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.)
CLOSED TERRARIUM PLANTING AND CARE
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.
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.
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.