Lids on terrariums

Way back in the 60's, the terrariums often had lids (still do, of course), and I thought it was so that they could be somewhat of a closed system, needing little interference. Now I have questions about the possible formation of mildew---does this depend on the plant species, the temperature, or what? I do realize that with all gardening endeavors, one has to just be observant, but are there any general principles to guide one on this? My terraria are all in my mind so far!

Depends on your indoor conditions. With mine, I have to keep the lid cracked all the time or mildew forms.

hmm ... I live in a VERY humid climate and I keep the lids on my terrarium's all the time with no problems of mildew or mold at all. Our air conditioning stays on almost year round though so it isn't so humid inside the house. I have jar terrariums that I made a couple of years ago and have never watered them because the lids fit tightly which keeps the moisture in. I have a 10 gal aquarium with a lid and light and that one I do have to add a teeny bit of water every few months because the lid has openings in the back so the soil does go dry after awhile. I know mildew and mold usually come from too much moisture so if you are having that problem you should keep the lid cracked a bit as threegardeners stated she does with hers.

Terrariums are a fun project!