I am not referring to how Sir Simon conducts his gardening - although his own view of himself as an enthusiastic amateur is probably over-modest - but how he conducts his life. Born, as he once confessed, with a silver spoon in his mouth, going to Eton, Oxford and the Grenadier Guards, he inherited at 47 the chairmanship of the high-street retail chain W H Smith - his grandfather having been an original partner.
Last month, after 12 successful years, he fulfilled his long-stated career plan of stepping down before he reached the age of 60 - and walked straight into the RHS post, which he describes as 'the most marvellous thing that could happen to me'.
We sat in his new office - smaller than his old chairman's office, but still handy - at W H Smith's Chelsea headquarters, a few hundred yards from the site of the May flower show that marks the high point of the RHS season. 'To me,' he observed, 'the most important thing in life is to enjoy yourself.
'I simply love gardening. If I had thought about what I wanted to do next I would have put this at the top of the list. In the event, it came right out of the blue and I've never been so excited by anything.' He should not be surprised if the rest of us, being mortal, view him as the man at the flower show who comes away with all the prizes.
In terms of gardening politics, though, it matters less why Sir Simon is glad to be president than why the RHS is glad to have him. He is not the first businessman in the post - his predecessor, Robin Herbert, holds a number of directorships - but he certainly brings to it the highest City profile. Before Herbert, presidents tended to be members of the aristocracy. Since the job was not taxing, it did not matter that they grew old in office or that the great majority were dislodged only by death - including Prince Albert, Queen Victoria's consort, who served from 1858-61.
The post is still part-time (Sir Simon estimates he will spend between one-and-a-half and two days a week on it) but is nowadays in no sense a sinecure. The RHS is big business. It has nearly 180,000 members, owns four large gardens - all with plant shop - and runs 23 shows a year. It publishes a monthly magazine and numerous books. It not only has a businessman for a president but also, since last year, for its full-time director-general: in July Gordon Rae, from the chemicals company ICI, replaced the plantsman Christopher Brickell.
Sir Simon believes the society already gives a good service. A recent survey of members found that they largely agree, but they think the Royal Horticultural Society falls down on communicating its knowledge and expertise. A separate report called for improvements in its educational and scientific work.
The show side, though highly professional, has inevitably suffered strains from its recent rapid expansion: in the last two years the RHS has assumed full or shared responsibility for six new events. The Hampton Court show, which Sir Simon believes has tremendous potential, just broke even last year, having made a loss under its previous operators. The co-operation with News International over an indoor Easter show at Wembley turned out to be a one-off adventure: this year the show is at Olympia and the RHS is not involved.
The new president, who will formally assume office at the society's annual general meeting on Tuesday, insists he is not going in with a brief to sharpen its commercial performance. 'It is a well-run organisation looking for improvement all the time,' he said. 'I'll be just touching the tiller rather than pulling the wheel around. But we're going to have to raise a lot more money - there are things we want to do to improve facilities.
'My experience has been in running a big organisation - marketing and relationships with customers. Our job is particularly to look after our members and the trade. I think I really do understand how to give good service, and service is critical.'
This soothing approach is calculated not to alarm his new colleagues at the RHS. Sir Simon has learnt discretion from one of the rare blips in his otherwise unruffled career, when he became embroiled in a bitter controversy shortly after becoming chairman of the Design Council seven years ago.
In an unguarded interview with Vogue magazine, he attacked the council as 'a faceless body', its Haymarket headquarters as 'a second-rate souvenir shop' and its magazine, Design, as 'impossible to read'. He was censured by the staff and apologised; but his initial vision was sound enough and by the time he stood down in 1992 he had introduced most of the radical reforms he had envisaged.
The criticism he levelled at Design magazine is, as it happens, sometimes made of the RHS monthly, the Garden, which is sent free to members as part of their pounds 23 annual subscription. Despite recent improvements to its format, many feel that the Garden is still too academic for its general readership. Sir Simon, though, will have none of it. The magazine, he says, is 'good and getting better'.
Since I was clearly not going to prise any Vogue-style indiscretions from him, I switched tack and asked about his horticultural background. He grew up in a large house, at Pusey in Oxfordshire, with a showpiece garden designed by the legendary Geoffrey Jellicoe in 1935. 'As a boy I loved working with my parents,' he said. 'We were a very happy family and they were both good gardeners. They encouraged and taught and enthused me. I liked being with them and we did it together.'
He took his interest to Eton, where he worked in the garden at a master's house. When, in 1968, he married Sheran - daughter of Peter Cazalet, the Queen Mother's racing trainer - they moved into the rectory near his parents' home and created a garden there from scratch. Three years ago they moved to a house near Wantage with 50 acres of grounds, and he instantly set about creating a new garden there.
'It's very exciting. We've planted a lot in a year and I'm pleased with what we've put in so far. I've concentrated on trees and shrubs at this stage because they take the longest to establish - interesting oaks, acers and sorbus. I'm going to create a walled garden which will be very formal, otherwise there will be a lot of shrubs and plants that like water - primulas and hostas and things. It has a lot of water.'
Sir Simon is lucky (again) to be next door to a racehorse trainer, Tim Forster, who supplies him with the soiled wood chippings that nowadays replace straw in many stables. With these, he has spread a 4in mulch over all his beds. 'I'm a great believer in mulching, and wood shavings are wonderful. Most plants are very greedy and amateurs don't feed them enough.'
This is one of many pithy opinions, passions and prejudices that, from 1983 to 1987, Sir Simon used to share with readers of Tatler in an entertaining and elegant gardening column. He was, for instance, one of the early apostles of the current horticultural correctness that abhors rose gardens.
'I adore roses,' he told me. 'They're probably my favourite plants - but I think rose gardens are incredibly boring. Roses should be planted among other things.'
Other pet hates include variegated plants ('occasionally they are wonderful but they have to be used very sparingly'), bright yellow flowers such as marigolds, and winter gardens: 'I'd rather be in front of the fire than walking in a garden in winter. I admire winter gardens as architecture, but I can't bear those tiny little irises that come out everywhere.'
Now that he has more time - although he is still on the board of several companies and public bodies - Sir Simon is thinking of taking up his pen again, not necessarily as a columnist but more probably to write a book.
'I'd like to do one about the first five years of my new garden. I think creating a garden is of great interest to people. What I find is that a huge number of my friends who have houses in the country, but know nothing about gardening, don't know where to start. I would take a lot of photographs and show it developing and describe how I did it.'
Envy is an unattractive quality; so I gnashed my teeth silently and forbore observing that, with his record, how could it turn out to be anything but a best-seller?-
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