Anna Pavord: Totally tropical

I've been in the Caribbean, sailing mostly, and marvelling at the moon, which went through a total eclipse while we were there. Otherworldly? I should say so: a warm night, water lapping against the hull of the barquentine, the sea blackening as the earth's shadow passed slowly over the silver disc in the sky. But the Caribbean itself is another world too – outrageous plants with leaves as big as tablecloths and flowers that might have been designed especially for Walt Disney's Fantasia. Orchids look almost natural there in a way that they never do with us in Britain. Here, they seem to peer disdainfully at their surroundings, wondering what misfortune has brought them to roost on this window ledge, in this chilly milieu. Cymbidiums are the only orchids that cheerfully get along with curtains, sofas and carpets.

But at Orchid World in St George's parish, Barbados, you don't ever see the terrestrial cymbidiums. Here, the focus is on epiphytic orchids, the ones that have their roots dangling in space, not anchored in the ground. That was a clever move. Instead of fighting off other plants on the ground, orchids took to the trees, and adapted the spongy tissue round their roots to pick up moisture and minerals from the air rather than the earth. How many million years did that take? And how many more million years before the orchids had adapted further and sorted out between themselves whether they were penthouse types (odontoglossums, oncidiums, masdevallias) that liked sun and a relatively open prospect, or basement dwellers, like the ascocendas, happiest in shade.

Orchid World used to be a pig farm, and the fairy godmother who transformed it is a Yorkshire landscape architect, Richard Coghlan, who went to Barbados to design a golf course and found several reasons to stay on there rather than return to Britain. The channels that once ran with slurry were turned into streams, pig pens became shade houses and coral stone rock (Barbados is unusual in the Caribbean in being a coral island rather than a volcanic one) disguised concrete walls. Orchids are hung in rows in the shade houses like washing on a line: vandas, epidendrums, phalaenopsis, cattleyas, oncidiums, calanthes.

Cattleyas were famous long before Marcel Proust went on about them at such length. They arrived over here almost by accident when a plant hunter, William Swanson, used the cattleya's stems and leaves as packaging in a consignment of Brazilian plants he was sending to William Cattley of Barnet. Cattley made his money as a merchant but his real passion was plants. "One of the most ardent collectors of his day" said a fellow enthusiast, describing how Cattley grew the packaging as well as the contents and the sensational moment in November 1818 when the then unknown plant put out its first pouting bloom.

So it was fair that the cattleya should have Cattley's name. That first species, Cattleya labiata, has spawned innumerable hybrids, but no plant hunter retracing Swanson's footsteps in Brazil will now find it there. Once so common, it could be used like straw, it's now gone forever from its home in the Organ Mountains. It's not where most of us would start though, if we wanted to grow orchids. Cattleyas are fussy. Light, humidity, temperature, food, water, all have to be exactly right – especially humidity. But at the same time, cattleyas need plenty of air flowing around them. During the day they demand a temperature around 20C and don't expect the night to be chillier than 14C. They like good light but not full sun, so inside, that means choosing an east or west facing window ledge rather than a north or south one.

Though a sitting room might seem to provide the right kind of day/night temperatures, the air is likely to be too dry to suit the cattleya. Mist the plant regularly and stand the pot on a tray of pebbles which you can damp down regularly. The free air flow is more difficult to arrange. Did Cattley hire a boy with a fan? And how did he work out that the plants like to dry out for a couple of months in winter, even if they are in flower?

New species of orchid are still being discovered in the rainforests of Central America, Africa, India and the Far East. Botanically, they are a daunting family with 25-30,000 different wild kinds, not counting the many more thousands of hybrids that have been bred from them. Where on earth do you start with a family as massive as that? "With cymbidiums," I'd say, because they are easy and profuse, with flowers that last for two months or more. I was introduced to them by our eldest daughter who, perched in London, scoops up orchids from the Columbia Road street market in the same carefree way she chucks bottled water into her Sainsbury's trolley. Both seem extraordinary extravagances to me. As a person who would prefer to go without food than flowers, she reasons differently. An orchid plant (£15) lasts in bloom for at least a month, sometimes six weeks. A bunch of cut flowers (£6) barely survives a week. Therefore the orchids are a bargain.

Cymbidiums are huge plants with strappy leaves a couple of feet long. The flower spikes can bear as many as a dozen blooms, in weird buffs and pinks and pale yellowy-green. They like a cooler climate than the cattleyas, somewhere around 16C or a maximum of 18C during the day, 10C at night. Keep them away from radiators; they hate being cooked. Inside, they are also best kept out of direct sunlight. The chief danger is overwatering. Let the pot dry out between drinks and don't be tempted to repot too often. Cymbidiums flower best when they are potbound. At the end of May (or when they have finished flowering) take the pot outside for the summer. At this point, cymbidiums need sun to ripen their growth and initiate the flower spikes for the following season. Remember to bring the pot inside before the first frosts.

Though at first they look very different, all orchid flowers conform to one basic pattern. The three outer petals stand at 12 o'clock, 5 o'clock and 7 o'clock. The three inner petals are usually showier than the outer ones, wider, frillier. One is set at 3 o'clock, the other at 9 o'clock. The third develops into a lip (labellum), a landing stage for beetles, flies or moths, and is marked with lines like a runway to point the pollinator in the right direction. At the centre of the flower is a finger-like column which contains the pollen. Millions of pollen grains are compressed into tiny parcels for insects to carry from one flower to next.

Orchids used to be staggeringly expensive because they were very difficult to propagate from seed. Then some observant person noticed that in the wild any surviving seedlings usually sprang up close to the mother plant. It turned out that a fungus in the roots of the mother plant was an essential catalyst. Seeds could not germinate without it.

By 1922 an American professor, Dr Lewis Knudson of Cornell University, was showing commercial growers how to inoculate their sowing medium with the nutrients provided by the fungus. For the next 40 years more than a million seedlings were successfully raised by this method. Then in 1964, Dr Georges Morel introduced the revolutionary technique of propagation by tissue culture – micropropagation. Most orchids now begin life in a laboratory and take four years to develop from a scrap of tissue in gel to a full-grown flowering plant. That has brought the average price down from £500 a plant to £15, though rarities are still wildly expensive. A Japanese-bred orchid Neofinetia falcata 'Brown Bear' was sold recently for £50,000.

There's a lot to learn with orchids and the place to do it is the London Orchid Show which is going on this weekend at the Royal Horticultural Society's Halls, Vincent Square, London. Among the exhibitors will be pupils from Writhlington Comprehensive School in Bath who, with their teacher Simon Pugh-Jones, renovated derelict greenhouses to house what is now an extensive collection of orchids.

The show is open today and tomorrow (10am-5pm). Admission £5. To book tickets call 08456 121253. If you can't get to the show, buy Easy Orchids by Liz Johnson (Collins & Brown £14.99) which is simply laid out and well illustrated.