You will come away convinced that, even if we and our flowers escape being parched to death by drought, we are still doomed because we let plant varieties perish and ignore bio-diversity - the new religion of the age.
At the same time, you will learn that those of us who, year after year, lovingly cram our gardens with delights from nurseries and garden centres, snipping away to keep them trimmed and tidy, have been doing it wrong. For the sake of the planet we should be letting wild flowers and weeds run riot over our backyards.
Whatever happened to the joy of gardening? You would have thought that the wettest June since Noah would wash some of the gloom from the long faces of the Cassandras of global warming. To judge from the sudden downpour on the show's preview day, leaving some marquees flooded, global drowning is a more real threat.
Almost the first person I ran into there was the editor of Gardening Which?, that alert watchdog for the horticultural consumer. Since the magazine had devoted its stand to water conservation, it was no surprise that he should inform me with relish that the June rains had made precious little difference to water reserves, because they had mostly evaporated.
And yet, I replied, the plants in my own garden are looking lush and healthy. Aha! Even that is not the good news it appears. Later in the summer, the prophet warned me, when the real drought comes, they will suffer even more because the early rains will have accustomed them to an extravagant supply of moisture.
Thames Water, sponsoring a post-modern garden made from recycled water pipes with solar-powered pumps, were busy plugging the same theme. They might have better spent the money plugging the leaks in their system that wastes millions of gallons a year.
The show's bleakest spot of all is Christian Aid's `Global Garden of Eden', with its warning of disaster unless we follow the bio-diverse path to salvation. If we ignore the plea, as many as 60,000 varieties of wild plants will be extinct by the middle of the next century.
"The result could be catastrophic, leading to food shortages and famine on an unprecedented scale," the accompanying leaflet explains. "Many plants which could potentially provide the cures to diseases such as Aids are also being lost."
The dramatic highlight of this Eden is a sandy desert, empty except for a scattering of tombstones commemorating celebrated botanical catastrophes: the Irish potato famine (600,000 dead); the Sophora Torimoro tree in Easter Island, last seen in 1962; Hubbardian grass, made extinct in India because of a hydro-electric scheme. The final tombstone reads "Homo Sapie . - ." with the two final letters - n and s - missing, symbolising, I suppose, the extinction of Man before the carver could finish the work.
It is as hard to argue against diversity - bio or any other kind - as it is to oppose motherhood; but it is also hard to justify such a cataclysmic approach. Many garden plants disappear simply because they are superseded by others that perform better. To conserve them all, regardless of their value, makes no more sense than preserving every building ever constructed.
Not all the show gardens at Hampton Court are sermons in stones and plants. Some still cater for the great majority of gardeners who aspire to little more than a patio, a bit of lawn and a few geraniums. But very few of those have ever found favour with the Royal Horticultural Society's judges.
This year's show marks the culmination of a creeping trend: wild flower mania. Both Tudor Rose awards for show gardens and water gardens - the top accolades in these sections - went to exhibits that can scarcely be described as gardens, in the conventional sense, at all.
The winner in the first category was `The Railway Children's Garden', containing a working steam locomotive, a level crossing, a signal box and a bridge. A studiedly unkempt cottage garden runs down to a stream, with a wild water meadow beyond it.
In the water garden section the award went to `A Garden Richer in Wildlife', sponsored by the London Wildlife Trust and based on the fallacy that wild creatures can thrive only in natural settings. Tell that to the squirrels of south London, who greatly prefer digging up my carefully potted plants to scrabbling about in the nearby weeds.
The second prize, and a gold medal, went to `Magical Ireland', another garden virtuously trying to pretend that it is nature in the raw. But this is not much more than a mound dotted with rocks beside a pond, its grass verges infested with daisies and celandine.
Today's designers have lost the plot. A small garden is supposed to look artificial. The point is not to replicate nature, but to tame it and create something you find restful, spectacular, or whatever takes your fancy.
I am not denying that there is artifice involved in designing a garden to look as though you have simply fenced in part of a field. My point is that in real life, as opposed to in RHS shows, it is not something that many gardeners do.
Gardens that reflect popular taste do not nowadays win the top awards. But you would expect Marks & Spencer to have its finger on the pulse of the high street, and the three gardens in its display confirm that they do. Stiff with hard landscaping and ablaze with bright colours, they illustrate the preferences of three types of ordinary householder.
On preview day, people queued patiently to catch a glimpse of them. There were no queues at all for `Magical Ireland'. Yet the judges placed M&S among the also-rans, with a paltry silver-gilt medal.
Enough is enough. It is time to return gardening to its roots. Give us back our bedding plants and pergolas. Why can't someone invent a spray (organic, naturally) that would eliminate horticultural fashion and taste as readily as a garden spray kills off green fly?
That underrated philosopher Sam Goldwyn said: "Pictures are for entertainment. Messages should be delivered by Western Union." With gardens substituted for pictures, those words should be embroidered and framed in the Royal Horticultural Society's offices.Reuse content