Go the flower shop in Goldenrod City. Talk to the lady there. She'll give you the Squirtbottle. Then go up to the odd, wiggly tree and press "A". When it asks if you want to use the Squirtbottle, say yes. The tree turns out to be Sudowoodo, a rock, I repeat ROCK, not grass, type. Either catch it or KO it. Or, simply use an Action Replay, "Walk Through Walls," or, "Move Anywhere," cheat, and walk through the tree. Whichever one appeals to you.