Can a garden be a work of conceptual art?

If the great gardens of the UK – Sissinghurst, Stourhead, Stowe and so on – can be regarded as the horticultural equivalents of British art history, then the conceptual gardens on show at the Royal Horticultural Society's Hampton Court Palace Flower Show are the YBAs – the Tracey Emins and the Damien Hirsts.

Hampton Court is the only mainstream flower show to champion conceptual gardens. It has been doing so since 2005, but the idea is still a controversial one. The simplest definition I can come up with is a garden that seeks to portray an idea, rather than provide a landscape design solution.

The term "conceptual garden" is attributed to Tim Richardson, author of Avant Gardeners, and a member of a relatively new genus in gardening – the landscape critic.

A growing number of people, including Richardson, believe that gardens should be subjected to the same critical scrutiny as any other form. There are inherent problems with the evaluation of conceptual gardens, however, both philosophical and horticultural.

As Matthew Wilson, the former curator of the RHS gardens at Hyde Hall and Harlow Carr, said to me: "I think the conceptual garden category is the best thing the RHS has initiated at their shows for years; but how would the judges at Chelsea respond to an upside-down garden? You'd never get away with it."

The garden he means is Excuse Me While I Kiss the Sky, designed by Anoushka Feiler. It features a circular steel structure, from which pots of plants hang upside down, above a semicircle planted with blue agapanthus. The structure is reflected in a mirrored ball, which adds to the sense of topsy-turviness. It won a silver-gilt medal, rather than a gold, which may reflect the judges' uneasiness about the upside-down bit.

The Best Conceptual Garden award went to Dan Lobb's Landscape Obscured, which at a casual glance doesn't consist of a garden at all, but a group of metal periscopes around a tilted rectangle of turf. My personal favourite was Picturesque, designed by Melissa Jolly as a gallery in which real plants were used to recreate famous artworks such as Monet's Water Lilies and a Rousseau jungle garden.

The pièce de résistance, however, was the "Damien Hirst" in the centre of the gallery. Three tillandsia, or air plants, were hung in a glass case, looking like some weird skeleton.

Picturesque, which I loved, won a gold medal, as did CoppaFeel! I think I would go as far as to say I hated this garden. It was beautifully executed, it was sponsored by a very worthy cause (CoppaFeel! is a breast-cancer awareness charity) and it was created by Hugo Bugg, who is an award-winning young designer. But it just made me cringe.

This idea that a conceptual garden can inspire emotions was also a theme in Enduring Freedom? by Nete Hojlund and Corinne Sharp. This took the form of an Afghan dirt road, and was designed to illustrate the rising number of military casualties as a result of improvised explosive devices (IEDs). An explosion, followed by clouds of smoke, went off every 20 minutes. That an ordinary dirt road, meticulously recreated with stones, sand and grasses, could possess such a sense of menace was a very powerful idea.

It's this, I think, that makes conceptual gardens so exciting, in the way that modern art is exciting and challenging. I'm a keen gardener, and although I've admired many conventional gardens, they have never inspired me to feel outraged, or emotionally stirred, or made me laugh out loud.

Can gardens be art? Perhaps we should turn that question round and ask why should gardens not be art? If we are capable of making an aesthetic judgement about conceptual gardens – one based on taste and emotion – I think we are getting very close to defining them as such. But we have to get over the idea that everything in the garden is "Lovely!"