One is carrying a heavy four-pronged fork, another slaps a trowel into his gloved palm, and the rest have their hands full of potted plants. They are half-menacing, half-comical; curious rodentine droogs from some green-fingered version of A Clockwork Orange.

The difference is that this gang comes in peace. Or at least comes with sweet peas. They are guerrilla gardeners, part of a growing international movement of individuals and groups who garden illegally in space they do not own. This lot are from Aberdeen, but there are 'troops' – to use their preferred term – all over the world.

More than 8,000 are registered on the website www.guerrillagardening.org – jardiniers sans frontières from Bordeaux to Bath to Berlin. There is even guerrilla gardening in China, where industry is gobbling up farmland and food prices are rising steeply, forcing some Chinese to grow crops covertly in any available fertile space. The first shoots of guerrilla gardening have begun springing up in Scotland during the past few months. At the moment, it's still underground, but start digging and you quickly hear guerrilla lore. It's rumoured that two young men are cutting about Fife dressed as gnomes, tidying neglected parks during the night. A mother of three in Prestonpans is rearing sunflowers beside streetlights. A man in the Dunfermline area is very quietly planting a forest of oak on council-owned waste ground near the estate where he lives.

According to Richard Reynolds, a spokesperson for the movement and the author of the influential, recently published On Guerrilla Gardening, "This is a fight against neglected land, against scarcity of land, and against public apathy. The enemy is not the councils, the enemy is all among us. Also, any enthusiastic gardener will recognise that gardening itself is a tussle and a battle with nature. It's not a hippyish thing where we just let nature do its thing. Gardening is about controlling nature and shaping it to fit a vision we have of what looks attractive."

Reynolds talks a good fight, but everyone has their own reasons for going guerrilla. The arty New Yorkers who invented the phrase 'guerrilla gardening' in 1973 were inspired by the tomato plants they saw growing in mounds of garbage to establish a large community garden amid the decaying Bowery slums. Most contemporary guerrillas want to make their neighbourhood less ugly and are fed up waiting for the local authorities to act. For others it's political – a statement about land ownership and taking control of public space out of the hands of the state. Some guerrilla gardeners are simply aggrieved that they don't own a garden, and see no reason why they shouldn't annex any green space that catches their eye.

There are guerrilla gardeners who use their talents to remember the dead, such as the man from Hampshire who planted daffodils on a roundabout near the spot where Milly Dowler was found murdered. The artist Paul Harfleet plants pansies at the sites of homophobic attacks and is planning to plant one in Perth's South Inch Park where James Kerr, a gay council worker, was killed in April last year.

The movement even has an extremist wing – anti-capitalists and eco-activists who drill into stretches of motorway and plant trees, or secrete ivy in the walls of banks in the hope that the façade will eventually be destroyed; this is what Fight Club would have been like had it starred Monty Don.

In Edinburgh, there are a lot of people who quite fancy a spot of guerrilla gardening but haven't got round to it yet. This runs contrary to the spirit of the movement, which is against procrastination and very much in favour of just getting out there and planting something – sort of feel the fir and do it anyway.

Glasgow is ground zero for guerrilla gardening in Scotland. This makes sense. It's our most populous and built-up city, with lots of high-rise accommodation and a tagliatelle tangle of motorways. Yet Glasgow also has a mythology of fertility – 'the dear green place' – and so its people, while living in what is in large parts a concrete jungle, always have at the back of their mind the idea of living in an actual one. You can see something of this tension in the recent battles over proposed commercial use of Pollok Park and the Botanic Gardens. People value their green space but feel it is under threat from state and corporate interest. Such a siege mentality is the ideal conditions for guerrilla gardening to sprout and bloom.

The de facto leader of the Glasgow guerrillas is Jennifer Calder, a 33-year-old refugee outreach worker unafraid to team a black leather jacket with floral wellies. She lives in a "tiny, tiny" flat in Glasgow where the Merchant City shades into the east end, and became inspired to guerrilla garden after reading Reynolds's book. In June, she decided to do something about the communal bin area behind her building, which had turned into a general dumping ground. She contacted the council on three occasions, asking for more bins, but to no avail. So one day she cleared the rubbish, put down compost and planted lavender, sunflowers, fuchsias and sage. It was, she says, a "horticultural coup".

"At first it was quite normal to come down in the morning to find black bags dumped on top of seedlings, plants knocked over, rubbish from over the weekend," she says. "But it's calmed down a lot. People have started to see it as part of the environment and something to respect.

"I felt a bit silly at first because I could be seen from the street, and there were drinkers outside the pub opposite while I was putting plants in and moving rubbish about. But there's been a really good response from the public. Passers-by have given me money and offered plants and cuttings." Two pensioners gave her seven quid which she spent on mint and a red cordyline.

