These days the average person spends more time in air-conditioning than the fresh air, especially in a Canberra winter. This is sad. Humans evolved with green and growth around us, and we are happier and more relaxed when surrounded by it.

I have the luck to work surrounded by gum trees, several hundred fruit trees, not to mention birds and snorting wombats - though it took a few decades of work as well as luck. But if you lack a garden within sight where you work, you need a terrarium.

Terrariums have five great virtues: they are cheap and easy to make, or even free; they look fabulous; they take almost no work; they can be left to look after themselves when you go on holiday; and they don't make a mess. They can even be sealed, if you use the right plants, so no mould, fungal or mildew spores escape from the soil, and no perfume annoys the sinuses.

Basically a terrarium is any glass container where you can grow a miniature garden. An ornate glass bowl makes a most elegant uncovered terrarium but an old jam jar can be the basis for a cheap/free one. The bigger the bowl the better, so you can put your hand in and not have to do the work with a long-handled teaspoon. Old apothecary jars, empty second-hand aquariums, that big glass jug or glass vase Aunt Agatha gave you for your birthday five years ago and you've never used - all are excellent.

A lid stops moisture evaporating, but isn't essential.

First of all, place a layer of pebbles in the container, at least at deep as your thumb minus its nail. The pebbles can be attractive pure white or blue grey or shiny black ones bought from garden centres, or snavel some excess gravel from the side of the road.

A same depth of "activated" charcoal goes on top of this. Activated charcoal is basically the black lumps left from wood burned at a high heat. You can buy it, or scavenge some from a fireplace next time you go on a bushwalk, or keep an eye out for a smoking chimney in this winter weather and put a polite note in the letter box with an empty jar below it: please could you fill this with charcoal?

Now add two layers of potting mix. At this stage you may decide you need a bigger jar, as if it's more than half full, your plants won't have room to grow.

The exciting bit comes next: plants. If you're broke, "borrow" a few leaves of someone else's African violets, or divide a clump of a low-growing ornamental grass like mondo grass. Plants for a terrarium need to be naturally low growing, or able to branch out when pruned for an open-mouthed terrarium.

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The artistic terrarium creator can add shells, driftwood, ornamental "boulders", or whatever their creativity suggests.

You might even make a fairy garden, with a small statuette of a fairy peeping through maidenhair ferns, or a small jungle of begonias for toy tyrannosaurs or velociraptors to lurk among - both ferns and begonias are excellent for closed terrariums, and so make excellent gifts for kids who won't remember to water them.

Kids won't remember to feed their terrarium either - hopefully. This is good, as terrariums don't need to be fed. Terrarium plants can't grow much and so get all their nutrients from the potting mix.

If you want more advice on what to plant, ask the nice person at the garden centre what they recommend. If the budget is strained, see what's in your friends' gardens/pots that you can pinch a cutting from, or divide. You might even boost your productivity with a small rosemary bush in a very large jar - break off the leaves and sniff them as they begin poke above the top of the jar. The scent of rosemary is supposed to increase memory.

Once you've planted your terrarium, the surface area is supposed to be "finished" with a layer of something ornamental. Try moss or you could also cover the exposed soil with more ornamental pebbles, or better still, plant a tiny ground cover like Corsican or creeping mint - the scent will seep out on warm days in delicious whiffs.

Your terrarium is now ready for a judicious watering, just enough to wet the soil. A sprayer is perfect for this and you can use it to wash away any mess you've made on the glass.

Now find a spot - not a hot sunny one but one with adequate light.

If the light is not adequate, your plants will tell you. Within a few weeks they will look pale, and more tellingly, any taller plants will be leaning towards the light. Accede to their request and find a better home for them. Terrariums that are too hot will give you brown leaves. Soggy terrariums will give you brown leaves and a smell of rot. You might just save your plants by pouring out excess water.

Most terrariums only need watering every couple of weeks, even those with no lid on them. Some can go for many weeks. Just keep the potting mix looking damp. Even if you are terrarium novice, you'll soon learn to tell the difference between wet and dry potting mix - the latter is darker. Over-watering means a layer of brownish liquid collects among the rocks. Don't add to it, and with luck and an airy spot it will evaporate with no harm to the plants - that's why the pebble layer is there. The charcoal is meant as a filter, to stop the potting mix slowly seeping into the pebbles - another symptom of overwatering.

Once you've assembled your terrarium, that's it, beyond a little water now and then. The only other rule is to keep it where you can see it grow, minute but fascinating changes as the Corsican mint spreads, or the black mondo grass grows taller and the begonias and African violets bloom.

Every office needs a terrarium, either on desks or a large one, preferably three metres long. So do hospital wards - closed terrariums, carefully engineered so no mould or fungal spores can emerge.

Making them is fun, and having a small enclosed (or not) ecosystem next to you while you work, study or fill tomorrow's lunch boxes, will not just add greenery but a tiny fragment of peace and beauty to your world.

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