The Torre Guinigi in Lucca, Italy, is a brick medieval tower – it’s handsome, but of a type common enough in historic Tuscan cities. What makes it special is a grove of holm oaks growing from its summit. Trees come with expectations, such that they are rooted in the ground, yet there they are, high in the air, apparently flourishing. The tower would be less interesting if it weren’t for the trees and the trees would be less interesting if it weren’t for the tower.

So there’s something compelling about trees in unexpected places. Hence at least part of the appeal of the High Line in New York, where gardens grow on an old elevated railway line, and of the ski slope on top of the Amager Bakke power plant in Copenhagen. There’s been a thing for wrapping towers in vegetation in recent years. Little Island, the micro-park recently created by Thomas Heatherwick over the Hudson, has a similar well-I-never, Instagram-able impact.

London’s Marble Arch Mound, a temporary artificial hill at the point where the end of Oxford Street meets the north-east corner of Hyde Park, is the latest attempt to harness the charm and power of the elevated tree. Visitors will be able to ascend to a viewing platform at the mound’s 25-metre-high summit and then go down into an events space buried inside it. They will see Lightfield, which is billed as “an incredible light exhibition put together by W1 Curates” that “alludes to the mycorrhizal nature of birch tree forests”.

The project has been commissioned by Westminster city council in the hope that it will help bring people, post-pandemic, back to the UK’s most famous shopping street. It is also a figurehead for a larger scheme, called Oxford Street District, which aims to crack what has long been an unsolvable problem, the Fermat’s theorem of town planning: how to make this well-known but none-too-pleasant thoroughfare more enjoyable and civilised. The plans include relatively unglamorous ideas such as part-pedestrianisation and upgrades to paving and planting.

The mound has been designed by MVRDV, a Rotterdam-based practice whose Dutch pavilion for Expo 2000 in Hanover was a multistorey garden with wind turbines on the roof. It was a memorable work, a celebration of the Dutch genius for constructing human-made landscapes, and a pioneer of the modern trend for aerial vegetation. MVRDV also created Tainan Spring in Taiwan, which converted the remnants of a shopping mall into a “lush lagoon”, and the Skygarden in Seoul, where conifers, cherries and maples, their colours changing with the seasons, grow on top of an old highway overpass.

But the very thing that makes hovering gardens attractive – their apparent impossibility – also makes them hard to achieve. Projects such as the Copenhagen ski slope can end up with a weary, rough-at-the-edges feel, so much energy having been expended in making the thing happen at all. Or they can be stupendously expensive, like Little Island, which cost $250m for 2.4 acres of garden. They might come with terms and conditions that take the edge off the joy of the concept, such as the spontaneity-killing necessity of making reservations if you want to visit Little Island after midday.

The Marble Arch Mound shows signs of struggle. For something billed as a summer attraction, it’s opening oddly late in the season, its completion having been postponed at least once. Last week, abseiling operatives were still frantically sticking slabs of turf to its sloping sides. The result looked parched and patchy, more like an ensemble of ill-matched carpet tiles than a greensward. The trees were looking skinnier and less luxuriant than the computer-generated promotional images had suggested. The finished project will have to pull off the trick of making you forget the scaffolding that holds the whole thing up, but the grass looked in danger of losing its argument with the metalwork.

If a landscape is temporary as well as gravity-defying, as the Marble Arch Mound is, it will also have trouble recreating the full range of experiences that go with plants. For a tree is not just a tree and grass is not just grass. They are also, in normal circumstances, accompanied by ecologies of birds and insects and by human activities: sitting in the shade, picnicking, snogging, children rolling down slopes. Anyone who tries rolling down the steep flanks of the mound will be in danger of continuing into the busy road that surrounds it and under the wheels of a bus. To avoid this hazard visitors will have to keep to the metal steps and walkways.

Without this range of experiences, a tree becomes a sign of itself rather than the real thing, a symbol of all the good stuff that goes with tree-ness more than its realisation. Foliage-wrapped towers and temporary vegetal installations give off an environmental buzz – it’s green so it must be good! – but in reality there is nothing very ecological about shipping living things to a given location and then shipping them away again. The City of Westminster say that the mound is “built to the highest sustainability standards and any materials used will be recycled where possible”, but this doesn’t mean that it will make a positive contribution to the environment.

It’s possible that the visible sweat and effort of the late stages of the mound’s construction will miraculously disappear from the finished product. It’s also possible that the views from the top and the sheer unlikeliness of the project will transcend whatever limitations it might have as a piece of constructed nature. But it’s hard to avoid the conclusion that the progression from High Line to mound is one of increasing sketchiness.

For the High Line is a work of time, whose highly considered planting and design is impossible in a temporary installation. So for that matter is the Torre Guinigi – that little grove on the top creates a delightful place that the metal deck on top of the mound will be unlikely to match. This in turn, in the case of the High Line, is a reflection of the political and financial environment that created it – it was made possible through trade-offs with private developers through a clever manipulation of New York’s sophisticated planning system.

The plan for funding the mound, by contrast, is to charge a GBP4.50 entry fee and raise profits from “M&S bespoke food trucks”. Like the Little Island timed entry, this seems to take some of the fun away: how many hills do you know that you have to pay to climb? Which, like the other contrivances necessary to make the mound happen, is a shame. For this climbable irruption of confected nature into the traffic-filled centre of a metropolis is a great idea at heart.

It would be magnificent if such a thing could be created permanently, with all the thought and attention necessary to make it ecologically rich and pleasurable to experience. In a better world, such a thing would happen. In an even better world, the creative and political energy that goes into something like the Marble Arch Mound would be directed at planting trees permanently in ordinary streets all over the country. That really would make a difference to the environment as well as enhancing the lives of thousands. But it wouldn’t get the same attention as a temporary mound.