"The pub over the road has offered me water," she continues. "One of the staff loves gardening, so she was really pleased to see what I was doing. The whole thing has been great because I've taken control of my own environment. You often hear people complaining about vandalism and litter, but they always expect someone else to do something about it. But when you begin to think about what you can do yourself, it becomes tremendously empowering. It takes on a life of its own. I did that small strip and then moved on to bigger sites."

Calder identified an expanse of neglected ground in the Townhead area, across the road from the Royal Infirmary and immediately behind Martyrs' School, one of Rennie Mackintosh's early buildings. She passes it most days on her way home from work and had often sighed over the state it was in – terribly overgrown with weeds. Last month, she and a group of around ten guerrillas, their ranks swollen by children and parents from a local school, worked from 8pm until after two in the morning, clearing the weeds and litter, and putting in a huge variety of plants including cyclamen, pansies and ornamental grasses. The transformation is incredible.

Just over a week later, the guerrillas returned to plant some spring flowers and tidy up a bit. They were armed with trowels and a big bag of bulbs, including crocuses and Russian snowdrops. Ewen Nicolson, 32, whose day job is in a solicitor's office, breaks off from weeding to enthuse about his interest in seed-bombing – hurling compressed clods of clay, compost and seeds into inaccessible areas of waste ground. Michael Gallacher, 35, a guerrilla gardener who works for the Glasgow East Regeneration Agency, darts around taking photographs. "We are slowly reclaiming the city for ourselves," he says.

Ewen Brown lives in Bridge of Weir but works in Glasgow. He has driven here in a dazzling yellow Fiat Seicento; the miniature daffodils he's planting will be lucky to achieve a similar shade. Brown, who's 41, has "nine or ten" cars, seven of them Fiat X1/9s, and is no one's idea of an eco-warrior. Yet he has been bitten by the guerrilla bug and is considering packing in his IT job and taking a course in horticulture at the Scottish Agriculture College, near Ayr. He has a "crazy idea" to turn the M8 motorway into an avenue lined by giant redwoods, but in the meantime his focus is on his collection of around 500 cacti which has become too big for his two greenhouses. Brown has been a cactus fan since primary school and is a long-standing member of the British Cactus and Succulent Society.

Why did he get involved with the guerrilla gardeners? "Driving around, I see a lot of Glasgow derelict, a lot of open spaces uncared for," he says. "I find it amazing that a city with a Victorian background and fantastic glasshouses like the Kibble Palace seems to have lost its way. There's a huge amount of green space lying unloved. What really bugs me is the amount of litter. I've travelled a lot in Europe – Germany, Austria, over the Alps to Zurich – and you just don't see the amount of crap you see in this city. Here, people know that nobody's going to pull them up for dumping stuff. There's a lot wrong with society and the way we police this. The parks are still well looked after, but the council has this blinkered approach that the parks are all that matter. We should be spending as much time on the inner cities."

Brown has plans for further action in Bridge of Weir. The Glasgow group, meanwhile, intend to guerrilla garden the overgrown planters on Wilson Street on September 27, during the Merchant City Festival. "What we're doing," says Calder, "is breaking the rules in a very positive way. And I think it would be hard for the police to take any action. How can you prosecute someone for tidying up and improving an area?"

While the Glasgow guerrillas have not yet had any run-ins with Strathclyde Police, Reynolds has not been so lucky with the Met. Gardening property you do not own is classed as criminal damage, and he was told this, in no uncertain terms, in April while weeding a roundabout near his home in the Elephant & Castle area of London. Police officers told him to stop or face arrest. They were unmoved by his attempts to get them to admire his forget-me-nots. Reynolds and his friends packed up their tools, went back to his place for a fortifying glass of red, and then returned to finish what they had started.

"That incident made me feel very depressed and incredibly disappointed," says Reynolds. "I'd been guerrilla gardening for four years, and although I'd been spoken to by the police between 15 and 20 times before, it had always been straightforward and quite amusing." He was once pulled over and searched under the Prevention of Terrorism Act when officers feared that the sacks of mulched wood chips on the back seat of his car were some sort of fertiliser bomb.

Reynolds is 31 with a thicket of dark curls, a plummy voice and rakish manner that are rather more Brideshead Revisited than South London. He grew up in Devon and, at an early age, was put to work in his parents' large garden. His guerrilla activities began in 2004 when he moved into a high-rise and, frustrated by not having a garden, began nipping out at 2am to tend the weed-choked flowerbeds at the foot of the block. Encouraged by his success, he ventured further afield, transforming roadside verges and other scrubby public areas, and gathering disciples around him. He has been personally involved in guerrilla gardening at 30 locations, mostly in London, though he came north for the recent Glasgow dig.

"The problem is that these landowners – and it's typically local authorities – have a huge number of other responsibilities such as schooling, traffic management or crime, which, quite rightly, are more important and more pressing, and which people care about more," he says. "So these pockets of land are low down on the list of priorities, or even forgotten altogether. Given that they are public land anyway, I feel that they should be taken back by local people, who should take responsibility for them instead.

