No Exact Tree(s) You Should Headbutt As All Trees Have A Possibility Of A Pokemon Appearing. The Pokemon Appearing Is Decided By The Game Itself So The Only Advice I Can Give You Is, Headbutt Until Something Pops Out.
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Headbutt a tree in the Viridian Forest, you might also get a shroomish.
Note: You need Headbutt.New Bark Town Headbutt Trees.CherryGrove City Headbutt Trees.search him on your pokedex and it will be red instead of green that means you need to headbutt a tree using headbutt.
headbutt a tree(close 2 ecruteak city)
When facing the tree, go to your Pokemon and select the one that has the move HEadbutt. Then use it.
Use headbutt on a tree and catch a pineco. then evolve it.Get a Pineco via Headbutting a tree. You can headbutt trees at places such as Ilex Forest to get one. Level it up to level 31 and it should evolve into a Forretress.