it is in the 3 way intersection above goldenrod city, you need the squirtbottle to make it wild which you get after you beat the gym leader and talk to the girl on the NE side of goldenrod
you get a water thingy and you use it on the tree
It's a sudowoodo
East of the National Forest.
a rock Pokemon disguised as a tree
get the squirt bottle and use it on the wiggling tree aka sudowoodo
you get the squirtbottle at the flower shop and water the tree(which is Sudowoodo)
in route 36 theres a tree get the squirtbottle after defeating Whitney in the flower shop next to the gym pour it on the odd tree and there you go theres the lv 20 sudowoodo
first you go on the route that has the strange tree and spray it with the squirt bottle and it will attack you and use a fast ball to catch it and there you go a sudowoodo
It can be caught at battle frontier, and a weird tree will be there, and when you water it. It'll battle you which means the tree is sudowoodo. But only one is available, so if you defeat it and save, it'll never come back.
you have to get the sprinkle water and sprinkle it on the tree
sudowoodo
The wiggly tree is actually Sudowoodo. You need to get the badge in Goldenrod City. Then you go to the Flower Shop (next to the gym) and get squirtbottle. Use the squirtbottle on Sudowoodo and it will attack you. Defeat it or catch it and you will be able to get past.