I started the achievement with my lv56 Mesmer a while back, then finals happened and I don't have a computer that can play the game on for the time being. Will I be able to log back in and complete the achievement if I still have the plant seeking item in my inventory?
No, the Fervid Censor can no longer be obtained because that part of the living story is over.
Edit: Since the achievements UI was changed, none of the "Historical" achievements can be completed.
There is a similar option back piece though. The Cutthroat Politics living story introduced a back piece with a similar appearance to the Fervid Censor, called the Desert Rose, which can be purchased on the trading post.