Christine, thank you. I currently only have the two aquarium/terrariums, the ones shown in the first two photos. One of these days I might get around to planting up some of the more decorative glass containers again.

Removing or eliminating mold

If you have a closed terrarium and it is developing mold the water inside might be too abundant. Remove the cap off the top for a few days to reduce the water and promote some air circulation. This might correct the situation. It is a good option.
Also - if there is limited mold you can use some tools to prune away the affected area. Still keep the cap off to reduce the humidity and water inside.
Also- If one or two plants are affected you might want to just remove them from the terrarium.
While you are removing or pruning plants you should remove any buds of mold that is on the soil. And irrigate the soil a bit by stirring the surface around. Scratch it to get air to it.

I'm not at all experienced with keeping a terrarium. I got one as a gift for Christmas. It's in a one gallon jug, very narrow opening. It got the white mold discussed here, and I instinctively took the cap off for about s week. Now everything in it has died. The soil, sand, gravel, etc layers are the more attractive part, so I don't really want to dump it all out. The mold itself seems to be almost gone, so I'm considering trying to start new plants and see what happens. But how do you get into it to remove the old and plant new? The opening is only 1.25 inches and the top of the soil is almost 8 inches down!

BSlatt - Nice gift, but apparently completely impractical to care for. I will let others list the many things that may have gone wrong. You will probably need very long tongs to be able to manipulate the plants and plant material down inside.

Welcome to NGA, @BSlatt .

Many years ago I bought a "bottle terrarium" kit, with a large -- 3-gallon, maybe? -- jug with a narrow neck. It took me a couple of tries but I was eventually able to get it planted and "balanced" (as far as moisture) and it thrived for quite a long time. At some point I decided to tear it all apart, because things were overgrown and no longer attractive, and then never "got around" to replanting it; I still have the jug and now that I'm thinking about it I might add that to this winter's to-do list.

You had the right idea about the mold -- if mold is forming, there is too much moisture and you need to take the cap off. Watering needs to be done very sparingly; I had my bottle planted horizontally and used a small spoon fastened to the end of a dowel stick to deliver small amounts of water to each plant. For a vertically planted jug, such as you have, I would try using a drinking straw, or maybe a piece of tubing like that used for aquariums, to direct the water to the individual plants. A fork fastened to the same type of stick is helpful for planting or adjusting your plants, as is a "grabber" thing like this: https://www.amazon.com/dp/B000RB7AGY/

It definitely takes a good bit of patience to deal with a terrarium planted in a jug, but it's fun to have once you've done so! If you don't want the hassle involved with trying to keep live plants healthy in it, you could always look for some realistic-looking artificial plants and use those!

BSlatt,

Is it possible to post a photo of your terrarium jug? I agree with WillC, unless one is an expert at the artistry of terrarium keeping, those types with the very narrow openings are quite impractical and difficult to keep healthy.

The possible reason for the mold and death of the plants contained in your terrarium may have been due to the fact that the "jar" has a small opening, not allowing proper air circulation; if the cap remained in place for extended periods, it probably caused the mold spores to flourish. Plants need air, nutrients and water to survive and if one of those elements is absent, plants won't live very long.

Depending on the height of the jar, you will need long tongs to remove old plant material and add new plants if you make the decision to do so. I'd suggest leaving the cap off the terrarium to allow the soil to dry thoroughly before adding new plants.

Pics - thanks for the help, folks.

My problem with your current set up is that the mold spores are in the soil. You really need to dump it all out and start again. Once your jar is cleaned out (washed and sterilized), you can make new layers with new medium. Once planted, I doubt you will need to water it for years. Make the bottom layer charcoal and gravel.

We used to grow terrariums in 5 gallon water jugs (before they went to plastic). After planting, they didn't need any water. Ever.