Standing at the top of a beautifully manicured 60ft man-made hill, I survey the landscape below with its sculptures, architectural structures and lakes.

This is a garden, yes - but not as we know it. Designed by landscape architect, theorist and critic Charles Jencks and his late wife Maggie Keswick Jencks, founders of the cancer care centres that bear her name, the Garden of Cosmic Speculation is a feast for the senses as well as the soul. Wherever you are in the garden you are invited to look, touch, hear and smell. You are also invited to contemplate the origins of the universe through philosophy and mathematics. And, crucially, you are invited to smile. To say it's quite a place is an understatement.

The garden sits in the grounds of Portrack House in Dumfriesshire, Maggie's family home, and is only open to the public for one day every year - this year it's May 3 - as part of the Scotland's Gardens Scheme, which sees green spaces across the country open to the public, with the proceeds going to charity.

The couple worked together on the Garden of Cosmic Speculation from 1988 until Maggie's death from breast cancer in 1995, after which Jencks, an American who has worked all over the world, continued to add sections and features. A celebration of nature and science, it seeks to stimulate the mind as well as the senses: a water cascade recounts the story of the universe, a terrace shows the distortion of space and time caused by a black hole, a whole section is devoted to DNA and the lakes represent fractal geometry.

So what can visitors to the garden expect? "When it's sunny, this garden can be one of the great experiences," says Jencks, 75, as he shows me round the 30 acres. "There are 40 parts, each with its own special, different quality. On the open day music, dancing and local festivities enliven and give deeper meaning to this garden.

"The greatest thing is seeing people from all over the world coming through and bringing their own enjoyment. And, of course, children just love it. We have signs telling people not to slide down the mounds, but of course sometimes you can't stop them. This is a very playful garden."

Not surprisingly, Maggie Keswick Jencks has a permanent presence in the garden - a statue of her sits serenely without fuss in the section that contemplates death and renewal.

"Nature is a co-creator in any garden," says Jencks. "Sometimes I'll design something new and it will snow and it looks better - nature has made it better. It's wonderful to design with mother nature. But things grow and then they die - death is a part of every garden. Even in death things can be beautiful - or they can be horrific. My work tries to communicate ideas. It can be hard to communicate ideas anyway, and then nature will just obliterate it with one flood."

Interestingly for someone who has spent much of his professional life as a theorist, the idea for which Jencks is now best known is one that has made a practical impact on tens of thousands of lives - the aforementioned Maggie's centres.

Using their own experiences, the couple realised people needed a caring environment where people with cancer could go to find out information, speak to others with the disease and get support - a place where they could be empowered. Maggie, also a landscape designer, wrote the blueprint for such a place in her final months and raised the money for the first centre, a converted stable block in the grounds of Edinburgh's Western General hospital which opened in 1996, a year after her death. It was hugely successful and a second soon followed in Glasgow, at the Western Infirmary.

Since then the charity has continued to expand and Maggie's now has 17 centres across Scotland, the UK and abroad, with more in the pipeline. There is also interest from around the world in replicating the format.

Fittingly, considering the Jenckses' backgrounds, architecture was key to the centres from the outset, and over the years the cream of the profession, including the world-renowned Frank Gehry, Zaha Hadid, Richard Rogers, Norman Foster and Glasgow-based Page and Park have designed the buildings. Does Jencks have a favourite centre?

"As you would expect, I like them all," he smiles. "In some ways I like those I didn't expect to like, like the Rem Koolhaas centre at Gartnavel [in Glasgow]. I expected to dislike it because there was so much white, so much abstract concrete. I thought, 'Oh dear, it's going to stain in the dreich weather,' but I was completely wrong. It fascinates me the way he has used space and light, and the garden which Lily, our daughter, designed. The building and the garden, the way they sit in that little woodland, is very nice. Since they're all so different, each one has been a surprise." The Gartnavel centre was shortlisted for the Stirling Prize in 2012.

I ask whether he thinks the Maggie's movement has widened people's knowledge and appreciation of architecture. "In a small way that's true," says Jencks, who designed the garden for the Maggie's centre in Inverness. "There haven't been many examples in Britain of a series of buildings, built by a single institution, which are different but all architecturally interesting. In a way we have helped raise the game of architecture to return it to an art form, which in Britain and America, for many reasons, it has not been for a long time.

"To a certain extent we've created a virtuous circle. As our reputation goes up, then so does the standard. Someone said it's like winning an Oscar to get a Maggie's commission. It raises their game and it raises our game."

Jencks, a father of four who is now married to the writer Louisa Lane Fox, speaks about the Maggie's movement with a mixture of passion and the detailed practical knowledge that only someone who has been involved in every aspect of such an endeavour over many years can.

He sees the future for the charity as one of reproduction in a controlled, manageable way. And he believes as people live longer, other branches of medicine will look to Maggie's for inspiration. "It's interesting," he says. "There are five chronic diseases: cancer, heart disease, stroke, Alzheimer's and obesity - these are major health problems and you need something like Maggie's for each of those.

"I see this extending out. We are living with more of these illnesses, and we are living for much longer. We may get four out of these five at some point in our lives.

"Maggie's is, I suppose, like a church, though we are not religious. We have a spiritual and a psychotherapeutic element. What I've learned is that hope is social. If you have hope that you will live longer, you also have fear that you won't. Hope is different from optimism. Hope depends on your action and willpower, your belief. Hope you have to work on, because you slip into fear immediately.

"Maggie's re-establishes hope socially. Where do you get the hope from? Other patients. They tell you all the things your doctor never told you - where to buy a wig, how to talk to your mother or your children about having cancer. A thousand things. Some of that you can teach, as professionals, but so much of that is given by the people you meet at the centres - your friends."

Jencks says Maggie would have been amazed and proud of the way the movement has grown in the years since they first discussed the idea behind it - though he smiles when he admits that she would have been "a little sceptical" that he named the charity after her.

"Maggie was extraordinary and I was amazed by her in her last months," explains Jencks. "I helped her a lot with the writing and the intellectual part of it, but it was her willpower that did it. She would have been really proud, I think, to see what the movement has done. How many ideas that you have in your life actually come off?

"She called her last year of life the best year - she told me that. She never felt better and she was on an up. But I think she had an inkling that this was the right idea at the right time.

"Maggie's is a good organisation, and we do a good job. But it's bigger than us - it's an idea whose time has come."

The Garden of Cosmic Speculation, Portrack House, Dumfriesshire, is open on May 3 from noon to 5pm; admission is £6. Visit and