The charity Gardening Leave uses therapy to draw lost soldiers out of their isolation. Now it needs help to reach more of them • Click here to donate to the Guardian and Observer Christmas Appeal

On Bobby’s arm is his story. Northern Ireland, Bosnia, the Gulf – the conflicts, uniforms and guns are etched on his flesh. “Sometimes I regret the tattoos,” he said, “they’re a reminder. But then I’m not likely to forget any time soon, am I?”

Bobby left the army in 1997 but, like an estimated one in five men and women serving in the armed forces, his experiences have left him with serious mental health problems which have damaged his ability to cope with civilian life, broken up his marriage, ruined friendships and eaten away at his physical health.

“I’d give my legs if my head would be OK. If I could just have a night’s sleep. It’s like I’m in a time warp, stuck in that time, going over and over things. I’m like a squirrel locked in a squirrel cage. After I came back from the Gulf, my mother said I was a different person. The anger, the dreams.”

In Bobby’s time, the response to soldiers suffering from trauma-related stress was too often “give yourself a shake, you’re a soldier,” said Heather Budge-Reid, chief executive of the charity Gardening Leave. It’s one of the reasons why it takes, on average, a forces veteran 15 years to seek help. The charity helps 400 veterans a year at five sites, two in London and three in Scotland, and would like to reach more. It aims to draw these lost soldiers out of their social isolation and help them start to tackle their mental wounds, by working in a garden alongside professional therapists.

Bobby has been coming to Auchincruive in Ayrshire since just after it was founded in 2007. It’s an old stove house – a Victorian glasshouse in the gardens of a big estate that was used to grow exotic fruits for the gentry. Now owned by an agricultural college, the building, glasshouse and small garden space have been restored by volunteers. Two full-time mental health therapists work alongside veterans who come to garden, learn new skills, socialise with other veterans and soak up the benefits of a green and calm space. “You feel safe here,” Bobby said. “This place helps me forget, the banter and a sense of belonging. I’m one of the lucky ones, having this place on my doorstep. It’s like a big oasis, a safety bubble. Being with the boys and not being judged,” he said.

Two-thirds of the ex-squaddies here will have been diagnosed with post-traumatic stress disorder and a third with anxiety and depression, said Budge-Reid. “They suffer a lot from the idea of stigma; even if the wider public are starting to understand PTSD, the veterans have their own internal stigma. It’s a kind of survivor’s guilt, so there’s shame, huge shame.” About 80% of the veterans will have or have had a problem with alcohol. Obesity-related diabetes is another big health issue – the high calorific diet of an active serviceman doesn’t transpose well into civilian life, let alone the lack of cooking skills.

“As one man said, ‘if it can’t ping or ring, I don’t eat it’ – meaning microwave or a one-ring,” said Budge-Reid. “We find veterans will have a frying pan or a pot, generally not both. Because they have issues with crowds or people being behind them, going out to the shops is difficult. Thank God for supermarkets with late-night opening.”

Michael is in the garden’s pergola with two volunteers, local men who used to walk their dogs past the Auchincruive for years before finally coming in. “This place has saved my life,” Michael said. “I can’t speak highly enough of it.” He calls Budge-Reid “ma’am” and, like all the veteran, speaks reverently of the therapists. A year ago he could not have sat in such a confined space, never mind close to another human being. “The steps forward are small often, but we see them and it’s wonderful,” said Budge-Reid.

On one wall is a simple little plaque commemorating those who haven’t made it. Suicide attempts are an ever-present risk. A line of small wooden crosses leans against a wall where the veterans have had their own small remembrance service, marked with the names and dates of their personal losses, mates and comrades. These are men who have seen too much blood spilled.

“Their stories are their own and they don’t have to tell us,” said Budge-Reid. “Most of them have issues of concentration and shake constantly, so doing things like pricking out seedlings can be very difficult.”

For people with PTSD, everyday objects can trigger flashbacks. “A pine cone can feel like a hand grenade and metal bin lids had to go – in Northern Ireland they were used by people to signal the approach of soldiers. The smell of barbecues, cooking meat and fireworks are obvious.”

Christmas will be tough. Holidays always are, when support staff take well-earned breaks and routines change. Longer opening hours and more centres are what Budge-Reid and her staff dream of. “I’d love to be able to open on Saturdays,” said Budge-Reid, “and to reach reservists. And, of course, we haven’t started to see anyone from Afghanistan yet, but we will, very soon.”

Colin, who spent 15 years in the army, is taking photographs in the garden. It took a long time before he could get a photo in focus, but now they are sharp and clear. “I’ve slowed down a lot; my life was always run at 100mph. Once I come through those gates it’s peace and quiet. You’re not looking over your shoulder the whole time, your head on a swivel, your moods changing like that,” he said, snapping his fingers. “It wasn’t until I came here and started talking to the other lads that I realised it wasn’t right. The army takes you in, breaks you, moulds you into what they want, but they don’t retune you to fit back into civilian life.”

Another Michael, a 49-year-old, has memory problems and has only recently been able to sit in a room with a closed door. He still sits close to an exit. He has become an expert grower of potatoes and leeks. “The staff here know how to talk to us; there’s no being judged. Coming through the door there’s a sense of relief.” Gardening Leave, he said, had given him “a new life”.

Budge-Reid beams at him. “We’re not here to cure them or to get them back to work,” she said. “We’re here to bring them back to life.”