Instructions
Play through the game until you have defeated Whitney, the Gym Leader in Goldenrod City, and acquired the Plain Badge. Goldenrod City is south of the tree on Route 36.
Go to the eastern corner of Goldenrod City, where you will find a small house. Go in and speak with the woman. She will give you a Squirtbottle as a reward for defeating Whitney.
Exit Goldenrod City via Route 35 to the north. Keep following this path until you get to the National Park, then head east to Route 36.
Follow the linear path along Route 36 until you reach the strange tree. Spray it with the Squirtbottle and it will attack you, revealing that it is actually the Pokemon Sudowoodo, a Rock-type Pokemon.
Fight Sudowoodo. You can either knock it out or try to capture it -- the latter is recommended because Sudowoodo is a unique-event Pokemon and is a good addition to your party if you don't have a Rock-type Pokemon yet. No matter what you decide, Sudowoodo will be gone when the battle is over.