How To Get To Ecruteak City In Pokemon Soul Silver?

Whilst in goldenrod city, take your time to explore the buildings, go to the "flower Shop"  Fairly self explanatory, and talk to the lady inside, she'll talk about a tree that moves when you squirt water onto it and will give you a spray can of some sorts. After defeating the gym travel north, through the National Park and walk to the tree, you will have seen it on your way to the second gym city from violet city, use the spray can, and fight the level 20 sudowoodo, you can catch it if you wish or defeat it. Hope this helps.

Go to the Poke mart near the third gym and buy water use it on the "tree" and you battle a lv. 20 sudowoodo

You have to get water from a poke shop right next to
the third gym and sprinkle it on suduwoodu(desigised
as a tree) then it will battle you and free the path to another
town and you can catch it level 20

Go to the flower shop and get sprinkling water and then go to the tree that won't move and put it on it.