You need to defeat the Gym Leader in Goldenrod City, then go to the flower shop and talk to the woman watering the flowers. She will give you a SquirtBottle, use that on the tree and it will attack you as a Sudowoodo Lv20.
I recommend that you catch it for the Pokedex, because that's the only place you can find it.
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Use your watering can on it. The shaking tree won't like it and it will be a Sudowoodo try and catch it then you will be able to get into Smeargle Cave.
A rare Pokemon is on a honey tree when you see the tree shaking. (the more it shakes the rare the Pokemon )
AnswerIf you use Headbutt on a tree in Pokemon Crystal, one of the following Pokemon may appear: SpearowHeracrossAipomEkansHoothootNoctowlSpinarakPinecoLedybaExeggcuteVenonatCaterpieMetapodButterfreeWeedleKakunaBeedrill
headbutt tree
Aipom is a pokemon occasionally found after checking a honey tree (yellow). You will need to have used honey on that tree. After a period of time (a few hours), if the tree is shaking, there is a pokemon there. The pokemon you find from honey trees can vary.