I suppose it is the contrast between what's happening inside and what's happening outside that makes the shows so special. Outside there is hooting and screeching and dust and litter, tarmac, brick and concrete. Then you pass through the doors of the curiously old- fashioned building (even the hall they call the New Hall was opened in 1928) and are enveloped in a different world.
There is the low, contented hum of a well- organised hive - gardeners on a spree. There is the distinctive smell of bruised foliage mixing with a hundred different flower scents. There is a magnificent sense of profusion. And a sensation of being some specially gifted horticultural time traveller as, with one turn of the head, you can take in Jim Jermyn's searing displays of gentians, grown in the acid soil of his Berwickshire nursery, together with streptocarpus from the Efenechtyd nursery in north Wales, hostas from Sandra Bond at Goldbrook Plants in Suffolk and Carol Klein's cottage garden plants brought up in a hired van from Devon.
It would take you several days' travelling to get around those four nurseries. At the Great Autumn Show, the next on the RHS calendar, you can expect to find at least 80 nurserymen exhibiting their plants, together with displays from specialist societies such as the Delphinium Society and the Brogdale Horticultural Trust, second-hand books from Mike Park and botanical prints from Deborah Cutler of Cranborne Antiques.
The Great Autumn Show, on 15-16 September, is one of my favourites. It is partly the time of year. There is the pleasurable anticipation of a last fling before the hatches are battened down. The mood is reckless, slightly decadent.
Flowers are ludicrously rich and luxuriant. Fruits have a lascivious air. 'Go for it,' they shout on every side. And mostly I do, although with incredible restraint I have so far managed to stop myself buying gentians. I could not make them happy and I could not bear to see them sad. The Great Autumn Show gives me my proxy gentian fix.
The fact that you can buy plants at these shows makes them wonderfully dangerous. No phone call, no letter-writing, no car journey stands between you and the object of your desire. And here you will always find plants you have never seen before. By far the greatest proportion of the nurserymen at the RHS shows run small specialist operations.
There are always a few big stands. Aylett's usually has one to show off its dahlias. So does Burncoose & Southdown Nurseries, which comes up from Redruth, Cornwall, with a seductive collection of West Country specialities. David Knuckey, who puts together its stand, has developed showmanship to a fine art.
'It's nothing like gardening,' says Carol Klein, who has already started planning her display for the Great Autumn Show - her third. 'You've got to have everything at the peak, all at the same time. That doesn't happen in my garden very often.'
And there are some plants that never show well. Penstemons are difficult, she says. They do not grow to best effect in pots. In contrast, Campanula takesimana, a rare beauty with lilac white bells and stippled inside with maroon, invariably stars on the show bench. It is unusual enough always to attract comment and has the sort of flower that shows itself off more effectively when it is raised from the ground.
Most exhibitors stage their displays on table tops about 8ft x 9ft (2.4m x 2.7m), which are covered with long skirts to the ground. The plants for sale are usually stored under the skirts, so conversation with nurserymen is more likely to be addressed to their trousers than their faces. Ms Klein reckons to bring at least 100 different types of plants to the show, plus a thousand more in 9cm (3 1/2 in) pots for sale. This year she will be showing unusual anemones and crocosmias among her cottage garden plants. And she will have a star verbascum, tawny-coloured 'Helen Johnson', for sale.
What makes her do it? It is a long way to Westminster from her nursery at Umberleigh in north Devon. She has to hire a van, be away from home for half the week and find accommodation in London. Is it worth it? 'Yes,' she says with the unequivocal conviction of the converted.
'I still remember the first Westminster show I ever went to. That was the Great Autumn Show. It was not long after I had started the nursery. I wasn't showing, just looking. I got a lift up and then went home on the bus with a huge black plastic sack of plants. The excitement was the chief thing I remember. And the richness of it all. Then I thought, 'I could show there.' I wanted to meet more people, show off my plants. It's quite an isolated business, the nursery business. It's good to see what everyone else is up to.'
Ms Klein usually travels up with her plants on a Sunday night and spends all day Monday, from seven in the morning to 10 at night, setting up her stand. 'I'm always the last out. Then I'm usually at it again first thing on Tuesday before the judging at nine,' she says. She tends to have a rough plan in mind but does not always stick to it: 'The plants themselves tell you where they want to go.'
When she can she slips off to see her own favourite nurseries: Potterton & Martin of Caistor in Lincolnshire, which specialises in alpines, dwarf bulbs, ferns and orchids; the Paradise Centre based at Bures, Suffolk, where Cees and Hedy Stapel-Valk grow alliums, crocuses, erythroniums and fritillaries.
I have favourites, too, although they are constantly changing. I never get tired of the calm beauty of sempervivums, so am glad that Alan Smith will be at the Great Autumn Show with his unrivalled collection. And I am always transfixed by Ingwersen's displays of alpines. I suspect that some of their show plants, the sort that grow a quarter of an inch a century, have made the trip to Westminster more often than I have.
The Independent has arranged a special private viewing of the Great Autumn Show for readers (see right for details). This will be held on the evening of the first day of the show. See you there. I'll be the one with too many carrier bags and a notebook between my teeth.
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