Terrarium planting is a specialized part of home gardening, but an interesting one. A great many woodland plants which prosper in their natural environments fail to grow at all in the house due to the lack of moisture in the air. Terrariums provide this humid atmosphere and allow you to bring the forest plants home.

The theory behind the terrarium is that in a closed container the moisture which the plants take up through their roots and transpire through their leaves is condensed and eventually returns to the soil again, keeping the plants watered and at the same time keeping the air within the container at a point just below saturation.

Containers to use. If you are thinking of starting your own terrarium you will probably find that you have all the material and equipment you need around the house. Fish tanks, especially those that are not too big and unwieldy, make very good terrariums. All you need for them is a glass top. If you are going to start from scratch, you can build your own container using a flat, low-sided box for the bottom, and fitting it with glass sides which can be taped together, and a glass top.

Although the theoretical airtight terrarium would have no openings whatever, it is safer to provide the container with several drainage holes so that you avoid the risk of over watering and rotting the roots of your plants. If you use a large brandy glass or some other container for which drainage holes cannot be provided, then you must be extra careful that there is a good layer of drainage material (pebbles, gravel, crocking, etc.) at the bottom, and that the soil is sweetened with a sprinkling of charcoal.

Laying the groundwork. The way you place the soil at the bottom of the terrarium is extremely important. It should always be well drained. Start out by placing large pieces of concave crocking over each of the drainage holes to prevent the soil from leaking out. Then add an inch or so of good drainage material.

Follow this with another layer of a couple of inches of a good potting mixture, such as those that can be bought in a garden-supply store. It should be filled in firmly, but not tamped down too tight. The soil should be slightly damp, but not soaking wet. Then place the plants, which should be ready nearby, by digging small holes large enough to accommodate their balls of earth in a pleasing arrangement. It is a good idea to work out the arrangement ahead of time so that you know in advance just where you want each plant to grow.

Watering, ventilation and sun. When your plants are all in position, the soil should be well watered. If you can do this in the kitchen sink or in the bathtub where you can give it a thorough wetting and then allow the excess to drain out, so much the better.

The frequency with which terrariums must be watered after they have been started depends on how much ventilation they are given. Although opinions vary, it has been our experience that the plants will prosper if they are kept covered during the day and the top is partially slid off at night. If this system is followed, we think you will find that your terrarium garden will need watering only once every few months.

Terrarium plants need sunlight, but in limited amounts. An east or west window will do the trick, or a south-facing location, if there is a glass curtain between the window and the container.

Plants to grow. In addition to lichens and mosses, a good many plants which grow in the open air of your living room will prosper as well in miniature form in the terrarium. Your florist or nursery supplier can give you many suggestions.

Now all you need to do is sit and watch your beautiful terrarium.