Urban Gardener: Murder most florid

A small exhibit at the Chelsea Flower Show this year, tucked quietly up in its north-west corner, was a landmark as far as show gardens are concerned. Entitled "More Questions Than Answers", the outline of a fallen figure in white roses with red petals for blood, questioned our obsession with Health and Safety laws, and the disparity between sending soldiers into conflict unprotected while not letting our children play conkers at school. Emotive, political, conceptual stuff, something the Royal Horticultural Society has been a bit nervous of in the past. The fact that Tony Smith managed to ensconce himself at an event that clearly didn't have a conceptual category suggests that they were beguiled by his triumph at Hampton Court last year where his garden "In Digestion" won Best in Show in the conceptual category. Needless to say it was a breath of fresh air – so Smith's exhibit this year at the Hampton Court Flower Show is eagerly awaited.

The conceptual category is a useful track for up-and-coming designers and artists to showcase their creative ideas. It also provides garden writers with either an infinite amount of rope to hang themselves with (trying to decipher what the gardens actually represent), or bags of material to draw inspiration from other than the usual, practical stuff (eg. telling readers when best to take softwood cuttings – which, by amazing coincidence, is now).

In last year's "In Digestion", Smith used four million lettuce seedlings to "explore the assimilation of both food and information in a decadent society". This year a mere million seedlings will help him express a very personal subject – bipolar disorder, a condition characterised by abnormal mood swings from mania to depression, and from which Smith suffers to a certain degree. The installation, as you might expect, is foreboding but quietly mesmerising. A 2.5-metre-high enclosure or "prison" is black but with the texture of an Aero. Peepholes allow glimpses of a large upturned box out of which a gloop of acid green (lettuce) oozes into a sea of unrolled Tarmacadam. "I'm fascinated by walls," said Smith as he showed me the finished enclosure waiting for its first lick of paint. "The explosion of colour from the lettuce, punctuated by coloured Perspex shards, is the ecstasy emerging from the gloom, accentuating the dramatic mood shifts that are characteristic of the condition. I want to touch on the complexity of bipolar disorder. There are many layers to it; the juxtaposition of materials is a way of expressing that."

Visitors will be kept a metre away on three sides of the enclosure, so tantalising views into "Ecstasy in a Very Black Box" will accentuate the frustration of not being able to really "see" the whole picture in one hit. Much hinges on the weather, whether the lettuce seedlings will germinate in time for judging, and Smith's having to carefully work around the growing element without damaging its tongue-like form. As I watched him entering his "prison" I wondered how the frustration of implementing the work, having to squeeze all 6' 6" of himself through an 85cm x 35cm hole every few minutes without pulling a hamstring could possibly be cathartic, but it seems it is.

If that isn't enough he also has an exhibit called "Star". Not a garden as such but a shrine-like arrangement of shiny new wheelbarrows with gold wheels around a central plinth on top of which is a pink wheelbarrow full to the brim with gold-sprayed coffee beans. This beacon of satirical comment focuses on the Western world's obsession with fashion, branding and conspicuous consumption. Of course you may have your own take on what it all means and Smith values the feedback. "The magic is that you create something and it takes on a life of its own," he says. "Like art, it opens a can of worms and everyone gets a worm."

www.tonysmithgallery.com. Hampton Court Flower Show starts Tuesday and runs until Sunday 13 July; visit www.rhs.org.uk/HamptonCourt/2008