IT IS the latest gang to explode in the concrete canyons of South London, bringing not knives, nor drugs, nor guns — but plants.

They congregate at night beside roundabouts and road junctions, armed with trowels and spades.

The authorities say there is little they can do to stop the rapidly expanding guerrilla movement from planting every neglected patch of soil with rows of hyacinths, rosemary and Day-Glo primulas, tidying up afterwards and returning regularly to water and weed.

They call themselves the Guerrilla Gardeners and in five months they have grown from one man with a passion for shrubs to more than five hundred. On a Thursday night they appeared at a triangular traffic island a mile south of Waterloo, carrying sacks of mulch, a water dispenser and tools. They started weeding.

“I’ve just joined,” said Anne Slater, 66, a human resources manager from Stockwell, South London. “We came here two weeks ago and it was absolutely covered in weeds.”

A night employee of Morley College, which overlooks the junction, said: “It was like a flash mob. Suddenly there was nearly a hundred people out there, gardening.”

The Guerrilla Gardeners were back the next night, to plant rows of lavender, hellebores and a holly bush.

This week it was a smaller detachment carrying out maintenance and more planting at the site they call “Project Nine”. By day all are urban professionals: charity officers, tax consultants and managers. “It’s anarchic but in a nice way,” said Mrs Slater. “My stepdaughter is standing by to bail me out in case we all get arrested.”

The police have questioned the guerrillas but in an area of London with one of the highest crime rates they usually have more pressing engagements.

The local authority is in a quandary. There may be legal issues but Southwark Council does not wish to clamp down on the gardening. It would prefer to work with the guerrillas, a spokeswoman said.

Richard Reynolds, 28, the group’s founder, thinks that would entail “a lot of red tape, rather than simply gardening”. On a lunchtime trip to water “Project Nine” he met the man from Transport for London who is officially charged with tending the site.

“He was only allowed to do something here once every three months,” Mr Reynolds said. “If he wanted to water a border in the Old Kent Road, health and safety wanted him to shut down the dual carriageway. It would cost £600 just to water. I can come and do it on my way to work.”

Mr Reynolds, the son of a vicar from North Devon, is an Oxford graduate, and an advertising account planner.

His urban gardening drive began two years ago when he moved to a tower block overlooking Elephant and Castle, South London, decorated with empty concrete pots.

He went out after midnight and replanted the beds in the entrance way. Similar midnight missions followed.

So it began. He set up a website to chart the progress and to call on repressed gardeners to join up. Cash donations flooded in. Members receive regular e-mails notifying them of the time and postcode of the next project.

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