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Also, should a terrarium be airtight?
Terrariums are fine if they are airtight, but we suggest removing the lid periodically (about once a week or even daily) to allow fresh air into your garden.
Also question is, does a terrarium need air?
Air. Even though a closed terrarium like a pop bottle does not get new air, the plants inside constantly recycle the air. During the day, plants generate sugars in the complicated process of photosynthesis. Since plants produce both air components, they do not need air holes in a pop bottle terrarium to survive.
In theory, a perfectly balanced closed terrarium – under the right conditions – should continue to thrive indefinitely. The longest known terrarium lasted on it's own for 53 years. They may even outlast us!