I remember the moment when I first understood why people were coming here. It was one spring when I was 10 or 11. I had been away at school and returned for the holidays. I walked out into the garden, just to have a look at it, to remember it, turned the corner into the Lime Walk and then down the long perspectives under the branches of the hazels in the Nuttery. The woodland floor beneath them was carpeted with multicolored polyanthuses, a Persian rug of flowers 55 m long and 27 m wide, the colors just hovering, like the colors of those rugs, between the bright and the rich, half sequin, half velvet. The trees were just letting the sun through in blobs and patches. This scene has gone now — by the 1970s the polyanthuses had exhausted essential nutrients in the soil so they can never again grow there — but I will never forget what I saw then: a vision of great simplicity, a single idea, transforming something seen in acre after acre of Kentish woodland into something no one had ever seen before.

This garden is more than just a diverting day out. For the modern visitors, as it was for my grandparents, Sissinghurst encapsulates an idea of beauty that goes beyond the everyday. Early in the morning, or on a soft summer evening, or moonlit, it can be otherworldly. But at any time, it stands apart from the ordinary. It shuts itself away in its series of hedged and walled enclosures and thrives on its sense of secrecy. From the outside, there is little to tell what you will find within. A long brick range, pierced by its central archway, conceals the garden from view. Even beyond that archway, within view of the Tower in which Vita worked, there is still an air of reticence and discretion. Only when you find your way in still further, through the hedges into the Spring and Rose gardens, on around another few corners to Sissinghurst's two core places — the flaming reds and oranges of the Cottage Garden or the cool green and white pool of the White Garden, where you will find the air as chill and as elegant as a glass of white wine — do you begin to feel you have arrived. Even then you might never quite feel that you have. All the best pilgrimages invite a return.

My father still lives there, and for the time being I live a few miles away. I will live there when he dies. But, after an adult life largely away, I am more convinced than ever that what my grandparents made is one of the great English works of art of the 20th century. They found it in 1930 as a wreck: a great 15th and 16th century house lay in ruins amid cabbage and potato patches. The only flower growing there was an old deep crimson Rosa gallica pushing its head up between the rusting bedsteads. But the fragments of the great house were the makings of the garden. The walls, the odds and ends of the buildings, the great double lighthouse of the tower all provided a bony structure within which plants could be allowed to flourish. The garden they made hovers, as they intended, in romantic ambiguity, in part redeemed ruin, part a great house that seems, in a soft and theatrical way, to be sinking back into the landscape it adorns. Large amounts of money, ingenuity, inspiration and labor were poured into Sissinghurst, but none of it is visible. Sissinghurst looks as if it has always been there.

Iris Murdoch said that what identifies great art is that it must be for everybody, and Sissinghurst is. The highest levels of horticultural skill are applied to its effects, but you could never tell. The plants are allowed to flop and ramble over the brick walls and paths as if they were there by chance. The late Romantic aesthetic conceals all art and effort and, ideally, you should come across Sissinghurst's gardens as though no one had ever noticed them before. In that way, Sissinghurst is immensely generous. It never imposes; its greatness is intimate and reproducible. You can do Sissinghurst at home in a way you can't do Versailles. Its simplicity of structure is combined with a gentle and painterly eye, a drifting of colors and forms, a playing between precision and relaxation, which is the English sensibility at its best: neither tight nor careless, to the highest standards, but for everyone. People make their pilgrimage to Sissinghurst in order to enjoy a dream of English perfection, half natural, half cultural, a place that shows how beautiful the world might be. Where earlier generations of the English might have gone to church to understand such things, they now garden, and Sissinghurst is one of their shrines.

It is tempting to think that Sissinghurst is most itself when the people have left, when the gates are finally shut on a summer evening and the garden sighs to itself, like a host after the last of his guests has gone. Of course that is a beautiful moment. But occasionally you find a couple, or even a little group, who have stayed on after that moment, and are still drinking the place in, sitting in a private corner. I don't think it is too pious to say that Sissinghurst is even better then; that's what its life is for. Landscapes need their figures.