Closed terrariums

dianamay(z9 So.TX)September 8, 2003

In the past, I built many closed-system landscaped terrariums, and I thought you might find my methods informative. I had one terrarium in my home that lasted for 30 years (After I retired and moved to Texas, my son told me the dog had knocked it over. Ah well, life goes on.)


Soil mixture

Do not fill the terrarium half-way up. Use as little mix as possible to give the effect you want. Styrofoam or other inert materials can be used under the mix to give height. The more mix you use, the more moisture it holds and the greater the chance of rot .

Use a mix that holds water and air, keeps plants firm, and discourages bacteria and mold. (Vermiculite holds water and air. Perlite holds air. Sand holds plants up). Use moderately coarse vermiculite alone, or a mixture of vermiculite and builder's sand or bird cage gravel. Or one third each of vermiculite, perlite and sand works well. Handle vermiculite carefully and wash hands well after use. When you water (see below), vermiculite will settle around the roots to hold a plant in place.

Keep as little of the plants soil as possible without breaking off too many roots. The small amount that is left will supply the plant with food for several years if the plants are kept within bounds. Do not use peat moss. It becomes waterproof when dry and requires too much water to rewet.

Do not press the mix down. Just firm material around plant enough to hold it up. Moist air needs to circulate through it.


When starting your terrarium, water lightly around individual plants. Water will spread throughout mix by itself. Nature equalizes wet and dry. Once the humidity is properly distributed, roots will grow in the air and on the glass. Err on the side of too little rather than too much water. The terrarium should then be covered tightly with a transparent material such as Saran Wrap or glass. Wait at least one week to see if a condensation cycle starts.

If no condensation forms on the coolest side of the terrarium, during the day in very good light, add a few tablespoons of water a day until it does. Check to see that plants are getting enough light. (Inadequate light will prevent life cycles from starting.) If condensation forms on more than 1/3 of the glass, wipe it off with a paper towel and seal the top again. Do not leave it open to dry it. Plants that like high humidity will suffer. If excess condensation continues, repeat daily until only 1/3 condenses. When it is right, seal tightly, under the cover, with moistened Saran Wrap, and leave it in good light. A terrarium may go for a year or more without additional water if the proper balance of water and light have been reached, provided that it is properly sealed. You will need to open it only for housekeeping and trimming.

Mold and decay

Various molds may grow on dead material in the terrarium. Black thin little fibers with tiny spore heads may grow on dead leaves that are moist. If it does not appear to be spreading, leave it. This is often the fore-runner of moss, but may also mean the terrarium is too wet. White furry mold should be rubbed gently with a finger to disturb it as soon as it is noticed. Usually it will not persist if you rub it down a few times. Dead leaves should be left unless they are rotting against the glass. This will leave etched marks on glass that is hard to remove. If a lot of leaves are dying, give more light and wipe condensation out to make it dryer. The larger and taller the terrarium, the less likely there is to be trouble.

Insect pests

If you use materials from the wild, you may or may not want to prevent anything from hatching. (I love seeing what grows). Do not use collected wood (termites). When using wild moss (no soil), you may want to spray the back of it with a diluted liquid systemic insecticide (one that is absorbed by the plant roots). Use it sparingly and handle carefully. However, most things that hatch are easy to eliminate without chemicals.

Pests that come in on bought plants are harder to deal with, so examine all plant materials. You can use the systemic mixture on any soil around each plant if you think it is needed, but be sure to wipe out extra condensation that forms from the additional moisture. If plants are badly infected with mealybug or whitefly, it is best to snip off all leaves on the infected plant, wipe out most of the condensation in the terrarium, and wait for a healthy new crop of leaves. The result is usually satisfactory. Springtails are tiny white insects that jump around on the top of the soil when you water a plant. They are harmless and live on decaying matter so don't bother battling them.


Light is the food of plants. "Plant food" is the equivalent of our minerals and vitamins. Without a good source of light, plants will gradually perish. A small terrarium cannot take sunlight because heat builds up too quickly in it. In a tall terrarium with a lot of air space this problem does not usually arise, and the sun can shine for up to three hours in the morning or late afternoon, and in winter at other times. (Moss, however, does its best in a very low, broad space like a punch bowl, with no sun.)

Do not move your plants around to follow the sun or to avoid it. Plants are oriented to the light and do not thrive if they have to repeatedly re-orient themselves. Do turn the terrarium gradually, over a period of time, if the plants are all growing to one side, or else tip the terrarium up to give the plants more even light.

Most flowering plants need sunlight to bloom, or at least to initiate bloom. Orchids and miniature sinningias and other gesneraids (members of the African violet family) can bloom easily if kept in a large enough terrarium. Keep gesneriads in small CLAY pots. They will not bloom if their roots are allowed to spread out. Use fish-tank pebbles or bird gravel to bury the little pots at an angle tipped towards the light so they will grow evenly. They need a bit of sunlight to start blooming, but usually keep it up for a long time afterwards. They will need occasional plant food and water if their pots dry out, but be sure to wipe out an equal amount of condensation.

