Out of this world: Jupiter Artland sculptural garden

• Robert and Nicky Wilson from Jupiter Artland with the finished mounds by Charles Jencks. Picture: Jayne Wright

THERE is a plasticine model tucked away in an upstairs office room at Jupiter Artland, the 80-acre sculpture estate centred on a Jacobean manor house near Edinburgh Airport, whose summer opening begins this week. It is a rough - very rough - working replica of the Cells of Life, the vast spread of eight stepped grass-covered mounds designed by landscape artist Charles Jencks.

Through five years of construction, the model has been thumbed and prodded and flattened out of shape as owners Robert and Nicky Wilson, head gardener Richard Irving, digger drivers and others in their impromptu building team have laboured to turn Jencks' vision into stunning reality. "It would be lovely to cast it in bronze, enamel it, turn it into art itself," said Robert.

Jupiter Artland opened its doors two years ago, with 20 works of sculpture, often vast in scale, from artists such as Anish Kapoor, Antony Gormley and Ian Hamilton Finlay. But the celebration this summer marks the completion of Mound Eight. At about 22 metres high, it is the last and most dramatic of the stepped mounds of Jencks' giant sculpture.

Jencks' "landforms" typically use vast swirling curves of earth to turn scientific discoveries into landscape. From his own Garden of Cosmic Speculation near Dumfries, to the Ueda at the Scottish National Gallery of Modern Art in Edinburgh, they represent, often in intricate detail, the patterns of cosmology or DNA. That's the theory. Then there's the matter of getting them built.

Jencks' Cells of Life already straddled the main drive to the Wilsons' home at Bonnington House, like some surreal green gate, when Jupiter Artland opened in 2009.

But Mound Eight remained unfinished until this year.

For starters, the Wilsons had to move it four metres from the original spot to get the sightlines right, rebuild it using different soils after one side slipped away, fix a broken irrigation system and returf it through two years of probably the worst snows in living memory.

"We had a digger, and we just dug a hole," said Nicky Wilson, recalling how the project started. The art college graduate and sculptor is the undeniable driving force behind turning Bonnington House and the estate the couple bought in 1999 into a major Scottish art attraction.

"We just built it, and then decided we were going to open it to the public because it was too big, and too good," added Nicky.

Jencks' vision is outlined in the sketches on show in Metaphysical Landscapes, an exhibition of schemes, models and small eccentric sculptures from his studio that opens at Jupiter Artland this year. The drawings show curving coloured outlines with scribbled notes about the overlapping cells they represent. But the plans had no scale, fly-through graphics or structural guidelines.

"He will argue that everything has to be done by eye," said Robert. "This is why changes have happened."

Visitors to the Life Mounds are walking on giant representations of cell membranes and nuclei, with the six steps of a fountain representing mitosis, the process of cell reproduction. More practically, they've required tens of thousands of tonnes of soil with 20 tonne excavators, coating them with sticky compost made from Edinburgh's green waste collections and several football pitches worth of turf.

"Mound Eight is absolutely massive, when you get to the top you realise how big it is," said Nicky. It has added the final graceful sweep to Jencks' design. It takes three people about a day to mow the Mounds, along the paths and vertical grass walls, essential to keeping the lines of Jencks' vision crisp and clear. The cocktails planned at last night's summer party for Jupiter Artland, for about 350 people, say it all. The "Jencks" is a sophisticated American Martini, in honour of the architect born in America to a Fife mother. The "Quinn" is a fruity number, in honour of Marc Quinn's gaudy Love Bomb sculpture, a 12-metre multi-headed orchid that is the first thing visitors to Jupiter will see. The "Goldsworthy" is a dark, earthy-coloured rum number named after sculptor friend Andy, who works with rock and wood.

But two years on from Jupiter Artland's grand opening, it was billed as a lower-key affair in a year of consolidation. With roughly 30,000 visitors in the past two summers, including 124 school groups, one addition is a new education centre, with a classroom lined with sections of coppice wood and a garden fenced in by Duke of Edinburgh volunteers. Another is an extraordinary, but as yet uncompleted new piece by Goldsworthy. It features floor-to-ceiling tree trunks built into a window-less, bothy-style stone building, becoming denser and denser towards the back wall, tempting visitors to see just how far they can squeeze in.

Jupiter Artland is not a cheap family day out - individual tickets, bookable on line are 8.50, up from about 5 when it first opened. For the first time this year, Edinburgh College of Art students are acting as invigilators on the Jencks mounds, to stop people sliding down them. The attraction is marketed in a subtle fashion, which confirms that the Wilsons are doing this for love, not money.

Robert Wilson heads up a family-owned business dedicated to the manufacture of homeopathic treatments, Nelson's Rescue Remedies. It sells natural health products in more than 70 countries, including Rescue Remedy, an anti-stress tincture made of flower essences. With a quarter of prescriptions in countries like France already in homeopathic medicines, and booming markets like India beckoning, the company has been growing at about 10 per cent a year. It is this success that has provided the funding for the park.In the long-term, the Artland is already backed by a charitable foundation, and new commissions are already in place for next year. They include a planned audio tour narrated by two Hollywood actors, a "proposal" drafted by Peter Liversidge. He is one of the Artland's artists whose planting of black tulips, laid out in the shape of the shadow of a tree, is now in full bloom.

Indeed, the success of the award-winning project has led to Robert Wilson being named chairman of the Edinburgh Art Festival this year.

Jupiter Artland is now likely to be a major Scottish art attraction for the rest of the Wilsons' lives, while its likely people will be admiring - or puzzling over - Jencks' creations for centuries.

The Wilsons have been collecting sculpture and art since before their marriage 16 years ago. Early purchases included a piece by the late Ian Hamilton Finlay. They joke about an original by the abstract artist Victor Passmore which they turned down the chance to buy at a few hundred pounds - probably worth more like a quarter of a million today.

Jencks first came to the site in 2004. Nicky Wilson had seen his Garden of Cosmic Speculation. He drew up ten rules for an artland, he said in an interview, and how it would differ from a "sculpture park". Even great sculpture collections, like the Storm King Art Centre outside New York City, he said, can get frozen in a particular period. There were 140 sculpture parks in Britain, and he wanted them to think before creating the 141st. "For me sculpture parks are wonderful but the more successful they are, they turn into parking lots for sculpture," he said.

"An artland creates parity between the the rules of landscape and the rules of sculpture. It's not architecture, it's not landscape, it's not gardening," he said. "Art should be equivalent to the landscape."

Where Jencks had used wooden siding in his own mounds at Portrack, the Wilsons opted for costlier corten steel, which rusts to a red finish. Jencks, who came in to check on construction every few weeks, praised the couple's "Herculean effort." "It's hard when you start from scratch with the site. They were committed to doing it come hell or high water, and they have stuck with it to keep its quality high."