True colours: green

The sum of light and shade is green, a fleeting colour, as all artists know. Green paints and dyes are known as "transient", "fugitive", "volatile", words that bring the colour to life, give it an outlaw's unstable personality. We feel an ambivalence about its uncontrollable nature, which "slows tools, chokes outlets", as the poet Alice Oswald writes in a recent collection, "like something struggling to be held". Green is sunlight forced through each leaf and wedged unwillingly in a narrow place. Just look what happens if we don't keep an eye on our patch, our slice of Arcadia.

The sum of light and shade is green, a fleeting colour, as all artists know. Green paints and dyes are known as "transient", "fugitive", "volatile", words that bring the colour to life, give it an outlaw's unstable personality. We feel an ambivalence about its uncontrollable nature, which "slows tools, chokes outlets", as the poet Alice Oswald writes in a recent collection, "like something struggling to be held". Green is sunlight forced through each leaf and wedged unwillingly in a narrow place. Just look what happens if we don't keep an eye on our patch, our slice of Arcadia.

Subverting the British love of gardens, turning the lawn upside down and inside out, is the speciality of artists Heather Ackroyd and Dan Harvey, who work with green's transient nature instead of against it. Their vertical lawns have covered every surface, from a church in Zürich to the vaults of the Palais de Chaillot in Paris and a skeletal blasted oak. Chlorophyll is their medium, and they manipulate its photosensitive powers by growing grass vertically in a dark studio and then exposing it to light, thereby achieving shades of green grass in their "living" photographs equivalent to the greys in black and white film.

For several years, the two artists had been searching for a way to arrest the ephemeral nature of their extraordinary pictures, inhibit the loss of chlorophyll and "fix" the photographic image permanently within its grassy skin. But until three years ago, when a Wellcome Trust Sci-Art award allowed a collaboration between Ackroyd, Harvey and scientists at IGER (Aberystwyth's Institute of Grassland and Environmental Research), the living canvases were destined to die as soon as their meagre supply of nutrients was depleted.

Now, using a stay-green seed developed with the help of IGER geneticists like Professor Howard Thomas, the life of these grass photographs can be extended. By the spells of their natural magic, as early photographic pioneer John Herschel put it, these artists and scientists together have managed to capture "that most transitory of things, a shadow, the emblem of all that is fleeting and momentary".

IGER's pioneering work has resonances far beyond the art world, of course, but for Ackroyd and Harvey it is the equivalent of the discovery that pigments ground from water, when "frescoed" onto a lime-plaster wall, would survive as long as the wall. Professor Thomas has confessed that this is the most creatively satisfying part of his research. "As you may imagine, I am inordinately fond of the colour green," he said.

A man as elusive as the colour he studies, he is not unlike the fugitive magnesium molecule at the heart of the chlorophyll molecule: invisible in the picture, but essential to it. He has weighed the colour green, put it on the scales before and after it faded to yellow, measured what we are losing day by day. And his obsession lies at the root of my new novel, Fish, Blood & Bone, which was grafted from the buds and stems of those real artists, real scientists. Impressed by meeting them three years ago, I noted: doesn't their need to "fix" the ephemeral in time go against the very beauty of green?

Oscar Wilde, with a green carnation in his buttonhole, thought so. He believed that the ambiguous, unstable energy of green was linked to creativity (Dorian Grey was evergreen while his portrait browned and cracked like an old leaf); it certainly was to intoxication: absinthe, a favourite drink among the artistic set of Wilde's time, was often called "the green fairy", and drunk during l'heure verte. A partiality for absinthe, or for the easily available opiates of the Victorian/Edwardian period when he was writing, might explain Lyman Frank Baum's green imagery in The Wonderful Wizard of Oz.

One consequence of taking hallucinogens, as Patrick Trevor-Roper explains in his classic study of artists' visual perception, The World Through Blunted Sight, is an increased perception of "the true shadow-colours - the blue shadow in snow, the green beneath the red object". Hallucinogens, by interrupting the "association fibres" in the posterior lobe of the brain, allow us to see a far truer image than our usual ordered stereotype. The Land of Oz at the end of the Yellow Brick Road was full of druggies' imagery, with a spaced-out wizard decreeing everyone must wear green glasses so that his shimmering City of Emeralds, all delusion and abracadabra, could remain intact.

Green is dangerous, rampant, as the painter Ben Nicholson suggested on seeing his first Cubist Picasso in 1921. Impressed most of all by "an absolutely miraculous green" in the middle of the composition, he remembered it as "very deep, very potent and absolutely real. In fact, none of the actual events of one's life have been more potent than that".

Perhaps the colour's potency is unsettling, for it has the ability to fur us in moss if we stand too long in the same spot, freckle us with lichen, to wrap a liana around our throat and pull it tight. From chlorophyll to murder may sound like the fixation of a thriller writer, but a bloody connection between red and green is no fiction.

"Green, which lies under the flesh colours, must always show through a little," writes Cennino d'Andrea Cennini in his 15th-century description of how to paint a dead man. Put the chlorophyll molecule under a microscope, Howard Thomas told me, and what you find is a striking architectural resemblance to the red pigment in human blood, the haem molecule. Without overstretching the connection, green might be called both the blood and paint of plants.

One result of Professor Thomas's green collaboration with Ackroyd and Harvey is a grassy "living" tapestry imprinted with the artists' Italian holiday snap of a family picnic, a description that does not do its mysterious radiance justice. Threaded with sunlight and chlorophyll, it could be the green ghost of all lost gardens. And it hangs, appropriately enough, in the darkened, cathedral-like room where the Victoria and Albert Museum keeps the 15th-century Devonshire Hunt tapestries.

Because foliage colours in tapestry are particularly susceptible to fading, the V&A's hunting scenes are notable for the absence of green. It was Ackroyd and Harvey's idea to reintroduce the living colour using IGER seed grown on hessian in such a way that it reflects the heavy weight of the earlier masterpieces.

By drying their work so that it starts green and then slowly loses its colour over the course of the exhibition, the artists can subtly blend the dun and gold of a late- summer cricket pitch into the shadowy blues of the antique woven tapestries. In this way, the grassy medium becomes the message: not to be evergreen and live forever, but to fade away with grace.

'Fish, Blood & Bone' by Leslie Forbes is published by Weidenfeld & Nicolson on 7 September