Artificial light

Daylight in combination of fluorescent light is great. With fluorescent light alone, have the lights not more than 6 or 8 inches from top of terrarium. Incandescent light may add more of the red spectrum needed for bloom, but their red is hot. Keep the lights on up to 16 hours a day, preferably on a timer. Plants like regularity. On a fluorescent bulb, the 10 inches at either end gives inadequate light for many plants, so a set of 24" tubes alone does not have much good light. Under a 48" or 72" set of 4 bulbs, you can grow plants with high light requirements in the center, lower light at the ends. Judge adequacy by degree of etiolation (stretching). There are many new lighting systems on the market that provide much higher light and make growing under lights much more satisfying.


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this is VERY good stuff! thank you diana! good to have you here! most helpful! :)

    Bookmark   September 15, 2003 at 4:40PM
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dianamay(z9 So.TX)

Just bumping this up to the top.

    Bookmark   October 1, 2003 at 10:27PM
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dianamay(z9 So.TX)

Notice the way water is added in a closed terrarium. Almost a teaspoon at a time, just around the individual plant, and judged adequate by the amount of condensation.

    Bookmark   October 11, 2003 at 8:08PM
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Okay, thanks Diana. I guess I will have to spend more time with it. Darn, I wanted to give it to him soon. But I'm sure you are right about this stuff so I will work slowly.

About the not turning the plants, can I ask why again? We are told to turn our plants that are outside of a terrarium a quarter turn every now and then to ensure symmetrical growth, why not ones inside a terrarium? If I don't turn some of my plants once every month or so, they will lean and I can't turn the whole terrarium since it also functions as my living room coffee table.

    Bookmark   October 11, 2003 at 9:43PM
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dianamay(z9 So.TX)

I was really thinking of landscaped terrariums when I wrote that. Certainly, if the plants are in pots there is no reason not to turn them as you mentioned. I was aiming at those people who hop their plants all over the house.

    Bookmark   October 11, 2003 at 10:56PM
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dianamay(z9 So.TX)

Bumping this up for ddot.

    Bookmark   October 19, 2003 at 4:31PM
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wdelaney(nh, usa)

i picked up a couple of ecology books at my universitys library, and i am trying to build a terrarium that is either self sustaining, or close to it. I think it can be done atleast for a certain amount of time. Currently i only open my bottle gardens when they look like they need some interventions other then that i leave them alone. I have though of a couple of expirments, and i will post the results and maby some people here can use them.

    Bookmark   October 20, 2003 at 12:10PM
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Hi Diana, thanks for the info on the saran wrap, if I would have read your posting before building my terrarium, it would have been make in a better way. But this is my first attempt at terrariums and with your info in this thread, there's more on the way. Thanks for the step by step how to on terrariums. Ken

    Bookmark   October 20, 2003 at 10:41PM
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dianamay(z9 So.TX)

Bumping up for Spunky.

    Bookmark   November 22, 2003 at 8:07PM
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ingrid_I_LUVmoss(5 Nebraska)

I started a closed terrarium about a month ago. It sits in the greenhouse here at work. I suppose it is not completely closed but it is a large bell jar with a 2" bottle neck opening at the top (about 2 and a half feet tall). I placed rocks on a base that I found for it, a large 14"ceramic plant tray, for a landscaping effect. I put large ugly, sturdy bricks in first, then the pretty rocks. Then I put the bell jar over the rocks with the plan of never taking it off. Water still ends up leaking slowly, but not alot, it still holds alot of moisture. Then I started putting media in using a funnel and a small spatula taped to a long skinny bamboo stick. First pebbles, then sand, then a potting soil. Then carefully placed the plants in with my long tools. I chose a fine fern, 4 species of moss, 1 liverwort (Marchantia), venus fly trap, 1 sundew, a lycopod (Sallengela) and a small log of lichen. I spray it with distilled water two times a week with a spray bottle. It looks great and all my plants are taking off!!

    Bookmark   December 12, 2003 at 3:05PM
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dianamay(z9 So.TX)

That sounds like a delightful project. The only thing I would be concerned about is the possibility of mold growing inside the bell, since the level of moisture is so high if water is seeping in. With such a closed situation there is no way for the moisture to escape and it might accumulate there. How do you have it lit? I hope you will put pictures in.

    Bookmark   December 12, 2003 at 3:15PM
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dianamay(z9 So.TX)

Bunping this up for beginners.

    Bookmark   December 28, 2003 at 8:35PM
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dianamay(z9 So.TX)

Bumping up for beginners.

    Bookmark   January 18, 2004 at 10:29AM
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    Bookmark   February 15, 2004 at 1:43AM
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cannahavana(z7a Knoxville)

Hi Diane,

This is wonderful information that I have been searching for. My son is a Cub Scout and one of his tasks require building a terrarium. We are going to work on a closed terrarium. Really my only question was what type of soil to use as I know that regular potting soil would sour and the amount of light. Your original post answered both for me. I have plenty of houseplants for him to take cuttings and experiment with, such as Fittonia and an assortment of ferns. I thought about growing some ferns from spores but I do not have any fresh spores available at the moment. Thank you so much for sharing your experience and knowledge with us.


