My students (elementary aged) read a passage about terrariums this year. Excited they did some research and we converted a 75 gallon fish tank into a terrarium. Things have gone well for the past 6 months, but since spring we've had some odd growth.

In the bottom of the soil some round, white spheres have shown up (a bit of a yellow tinge). They have invaded the areas under the charcoal. The plants started out small, and have grown nicely. All plants came from a reputable nursery that specializes in indoor plants and fairy gardens.

We used cleaned pebbles, and potting soil from a local large nursery. So far nothing has grown on the plants. We water once a week, and keep one side of terrarium open. the only odd experience came from a student who thought to add grass seed from a science experiment. You can see the little wisps of grass by the moss.

Any idea what the growth is? How to combat it?

Thank You

Looking over the photos, I see that the growth has now invaded the soil, not just the pebbles. This is a new development.

Hi RebNew,

I can't tell for sure but it looks like salt build up to me. If you use tap water containing high levels of sodium bicarbonates for watering the terrarium, that might likely be the issue. Salt buildup can be common in plants with high pH or alkaline soils, making it difficult for plants to take up the necessary nutrients to thrive.

With potted-container plants, it's fairly easy to leach the salt build up by flushing with fresh water but of course that isn't possible with an enclosed terrarium. The issue may be correctable with the addition of calcium or other additives to the soil but I'm not knowledgeable enough to offer advice on the best way to achieve that. I'm sure others who are more knowledgeable with soil issues will be able to offer suggestions.

You could do a complete renovation of the terrarium by removing everything and starting over from scratch; thoroughly cleaning the inside of the tank and flushing the pebbles with distilled or purified water before replacing them and adding fresh soil back to the terrarium. If you decided to go that route, I'd suggest adding a good amount of perlite to the potting medium to allow for adequate air circulation around the roots of the plants and watering only with rain water or purified water in the future.

Most tap water contains chlorine, metals, and salts. At this point I think a makeover is the best option...tell the students it's a good learning experience.
After the re-do use only rainwater, or if that is not available, use filtered water. Hope this helps.

It looks like a type of mold or fungus that was introduced with the potting soil. Soil is not sterile, but a potting mix composed only of peat moss and perlite is sterile. Mold thrives in dim, constantly amp conditions, which is what you have in your terrarium.

The only antidotes would be to allow the soil to dry out, but that would also kill many of the plants or to use a fungicide that I would not advise using in a classroom.

The good news is that the mold is not likely to harm the plants that you have. I suggest that you leave it and have your class do some research on fungi and their role in nature, particularly their symbiotic relationship with plants. A teachable moment!

Thank you all! I think we'll leave it for now and watch what happens. I'll care for the terrarium over the summer and then the students and I can do some research and make a decision on what to do next. I'll have many of the same students. I am using city water, but since we are trying to add a rain barrel to our prairie restoration garden, that might be a good source for water. Here in MN we add extra salt to soften the water, so that makes sense.

If I start over, are the plants salvageable? Or should I start with new plants as well?

If you start over, use a sterile soilless potting mix from the outset.

If the plants survive the summer, they will be fine to re-use in the terrarium. Just be certain to use a sterile potting medium as WillC suggested and I'd also still add perlite to the mix to help with aeration.

Rainwater will be perfect to use and it saves the expense of having to purchase distilled/purified water.

I believe that the terrarium has a case of Leucocoprinus birnbaumii "invadicus" Came in this morning and found these guys in the terrarium. I think we'll be starting over. Can anyone suggest a place to purchase sterile potting medium? I am using my personal funds for the project so need to try and get by on the cheap if possible.

Mushrooms are a type of fungus, generally harmless if not eaten, and a potential subject for a science lesson.

The most cost-effective sterile potting mix is plain peat moss and perlite mixed together in a 4:1 ratio. You may also want to add an inch of clean gravel or pebbles at the very bottom.

I belive, another good sterile potting mix is, equal parts of : sphagnum moss, vermiculite, and perlite.
See which is cheaper for you.
😎😎😎

Vermiculite tends to collapse and compact over time. Perlite is a better way to add porosity.

The mushrooms are actually rather lovely to look at, however since I work with children I suppose I should be safe. Thank you to everyone who responded. I shared responses with students and we did some research today. With only a few days left we decided to put off major changes until fall. I'll care for the terrarium and if the fungus is too much will empty it early.

Thanks again.

I think the mushrooms are a nice addition. I wish you could leave them. But I guess you can't take any chances- they might fall in the category of attractive nuisance of sorts. Why not just pick them, study them, and dispose of them as they grow?

@WillC : Howdy 😀

Thanks for info on vermiculite getting compacted. 👍👍

I'd like to ask you a couple dumb questions, if I may.

When you say, plain peat moss. Are you talking sphagnum peat moss ?

Also your ratio from above. I'm guessing it's 4 parts peat moss, to one part perlite. I'm I correct ?

Thank you 👍
😎😎😎

Philip - Yes, sphagnum peat moss. What other kind of peat moss is there? Yes, 4 parts peat to 1 part perlite.

Thank you all for the responses, now I have another series of questions that the students brought up. We did some reading today and wonder, since the mushrooms are fungus and depend on decomposing bio matter, will they self limit in the terrarium?

Will they die, or will the spores just remain dormant?

This is my first foray into terrariums, I apologize for my lack of knowledge.

I'm sure Fungi are a whole new subject to study over the summer.

Any potting medium that is watered regularly will be decomposing organic matter and a potential food source for fungi. assuming spores are there in the first place. Drying out the existing soil is no guarantee that the spores will die. If you start over and do not want any fungi, then use a fresh potting mix that is sterile - peat moss, perlite, and coir only.