Romans in togas, shepherds in saunas and the Bridgerton garden in bloom … my wild day at Chelsea flower show

<span>Quite a view from the sofa … the treehouse by the artificial waterfall.</span><span>Photograph: Oliver Wainwright</span>
Quite a view from the sofa … the treehouse by the artificial waterfall.Photograph: Oliver Wainwright

A gigantic Chinese dragon made of gnarled chunks of driftwood towers over a display of bog plants, puffing steam from its nostrils and clutching a ceramic pearl that gushes with water. Nearby, men dressed in togas patrol the courtyard of a pretend Roman villa, where simulated rain pours into the garden from a pantiled roof. Around the corner, a waterfall cascades down an artificial rock face, creating an arresting backdrop to a display of luxury outdoor sofa cushions.

Welcome to the RHS Chelsea flower show, a surreal phenomenon that has gone from an annual fair of prized blooms to a multimillion-pound Disneyfied spectacular, where the flowers now struggle to hold their own against ever more elaborate pieces of set design.

Every year, in the space of just three weeks, the grounds of Christopher Wren’s Royal hospital in London are transformed into an unrecognisable wonderland of horticultural fantasies. It is a place where elfin treehouses compete for attention with pixie grottos, and sculpted clay stupas loom above moss-encrusted ruins. It feels like wandering around a themed food court, with Moroccan tiled courtyards jostling with Japanese bridges, thatched Burmese stilt houses vying with Welsh dry-stone walls. The cuisine on offer might not be as international, but you can wash down the global garden safari with a £15 Pimm’s.

Begun in 1913, in a modest marquee, the Chelsea flower show has mushroomed into a town-sized endeavour. It has become a festival of terraforming as much as flowers, seeing more than 2,000 tonnes of soil moved around the 11-acre site each year, and hundreds more tonnes of rocks, concrete, trees and scenery trucked in from miles around – all for just five days of floricultural theatre. Now, for the first time, this year there is a “green medal” for the garden with the lowest carbon footprint, which feels a bit like holding an exhibition of bonfires, then awarding a prize to the one that produces the least smoke.

For the first-timer, the fixture is a bewildering spectacle, like a trade show crossed with a wedding. Gaggles of visitors in floral dresses and straw boaters peruse avenues lined with stands selling secateurs, lawnmowers and ladders, along with hand-woven sun-loungers and sculptures of rabbits in comical poses. There are purveyors of thatched gazebos for outdoor entertaining, and makers of luxury shepherd’s huts, kitted out with saunas and roll-top baths, yours for £160,000. There is bad art aplenty for gardens of all sizes, from animal figurines made of bundles of wire, to big copper trees that spout water from their leaves. Meanwhile, outdoor tables erupt with decorative gas flames – mains connection optional – so you can enjoy the spectacle of global heating all night long, all year round.

Then there is the main attraction: the 35 show gardens, arranged along two avenues, where designers’ wildest dreams are unleashed. This year, one of the most elaborate pieces of set-dressing comes from landscape architect Robert Myers, who has summoned the ghost of Wren to recreate an entire chunk of St James’s church in Piccadilly. Two-storey high brick walls, replete with Portland stone copings and arched windows, tower above the garden, where a water feature trickles next to a round wooden cabin, crowned with a tall conical roof shaped like a wizard’s hat, as if plucked from a Studio Ghibli animation.

The design is apparently “illustrating a sense of hope and recovery and inspiring future generations to imagine the world to be different”. One might hope that future generations will question the wisdom of digging substantial foundations, laying stone paving, and erecting steel-framed walls clad with clay bricks, just to provide a diverting backdrop to some plants for a few days.

The grandiose ecclesiastical ruin stands opposite an equally ambitious installation by architect and cork enthusiast Dido Milne who won plaudits for the Stirling Prize-shortlisted Cork House in 2019. Working with garden designer Sophie Parmenter, Milne has created chunky structures made of big blocks of cork, which step out as they rise, topped with overhanging roofs supported by layered timber joinery, with a faintly Japanese air.

Designed for the National Autistic Society, the structures are intended to embody the strategy of “masking” – the practice of consciously or unconsciously hiding autistic characteristics in order to fit in. Perhaps fittingly, they effectively mask views of the garden from the main avenue. When you get closer, you find a tempting wooden walkway that leads into a sunken dell of ferns but, as with all of the show gardens here, it is off-limits to the paying public. For the £120 day ticket, visitors merely get to gawp at these creations from behind a rope fence.

Perhaps due to the spectacularly un-green nature of the whole bonanza, there are a few nods to sustainability in some gardens, particularly in terms of water management, responding to the increased frequency of flooding and drought. Charity WaterAid has commissioned garden designer Tom Massey and architect Je Ahn of Studio Weave to create a water-harvesting planted roof, supported by funnel-shaped cor-ten steel columns, which filter rainwater into a wetland garden below. The design creates an alluring vaulted space to explore (you’ll have to take my word for it because, once again, it’s off limits) and it will be relocated to a public garden in the north of England after the show.

Another flood resilient garden, by Naomi Slade and Ed Barsley, shows how rainwater can be channelled from domestic rooftops into the soil, through tanks, swales and ponds. But did these teams really need to build a great steel structure, and the facade of a brick house, to get their water-saving message across?

After passing a jaunty pink champagne stand (jarringly positioned next to the Freedom from Torture garden, the giddy parade reaches “peak Chelsea” with a Bridgerton-themed garden. Sponsored by Netflix, its stone walls and dwarf elms are apparently “a visual metaphor for Bridgerton character Penelope Featherington’s evolution from wallflower to muse”.

This might only be trumped by the collaboration between luxury jeweller Boodles and the National Gallery, where a clumsy row of steel pergolas is supposedly “influenced by the repetition and perspective elements in Canaletto and Claude paintings”. You might have to squint hard to see the resemblance.

If the sponsored stage sets become too much, you can always retreat to the Great Pavilion, a gigantic 12,500-square-metre marquee where besuited horticulturalists show off their prize varieties in time-honoured fashion. It is a visual and aromatic feast of delphiniums and pelargoniums, orchids and gladioli, salvias and sweet peas, along with displays brimming with bonsai, cacti and carnivorous specimens.

There are some clever plant-focused gardens here too, like the Size of Wales by Snowdonia-based designer Dan Bristow, inspired by the abundance of life that occurs in tropical rainforests. Bristow has filled his space with a lush plethora of 313 plant species, reflecting the number that can occur in just one hectare of tropical forest, from gangly trees to miniature alpine flowers that spill around the rocks, which were salvaged from a Welsh slag-heap.

The garden is bordered by a “fungus fence”, where edible mushrooms sprout from a wall made of trees felled due to ash dieback, and hazel hurdles rendered with hemp waste mixed with lime putty. After the show, the wood will be left in a heap, where the mushrooms will gradually eat their way through it all, providing food in the process, while the rest of the garden will be transferred to Treborth botanic garden in north Wales.

“It’s about using old techniques in new ways,” says Bristow, who won a gold medal for his garden. “We shouldn’t be ‘tech-ing’ our way out of the climate crisis. But that doesn’t mean everything has to look like a Hobbit house.”

• The Chelsea flower show runs until 25 May.