Kimselv

Hello, I'm currently in Akora Forest looking for a shroomish and headbutting every tree in my way. However I fear my roggenrola is going to have a concussion before i can get one. So my question is : do you get shroomish by headbutting any tree in Akora Forest or is there a specific way to obtain it ? Or maybe the encounter rate is veryyyy low ? Thanks in advance !