A mere 25 yards from Glasgow's thundering M8 motorway a small miracle is taking place. Next to the busy intersection that runs by Glasgow Royal Infirmary, the green shoots of potatoes, onions, marrows, rosemary and garlic are coming to fruition in a secret garden.

Look towards the trees high up the Townhead embankment and you might just see this urban oasis. Where once there were overgrown weeds and the debris of decades, this little kitchen garden has happily and easily taken root. But who has created it?

Just minutes down the road in the heart of the Merchant City a raised bed of colourful begonia, lavender, geraniums and hydrangea is blooming in the forgotten space between a derelict Victorian gents' lavatory and a police telephone box. Wasteground in Drumchapel at Kinfauns Drive is now blooming, too. And on the South Side of the city, under a billboard site at the corner of Pollokshaws Road and Calder Street, a row of badly neglected flower beds has been given a new lease of life.

But who did the planting, and who does the weeding and watering?

The simple answer is, well, nobody. It's not that Glasgow Guerrilla Gardening, a network of 40 or so volunteers who first got together a year ago with the collective aim of beautifying the environment, don't want to be identified. But they know that clearing, weeding, digging, planting and watering neglected spaces that belong to the city council could potentially be labelled criminal damage because, technically, it's illegal.

So the guerrilla gardeners believe that working under cover of dusk or darkness is the only way to get things done. They communicate with each other by email and meet up every week or so, giving their services for free and using donated or cheaply purchased utensils and plants.

The public face of the group is Jennifer Calder, 34, an advice worker with Glasgow South East Regeneration Agency and Glasgow guerrilla gardeners' founder. She lives in a tenement which doesn't have a back garden but does have a bin area, which she has cleared and planted with flowers.

But Calder now feels she's outgrown its confines and wants more space. It was she who identified the Townhead site, the Glasgow guerrillas' first.

"I used to pass it on my way home from work and thought it would be a good spot," she says. "I live on the High Street and don't have a garden and was looking locally for a bit of ground to tend. Now I come here every day - local people are very supportive, and residents stop for a chat. Even in the pouring rain it's possible to get something done.

"So far, Glasgow City Council has turned a blind eye; we've had no contact at all. But we can't plant veggies that grow too tall, like beans and peas, because they'd be too visible and might attract vandals."

Calder has joined forces with Michael Gallacher, 36, an IT support worker for Glasgow East Regeneration Agency and now the guerrillas' co-ordinator. His website catchphrase is "resistance is fertile", and he has managed to galvanise 40 regular users. Gallacher, too, lives in a tenement with a shared back garden, where he does most of the communal gardening, but wants to stretch his wings.

"When you consider that the Townhead site was smothered in seven-foot-high weeds and shrubs and that we only started working on it in August last year, doing everything by hand, and bringing water in bottles, I think we have every right to feel as pleased as punch over how it's doing," says Gallacher.

He's also proud of how their work with the raised beds in the city centre has helped wildlife. "I'm particularly pleased with the number of bees that have taken to hanging round the Merchant City site," he says.

Both defend guerrilla gardening as a sociable activity which is aesthetically rewarding. The Townhead site - roughly twice the size of a tenement drying green - was given a major boost of £200 worth of plants and shrubs from Sky TV, when it arrived here last August to make a short film about guerrilla gardening. It was presented by Noel Edmonds, and that inaugural event was organised by Richard Reynolds, the London-based freelance advertising planner who began his guerrilla gardening blog in 2004. It now has more than 4000 registered users from around the world.

Guerrilla gardening has been active in London, Brighton, Dublin, Essex, Amsterdam, Toronto, Turin, Tokyo and Zurich and elsewhere for 30 years. Reynolds's blog sign-off, "let's fight the filth with forks and flowers", appears to have taken root. He describes guerrilla gardening as "the cultivation of someone else's land without permission" and although the definition is not his, he has become a self-appointed publicist for the movement in the UK.

With a band of fellow guerrillas, Reynolds has helped build 28 guerrilla gardens around London over the last five years, and currently looks after half a dozen or so. His intention is not to build big community gardens, grow food or hijack neglected, privately owned land as a campaign against the capitalist system. He simply identifies neglected civic spaces such as roundabouts and roadside verges, clears them of debris and weeds and replants with colourful flowers and shrubs with the aim of "cheering people up and hopefully inspiring them".

In his book on the subject, On Guerrilla Gardening, published last year, Reynolds calls such neglected plots "orphaned land". He is a huge fan of what he simply regards as gardening in public. "We're so pre-occupied with our private spaces and possessions, but if we step outside we will get more out of living here," he says. "Gardening in public helps us communicate with other people."

Reynolds records that the term guerrilla gardening originated in New York in the 1970s during a financial crisis. The civic authorities eventually gave in and allowed the guerrilla gardeners to continue working.

For its part, Glasgow City Council has been largely silent on the subject of guerrilla gardening, except to say that it gave permission for the Townhead dig for the purposes of the Sky programme. "Other than that, we don't really have a policy as such," said a spokeswoman, who professed not to have heard of the term guerrilla gardening.

