Dutch King Willem-Alexander opened the gates to one of Europe's largest gardening fairs, a once-in-a-decade show focusing this year on how to make cities greener.

But critics have denounced the show, which features displays by 200 participants from 25 countries, as a "money pit" that has massively over-run its budget.

The Floriade 2022 exposition, which runs until early October, is expected to draw more than two million enthusiasts to the central city of Almere.

The Floriade 2022 shows "what a green city could be like in the future... what kind of materials could be used for this and what role the horticultural sector could play in it", said its curator, Annemarie Jorritsma, a former mayor of the city.

"On top of that, when the show is finished it will be a fantastic residential area," she told AFP.

Each decade, a different Dutch city gets to host the gardening extravaganza. Almere, the latest, is a city that was itself created by the Dutch by draining part of the former Zuiderzee bay to reclaim land.

As well as being a showcase for Dutch horticulture, each participating country has its own pavilion.

China's is showing "new ways of using bamboo", said Jorritsma. Italy is focusing on permaculture, while France shows how metallic imitations of trees can be used to cool cities.

The German pavilion is decorated with plants including garden plants, trees, food crops and wildflowers to form a "living ecosystem whose appearance would change throughout the exhibition", organisers said.

"The Floriade is the best place to show what countries have to change their cities," Detlef Wintzen, one of the exhibitors at the German pavilion, told AFP.

- Cost controversy -
The event has however been criticised for budget overruns that threatened its very existence.

Dutch media have reported that Almere has significantly over-run its 10-million-euro budget ($10.8 million) for the project.

Financial daily Financieele Dagblad estimated that costs could be as high as 200 million euros -- with losses of up to 100 million -- but said there was a "thick fog" hanging over the official costs.

First held in Rotterdam in 1960, the organisers of the last three Floriades -- 1992, 2002 and 2012 -- have all been criticised for losses totalling millions of euros.

And some media reports have even suggested that this could be the last-ever edition of the show.

Almere -- the country's youngest city -- plans to have a "green residential area by the water" after the end of the Floriade designed by Dutch architect Winy Maas.

Some 660 homes will be built in the "Hortus" district, many of them made from durable materials such as moss and mushrooms.

An imposing colourful building in the middle of the exhibition will eventually serve as social housing, and "floating homes" are also planned.

Members of the public can visit the Floriade from Thursday onwards.

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