"In the last 50 years or so, we've got too used to expecting someone else to do things for us. We have been encouraged to think about our house and our garden, our private property – 'the Englishman's home is his castle'. But I think we should be more like the lord of the manor and instead think about the village and the common grounds among us. There's a lot of fun to be had by stepping outside."

That's an unusual analogy for him. In his book, he tends to reference icons of the left such as Chairman Mao and Che Guevara, whom he admires for their guerrilla tactics rather than their politics. He is also careful to place guerrilla gardening within a historical context, tracing it back to the Diggers of 17th-century England who planted vegetables on common land in response to rising food prices and widespread hunger. Reynolds sees a connection between this and the present day, where the price of food is rising sharply and the majority of UK citizens live in towns and cities, many without green space of their own. According to the Centre for Cities, an independent research unit, 67% of Britons live in urban areas. Around 2,500 of them are signed up to Reynolds's website, and there are more people getting into this all the time.

THE ABERDEEN GUERRILLAS in their guinea-pig masks are unusual in that they are an off-shoot of an existing organisation. Reach Out provides informal educational opportunities to vulnerable and marginalised adults – recovering alcoholics and drug addicts, and people with mental-health problems and learning difficulties, for example. Getting them involved in guerrilla gardening is one way for them to learn skills and become used to mixing with others. It's therapeutic and fun. The reason Reach Out has chosen guerrilla gardening, as opposed to the regular variety, is that its illicit nature gives it an edge that appeals to the students. As one man who admits to a "chequered past" says, "Gardening has never really been my kind of thing, but I liked the idea of doing something we weren't supposed to be doing." He also says, "The last time I wore a mask, it was a balaclava and there was a getaway driver outside the bank."

The group walk a short distance from the office on King Street to a neglected play park at the bottom of Lemon Street. Annie McIntosh, 45, a keen gardener, is the project co-ordinator. She's wearing a blue vest, a Pictish tattoo visible above her left shoulder-blade, and is carrying a bag of bulbs picked up from Poundland. Doing things on the cheap is an important guerrilla principle.

The nine students walking along behind include Mike Buchan, 48, rake-thin but very strong. In his large rucksack he has four assorted forks and spades. He has a gingery beard, a black beanie and broken veins on his nose. The fact that he is wearing a mask and is recovering from throat cancer isn't enough to stop him lighting a roll-up. He tells me he used to work in the oil industry until his alcoholism cost him his job. He has depression and also suffers from agoraphobia, an ironic condition for a guerrilla gardener and, he laughs, even more so for a mountain climbing instructor, which is what he hopes to become. In short, Buchan has had his share of troubles and regards gardening as part of his recovery. "Coming from the life I had to the life I've got now, the hardest job was finding something useful to do. Now I feel I'm helping folk."

We're in a park on a housing estate not far from the beach. Seagulls shriek like car alarms. Reach Out has been working here sporadically during the past six months. It's quite a grim spot. The single concession to pleasure – a rusty climbing frame and slide – is outnumbered three to one by abandoned shopping trollies. Ross Weatherby, a support worker in a Joy Division T-shirt, wanders round with a black bin bag picking up dog dirt, broken glass, crisp bags and a used condom; it's hard to imagine who might consider this an ideal erotic nook.

The team get to work. They weed, cut turf for a new flower bed, and add tulips, crocuses and irises to the foxgloves and ox-eye daisies they had planted previously. "We take a lot of walks so we're often seeing places that are pretty messed up," says Weatherby. "If you get off the train in Aberdeen, you'll see some nice hanging baskets put in for when the Queen visited, but that's not where people live. People live in these wee estates. Their kids play in these parks. So why not do them up? The people who live round here respect it when we do it."

That certainly seems true. While they are busy working, a man comes over to say he's 100% behind them. He wouldn't normally bring his grandchildren to this park because it's full of dealers and the bit they are weeding is where the junkies dump their syringes. Maybe in the future, though, now that someone is making a difference.

There is a passage in On Guerrilla Gardening in which Reynolds describes visiting a friend in Zurich for dinner, taking two chilled glasses of pinot grigio outside while they planted some golden tulips by a bus stop, and then returning indoors for pudding. It's pretty clear, however, that guerrilla gardening is rarely so sensuous and cosmopolitan. It's sweaty, mucky, sometimes risky work; you are unlikely to dig up drug paraphernalia or evidence of al fresco sex while toiling in your own garden. Yet stepping outside their homes and society's norms has granted the guerrillas access to an altruistic way of life which, like a new breed of orchid, is tricky to develop but ultimately rewarding.

That's one way of putting it, anyway. But Buchan, putting down his black bag and blowing a curtain of fag smoke from beneath his guinea pig mask, says it better: "I always find things that are easy to do are awfa', awfa' boring."

To see the guerrilla gardeners in action, log on to www.guerrillagardening.org/ogg_video.html