An urban tree-planting scheme is attracting everyone from hoodies to catwalk models

Mulch ado: Emma Townshend gets down and dirty with the Trees for Cities helpers © Andy Paradise

Recognising a catwalk model is not always easy – and it becomes especially difficult when she's swathed in worker gear and wearing outsize gloves while engaged in a spot of tree-planting. At Lant Street, in Southwark, south-east London this sunny Saturday morning, the whole estate has turned out for a midwinter tree-planting session. In these surroundings, the sight of a skinny blonde girl helping a local estate kid could easily go unnoticed.

Recognising a catwalk model is not always easy – and it becomes especially difficult when she's swathed in worker gear and wearing outsize gloves while engaged in a spot of tree-planting. At Lant Street, in Southwark, south-east London this sunny Saturday morning, the whole estate has turned out for a midwinter tree-planting session. In these surroundings, the sight of a skinny blonde girl helping a local estate kid could easily go unnoticed.

As the little boy finishes his mighty struggle to fill the hole around a silver birch tree with a grown-man-sized spade, I realise that the girl helping him is actually about 5ft 9in, nine stone, and has perfect dewy skin. It turns out that Katherine Poulton's normal job is on the runway and that she lives mostly in New York. That doesn't stop her having a passion for Trees for Cities, though: at the charity's annual Tree-Athlon in Battersea Park (held last year on 15 September and returning this year on 20 September), she helps to organise a fashion swap of glamorous clothing to raise money to pay for just this sort of planting day.

Poulton is such a dedicated volunteer because the charity is such an interesting one. "What are you all doing here?" ask some passers-by giving us curious looks on their way back from Sainsbury's. Four or five members of the digging crew stop to explain, cheeks red with excitement, effort and cold.

Trees for Cities won't be cross about this dereliction of duty, though. Liaising with the local community, telling people about what they are doing, is a central part of the job. "If we involve people in the planting – especially when it's teenagers – they become protective about the trees, rather than seeing them as something that's been foisted on them from outside," says Jo Hurst, the project manager. "We often get kids who hang about at the beginning, sometimes even being a bit threatening. By the end of the day, though, the chances are that they will have joined in."

The joining-in spirit is helped by the best efforts of the events organisers, who have got together with the residents' association to provide a tent for shelter, drawing crayons, a session of drumming on various exotic-looking instruments, and most importantly, home-made leek and potato soup with huge fluffy white rolls covered in butter. I tuck in, starving, and watch sadly as my new model friend admirably restrains herself.

The scheme at Lant Street is not limited to trees. Some 400 smaller shrubs have also been planted this morning; all carefully mulched and ranging from Ceratostigma with their intense blue flowers, to briar roses and lavender. "We respond to what the residents tell us they want," says Hurst. "It's only after there have been plenty of consultations that I will even draw up plans."

"And we don't restrict ourselves to London," adds the charity's chief executive, Graham Simmonds, a moment later, trying to talk to me while keeping an eye on his face-painted daughter Alice, and her schoolfriend Nicola. "In Reading, we are planting trees in co-operation with local businesses and organisations." This includes the department-store chain John Lewis – which donated a tree to be planted in the nearby St Mary's Churchyard last December – and the much-loved Purple Turtle Bar, which organised a fundraiser featuring local bands in October.

Even further afield, Trees for Cities champions re-greening in cities as far away as Addis Ababa in Ethiopia, where it has initiated orchard-planting in partnership with the Ethiopian Wildlife and Natural History Society, and Nairobi in Kenya, where it is working with the Green Belt Movement founded by Wangari Maathai, the activist who, in 2004, became the first African woman to win the Nobel Peace Prize.

But wherever Trees for Cities is operating in the world, the charity responds to local enthusiasms. At Lant Street, the prime movers and shakers have been the residents' association, who are vocal in their delight for what's going on. This morning is their moment of reward, after wrestling with the bureaucracy of trying to get trees planted. Each committee member grabs me by the elbow in turn and tries to express their pleasure. "We have a wonderful tree officer in Southwark," says Alisar Alkhersan, the treasurer. "The council's response was really good. With Trees for Cities, we worked with Jo Hurst to try to plant this particular area in such a way that people stop using it for their dogs."

Trees for Cities will carry on organising glamorous fundraisers (one, for example, offers City boys the chance to pay for a day's tree-climbing at Kew). And its Tree-Athlon in September is now a regular event in the capital, taking over Battersea Park to involve almost 2,000 runners. But all the money ends up here, in the form of the trees.

James Biddlecombe, from the Lant Street residents' association, pauses for a moment, clipboard underarm, gazing at what has been achieved in a morning with delight. "It's just such a simple idea, planting trees. But it really needed the willpower, and somehow it's all come together. It's just going to make such a big difference."

Trees for Cities' next tree plantings are in Tower Hamlets on 1 March and Kennington Park on 15 March. For more information, see www.treesforcities.org

How to plant an urban tree

Jo Hurst, London project manager, Trees for Cities

We chose the trees on the basis of what the Lant Street residents told us in their survey – they weren't keen to have anything that would go sticky and squashy, such as crab apples, which can be slippy underfoot and attract wasps. We chose ornamental pears because the fruits are almost non-existent.

You need the right tree for the situation. In the playground, I chose a variegated tulip tree, Aureomarginata, which has this clowny look, as it's so outrageous and colourful. With trees for London streets you mostly have to choose ones that will stay relatively small, so we went for silver birch, Betula utilis, in a variety called Jacquemontii that has beautiful white bark. When it's starting to look a bit ragged, you can clean the bark with a mild solution of Jeyes Fluid, and it comes up really white.

On the day itself, make sure you've got everything you need. Tree, mulch, water, spades. You can't be too picky about the weather. Ideally, I'd like it to have rained in the week, be sunny on the day, and to rain the day after.

We dig a relatively small hole, about twice the size of the rootball. Monty Don digs a big pit, but in street conditions that could just create a bucket effect and drown the roots. We stab the sides of the hole to give the roots a bit of a head start.

Next, the tree goes in and then the watering tube, which snakes down into the pit around the rootball. This enables us to go back and water in the summer, especially in the first three years when the tree will need a bit of looking after. At home, you can do it with a hosepipe. For the first summer, it needs to be watered at least five minutes every two weeks. That's 50 litres a time – 10 watering cans.

Now we begin to fill the hole with the soil we took out to start with. At each stage, firm down the soil you're putting back in with one foot, steadying the tree with the other.

When the hole is almost full, we put on mulch. We put this on really thick, because we know we aren't going to be back – maybe six inches or so. Even grass can be quite a threat to a growing tree, in terms of taking its water, so this is really important.

I can go around London now and look at trees I've chosen to plant and feel that pride; hopefully, I'll be able to take my grandchildren one day.