    Bookmark   February 17, 2004 at 12:31PM
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wdelaney(nh, usa)

is there any restrictions on certain kinds of plants/speacies that he can have in the terrarium, is it sup[osto be a certain kind of terrarium, like bottle, tank, etc?

    Bookmark   February 17, 2004 at 12:56PM
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garyfla_gw(10 Florida)

Very well written..This should help a lot of people start the right way
Have you ever attempted a desert type setup?? For cactus or Lithops or succulents?? I've never had any luck with this type

    Bookmark   February 17, 2004 at 5:14PM
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mrbreeze(z6/7 OK)

My sister would sure love to know how to grow lithops. They're one of her favorite plants and she's probably killed a veritable forrest of them over the years. Thanks in no small part to her dumb cats.

    Bookmark   February 18, 2004 at 12:39AM
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dianamay(z9 So.TX)

The trouble with trying to put cacti or succulents in a totally enclosed space is that they require sun to flourish, and that will build up too much heat. If you leave the terrarium open, you really are just using it as a glass pot, and none of the closed terrarium rules apply.

Mr. Breeze: At the Brooklyn Botanic Garden they had quite an extensive collection of lithops growing on waist-high benches covered with what looked like small marble or limestone chunks. The benches were enclosed in wire fencing because people kept stealing the lithops. They had to make the screens finer and finer because people would even steal them thru the screen. In any event, the succulent books I have say to grow them in full sun (or slightly shaded if under glass) in a wide pan or large pot (with other succulents if desired). They do not do well in a small pot. In spring they need a very fine overhead misting daily, with slightly less in summer. Cool and dry in winter.

    Bookmark   February 19, 2004 at 10:48AM
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garyfla_gw(10 Florida)

Thanks for the info on the lithops. I can see that you're a purist when it come to defining "terrarium"When this list first started we had a discussion on the various
enclosures.Terrarium,Vivarium,Aquarium, Paludarium.Doesn't seem to be a definition.To me,it's gardening within an enclosed space.
Living in S.Florida I have to keep my terrariums open or they turn into vegetable steamers!! lol

    Bookmark   February 19, 2004 at 4:54PM
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dianamay(z9 So.TX)

And here I was, hoping to clear up the mixed messages that a beginner might have, that you can plant a terrarium as I describe, which is self-sustaining, but that it can be partially open and be watered. Once you lose and add water you simply have a whole different set of circumstances which need to be handled in a different way. Water is heavy and compacts the media, which needs to be taken into account when planting in an open terrarium, etc. Having additional water availability changes the humidity and starts requiring air circulation, something unnecessary in a sealed system. I have no expertise in dealing with these situations and don't want anyone innocently mistaking what I suggest as being rules for a different system. (In Florida you might use a closed system to keep humidity OUT.)

    Bookmark   February 20, 2004 at 3:37PM
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What is the best/safest way to sterilize the inside of a container that will be sealed? I've seen diluted bleach and lysol as options ... how long would residual chemicals be present?


    Bookmark   August 10, 2004 at 11:28PM
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sahoyaref(Alberta z3a)

I would think that if you rinse it REALLY well, using distilled water for the final rinse(s), that you'd be okay. I'd go with the bleach.

    Bookmark   August 11, 2004 at 11:53PM
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ladybug_guam(z11 Guam)

I'm soo thankful for your post, just now, I was going to ask few questions, which you answered, I wish I read this before I set up my terrarium and would of made it closed, but now, I leave about 1/2" open on the glass top for air circulation, (I also have a small waterfall in it, the ones you buy as a unit, not with a false bottom) but noticed some mold, I sprayed it but I will have to remove the top for few days to dry the soil and just mist very lightly (for that period)the moss and my mini orchids. My next one will be done following your instructions to the letter, I made a copy of them for my file.
Thanks again, excellent and simple to follow instructions!
Ana :)

    Bookmark   August 12, 2004 at 5:31AM
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I have a relatively new 20gal tropical terrarium with a clay pellet layer underneath the substrate for drainage reserve. I keep the glass top closed except for 1/8" for heat escape. My tropicals have looked great for two weeks but I cannot get the humidity above the 50-60% range. I have a variable speed internal terrarium fan for air circulation which lowered my humidity by less than 10%. I figured the air movement was worth the trade off. I have no condensation with minimal r/o water in the drainage layer. I just figure that I will keep misting a few times daily until the humidity keeps on increasing. I have not been adding water directly to the substrate around the plants since the substrate is slightly evenly moist. I am afraid to add water directly to fill the drainage layer because it is liable to keep my plants wet all the time without any time for slight drying.

Any humidity help or other comments would be appreciated.

    Bookmark   October 21, 2005 at 1:51AM
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I'm worried about using perlite and vermiculite because of all the cons on them, especially about the health issues related to perlite. Any take on this?

    Bookmark   October 22, 2013 at 1:15AM
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Great information, thank You, Diana!

    Bookmark   June 2, 2014 at 7:21AM
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warpiper(8 Charleston, SC)

Thanks Diane for sharing your experience with terrariums. I have tried doing terrariums many times and always failed at it. I'll have to give it one more try now.


    Bookmark   June 26, 2014 at 9:19PM
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