In Glasgow, only 27% of the population has access to a garden, and in some areas there is a nine-year waiting list for an allotment; the situation is similar in Edinburgh. Glasgow has 24 allotment sites, translating into 1320 plots; the most recent survey, in 2007, showed 652 people on waiting lists.

Glasgow City Council has recently published its Allotment Strategy and Action Plan, which states a commitment to improving allotment sites and increasing their availability. "We know how important allotments are in Glasgow and the contribution they make to the health and wellbeing of the local community," said James McNally, executive member of the council's Land and Environmental Services.

Yet Glasgow City Council last week took two residents from North Kelvinside to the sheriff court in an attempt to stop them growing vegetables on council-owned vacant ground that has been derelict for more than 20 years.

The council wants to sell the land, which lies between Clouston Street and Kelbourne Street, to private developer New City Vision which plans to build 115 flats.

Last Friday, the sherrif ruled in the council's favour and granted the continuation of an interdict which prevents Karen Chung and Douglas Peacock, treasurer and chair respectively of the North Kelvin Meadow Campaign, from putting new vegetable patches on the land, which used to be playing fields for the local school.

Despite the setback, the campaigners have vowed to continue their legal fight. Chung said her case will be based on a motion passed by the council in October 2008, which encourages the public to make use of derelict land while new developments are delayed in the current recession.

The Glasgow guerrilla gardeners supported the North Kelvinside residents in court, even though strictly speaking they are not guerrilla gardeners. Gallacher explains: "It's only through the efforts of the local people that the North Kelvin meadow has become the attractive space it is."

The Glasgow guerrillas remain undaunted. One year on from the very first Townhead dig, there are now sites across Glasgow, with more being identified every day, plus a growing network of Scottish guerrillas that reaches as far afield as Aberdeen, Dumfries, Dundee, Edinburgh, Fife, Renfrewshire and South Ayrshire. Erskine and Dennistoun are next to be "attacked" and Sighthill, Dennistoun and West Princes Street are all in his plans for next year.

Even as the growing season comes to an end, Gallacher says he hopes to continue to build an army of guerrilla gardeners for all areas of the city - and to learn more about gardening from more experienced people such as asylum seekers and immigrants, who he has noticed are especially skilled in growing plants.

Design graduate and fellow guerrilla gardener Darren Wilson is taking the "war on weeds" theme further with the launch of his Seedbom - a grenade-shaped package combining organic compost, fertiliser, and a mix of Scottish wildflower seeds and perennial favourites such as nasturtiums and sunflowers, encased in recycled materials such as old egg boxes.. Users are encouraged to "throw Seedboms into derelict land, neglected spaces or even your neighbour's messy garden and watch them grow".

"Seedboms exist to promote environmental awareness, help join forces with nature and brighten up dull and lifeless places in your local area," says Wilson, a graduate in 3-D design from Gray's School of Art in Aberdeen. "They are a tangible statement for eco-warriors like guerrilla gardeners, and help people understand what guerrilla gardening is all about, which is giving over control to nature. They're fun but with a slightly subversive undertone."

Wilson's Seedboms are a modern take on the very first "seed bombs" thrown by the New York pioneers in the 1970s, which could be anything from Christmas ornaments to condoms filled with tomato seeds, water and fertiliser.

Wilson lives in a tenement flat with his photographer partner Susan Castillo on the south side of Glasgow, and took part in the original guerrilla dig at Townhead as well as in Drumchapel.

He has orders for Seedboms from shops such as Entrading and Felix and Oscar in Glasgow, and Flux in Edinburgh, where they cost £9.99 for a pack of four.

He would consider vegetable seeds for a new version of Seedbom in the future, but acknowledges that they would need follow-up maintenance and nurturing which wildflowers do not.

"Sometimes we throw spare flower bulbs into empty ground, which does feel a bit naughty," says guerrilla gardener and solicitor's assistant Ewan Nicolson, 33. "But how can you say flowers are subversive?"

The Glasgow west-ender, who lives in a ground-floor flat with a small garden of his own, took part in the Townhead project and enjoys guerrilla gardening for its socialism as well as the socialising it affords, but he believes there is another element to it. "Planting on fertile ground or in places that would look good with plants in them is all very well, but the hard bit is when you're up against rocks, stones and tree roots and difficult growing conditions," he says.

"Taking control of something like that takes huge commitment in terms of time, effort and strategic planning. Guerrilla gardening is open to interpretation. It will be interesting to see how this develops."

Meanwhile, back at Townhead, the potatoes are about to be lifted - and may not be planted again. Gallacher has his doubts: "The vegetables were just an experiment," he says. "They tend to engender a feeling of individual ownership, so I'm not sure they really are in the spirit of guerrilla gardening."

Even as the flowers start to fade, and the growing season comes to an end, the guerrilla gardeners' roots of revolution are continuing to spread.