How do you transplant a garden from the Chelsea Flower Show to the roof of a London hospital?

One of the great lessons in gardening is to look around and see what’s growing naturally because that will indicate the growing conditions. On this roof there was nothing but air conditioning units. Nonetheless, I was inspired to create something that would, in a small way, help people going through some of the most difficult times in their life.

The sponsor, Morgan Stanley, set a demanding brief – a permanent roof garden at the hospital, but first it had to be displayed at the Chelsea Flower Show. In truth, showing at Chelsea had to be a secondary concern, although it gave me a date to work towards: a year from conception to delivery of both projects.

The shady, windless, rather humid environment immediately directed the scheme towards a woodland garden of dappled shade, varied leaf texture under-storey and jewels of blooms. Japanese maples, with their delicate foliage, form the centrepiece while buildings are masked by pruned hornbeam, oak and pear trees.

Overall, I wanted to ensure people saw something different each time they ventured outside – the way the light was catching the trees, seasonal blooms or frost on stems.
A lot of the design challenge, though, was dictated by what you can’t see. The roof wasn’t designed to be weight-bearing, which in turn dictated the placement of each of the three-tonne trees, as each had to be supported by a weight-bearing column. And everything had to be lifted over the hospital buildings by crane.

The whole project was a monumental challenge. Occasionally, when we were on site, I’d forget where I was, but then I’d step into the hospital corridors and collide with the daily world of noises and stress. Now, people can leave that behind, if only for a few minutes, and walk into this green church. It’s a place where parents can recharge themselves emotionally and spiritually. I’m enormously proud of what we achieved.”

Nick Knowles, mover and fixer: "Beneath was an MRI machine worth millions"

The first morning I came to work on the garden at Great Ormond Street Hospital I was drinking a coffee outside when an ambulance arrived with a tiny baby in an incubator accompanied by its two terrified-looking parents.

It was the starkest of reminders of the importance of what we were doing there: every day you would pass children on stretchers and in the hospital wards. People are going through hell, so taking on this project was always a no-brainer – although it did give us the toughest technical and logistical difficulties that we’ve ever had on the show.

Luckily, we had an army of volunteers to help – up to 200 – and from the moment the Chelsea Flower Show finished they swooped in, unearthing hundreds of plants and taking them to nursery storage. The enormous trees, meanwhile, had to be wrapped and put on the back of trucks to be driven across London.

Back at Great Ormond Street, work had been underway for weeks to prepare the space, a mammoth effort centred on reinforcing the roof and making sure the water barrier was secure. All the while we were aware that directly below was a multi-million pound MRI machine to which even a teaspoon of water could do serious damage.

Time was constrained: Great Ormond Street is a working hospital so we weren’t allowed to do any heavy lifting during the week. The cranes we needed to lift a five-metre, three-tonne tree over a fifteen-storey building could only be used at weekends. We used special collapsible cranes that could be dismantled, but it was still quite a feat of co-ordination.

There were endless other complications: we had to replace soil with specialist granules made from lighter volcanic ash to lessen the weight burden on the roof.

We even made a small miscalculation on the amount of “soil” needed – the last four or five tonnes had to be brought in by a chain of people with buckets – it felt like we were building the pyramids at that stage.

The whole process was incredibly emotional. So many people here have to absorb tough information about their children’s health or make life-changing decisions and they told us this garden would help in those moments of contemplation. It’s so much more than a green space.”

Clare Langer, clinical nurse: "It's now so green and peaceful"

The families we look after at Great Ormond Street are facing so many challenges. A lot of parents have children with rare and complex diagnoses, and more than half of them live outside London, so they are having to travel back and forth while juggling jobs and other children. There are huge practical and emotional strains on them.

Obviously, we all work very hard to make the wards nice spaces, but you can’t escape the fact that the hospital is a very intense, often noisy, environment with beeping machines and lots of hustle and bustle.

Previously, aside from the café and some quiet rooms there really wasn’t anywhere restorative where parents could go to get away and have a proper break. We used to suggest going out to the shops or to one of the nearby parks, but for many parents, leaving the building is a step too far when they have a sick child in a ward.

That’s why this garden is such a wonderful addition – it’s so green and peaceful. The first time I walked out there it felt like emerging into the fresh air, even though we are in the heart of the hospital at the centre of a busy, smoggy city. I know the parents will feel the same way.”

DIY SOS at Great Ormond Street Hospital is on tonight at 8pm on BBC1