Urban gardener: Gravitational pull

Well, it's over for another year. The dust, and there was plenty of that at Chelsea this year, has settled and we can be pleased that not only did we win an RHS Gold Medal, but that our garden for Bupa will now be transferred to Meadbank care home in Battersea, a short distance across the river Thames from the Royal Hospital. As ever, it was a team effort. So many people play a vital part in the success of a show garden. They are too numerous to mention here, but they know who they are and how grateful I am for their help. I must, however, tell you about La Boule, the sculptural element of the garden that seemed to capture the imagination of everyone who saw it.

Serge and Agnès Bottagisio-Decoux, who provided sculpture for my 2006 Chelsea garden, sent me a photo of La Boule, their latest creation, not long after Bupa had accepted my initial design in July last year. It took less than 10 minutes of staring at it to realise that I would have to re-design the garden from scratch. Changing the plan after it had been approved must have had Bupa doubting my judgement but, on presenting a picture of the giant sphere, it took them even less time to agree that this was the show-stopper we wanted.

At 2.2m in diameter and weighing over three tons it's quite a statement to place in a garden, especially for anyone weaned on Indiana Jones or even worse, Patrick McGoohan's nightmare, The Prisoner. But on coming face to face with it at the artists' home in Demu in Gascony, France, it was as beguiling as any sculptural element I have ever seen. Even the axed scars on its surface provided not only the illusion of sensuous movement but an important tactile element in what was to be a sensory garden.

Spheres carry an almost mystical energy. The sun and moon have captivated generations since the dawn of mankind. Technology has embedded in our psyche images of the earth seen from the lunar surface, as well as other planets and their satellites in the solar system – powerful imagery, but incredibly calming. It's hardly surprising, therefore, that spheres have such a pull on us and that we use them regularly in gardens as simple but effective topiary.

The scale of La Boule was important, but with that came the problem of moving it. It was heavy and, being hollow, could still be damaged if not handled properly. So when it lost patience with our inch-by-inch manoeuvres and broke its straps, I had images of it careering into the neighbouring Marshalls Garden like some giant wrecking ball. Three of us made an instinctive (and stupid) attempt to slow it but realised within a nano-second that an undignified retreat was far preferable to a comical demise. Fortunately, the concrete pad we had prepared for it cracked and cushioned its fall, enabling it to roll comfortably into its intended resting place. Cue nervous laughter all round.

During the show, the gravitational effect of La Boule could be felt throughout the garden. It was the dominant force and, without doubt, the focal point, but it didn't suffocate the space or overpower other elements in the garden. It was also the fulcrum for a series of paths that took one on a journey to explore the whole space without ever reaching a dead end. The irrigation and lighting experts Waterwell ( waterwell.co.uk) worked hard to source an LED lighting device smart enough to exploit the sculpture's obvious planetary qualities without making it looking too cheesy. The lighting exceeded all expectations: encapsulating the solar system, it had us bewitched well beyond closing time. Even the security dogs were reluctant to chase us out.

Cut adrift from the effect of La Boule, its raison d'être, the garden would be lost. It brought to mind a quote from the great Mexican architect Luis Barragan: "A perfect garden – no matter its size – should enclose nothing less than the entire universe."

Now, I'm not saying the Bupa Garden was perfect by any stretch of the imagination but, thanks to a slightly imperfect concrete sphere, it did allow us a glimpse of cosmic harmony.