I have tried before to use items like glass jars as miniature greenhouses to grow plants. However, every time since it was moist inside the "greenhouse," mold began to grow inside.

My question is regarding a plastic bag greenhouse I constructed to winter my bonsai tree in. I was wondering if anyone could tell me how to prevent mold growth inside the greenhouse while still retaining moisture and heat so that the plant will not freeze or dry out.

Thank you for your help.