Keep it in the plastic jar. My sour dough starter (called Boris) has been living in a plastic jar in my dark cold frig for 8 years now. I do have to clean out the jar about once a year (warm water and detergent). Glass jars are easier to establish (to make) the starter the first time because you can make them cleaner by using boiling water and soap. An old plastic jar may have "bad" germs and this makes it hard to begin a "good" starter.

Hi there alyaman, in my experience, glass jars are preferable because it's much easier to clean. However, if you're planning to have your starter endure longer, a plastic jar would be ideal for the job.