Even Gertrude Jekyll, that ace planter, thought that gardens should be more than the sum of their plants. Gardens that you have to inspect plant by plant are lovely, but are hardly works of art. A kick in the solar plexus is what most of us demand from a masterpiece, and it may be news to some that gardening can deliver just that. With an edge. Gardens are, after all, the only art form you can be part of. You can walk through a landscape, which turns it into kinetic art, you can work in it, or watch it change with the seasons, which makes it performance art, or you can simply sit and absorb its beauty, as you might gaze at a painting.

In the 18th century, gardens had status. Alexander Pope and his contemporaries ranked them with poetry and painting, because their role was to provoke thought. In those days, horticultural skill was the paint on the canvas, not the focus of interest as it is now. When gardens rise above the techniques needed to create them, that’s when they sing to us. It’s the singing that makes them art. There are many gardens that can prompt thoughts or strong feelings. In some examples, we may lack the receivers to distinguish their message, but we feel something important in the air. The gardens where we sense that we are somewhere special, without knowing quite why, are places where the owner or designer has breathed life and meaning into it, where the ghost of their inspiration lingers, so that the visitor goes home feeling changed.

Feeling is what counts. The experience of being in a garden can be transforming, but you need to be out there for the magic to work. Just as we do not accept a reproduction of a famous painting, we should not rely on images of gardens to convey a charge of emotion. What works on the imagination is the experience of being in a wonderful place, of trying to understand it. Unity of vision is critical for any artwork and it was that unity that made Pevsner describe 18th-century landscape gardens as our greatest contribution to western art.

Take Henry Hoare, the creator of Stourhead in Wiltshire, who believed that looking at paintings and books was the mark of a civilised being. Stourhead represents the story of his life and aspirations, inspired by his admiration for Virgil and the pastoral idylls of Claude. Geoffrey Jellicoe’s work at Sutton Place, in Surrey, is visually unlike Stourhead, but what it has in common with Hoare’s landscape is that it was designed to work on the mind. Beauty can be disappointing unless it engages with the imagination. Jellicoe believed that any visitor to a garden should go on thinking about the place long after leaving it. Not in the “I-wish-I-could-grow-hellebores-like-that” way that is now the norm, but to prompt a dialogue with the soul on the things that really matter. Eternal truths are common to everyone.

We are all bound for the dark, and 18th-century gardens were filled with memento mori, represented by urns, which have lost their potency now. But in the garden that the film-maker Derek Jarman created in Dungeness in Kent during the late 1980s, the flotsam and jetsam of the seashore provided a context of decay to underline the preciousness of life. In Scotland, in the 1960s, the artist and poet Ian Hamilton Finlay created Little Sparta, a garden that was part autobiographical, part polemic and part poetic — among a riot of puns. Little Sparta is intended as a neoclassical model of society, and even if the revolutionary politics are not to everyone’s liking, it has an undeniable force.

The spell of significant content, the connections and associations with people or places, are what give a garden resonance. The fact that anything grows is what counts; the optimism of nature is the message of every garden. The return of spring, the resurrection of life each year is an extraordinary realisation. In Barbara Hepworth’s tiny patch, run by the Tate Gallery at St Ives in Cornwall, space and light are everything. In it, her sculptures are an overwhelming presence, but one also feels the sense of the woman who was concerned above all with man’s position in the landscape and the way in which humans relate to forms that are eternal in their significance.

It is hard to follow Charles Jencks’s explanations of the remarkable Garden of Cosmic Speculation that he and his late wife Maggie Keswick created in Dumfriesshire. His talk of feng shui and fractals is bewildering. But we don’t need to understand everything to respond to a place, only to respect the creator’s vision. We may not all be gifted with the talent, scope and money lavished on masterpiece gardens, but nor do we expect to paint like Turner or write like Shakespeare. Most of us recognise that there is a difference between the paintings of a gifted amateur and those in the National Gallery. Where did we lose the ability to tell the difference between a garden that is a work of art and one that is a private hobby?

OXFORD LITERARY FESTIVAL

Mary Keen considers the garden as a work of art, at the festival, on Sunday, March 25, at 10am

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