If you want to say something, the expression goes, say it with flowers. It is a concept that will take on new meaning in China this week, which on 29 April opens the doors to the world’s largest ever gardening show, a mammoth exhibition of plants, pavilions and soft power that forms part of celebrations marking the 70th birthday of the People’s Republic of China.

At the foot of the Taihang mountains in the Beijing suburb of Yanqing, an area the size of 500 football pitches has been fenced for the massive Beijing international horticultural exhibition, which dwarfs the Chelsea flower show by an eye-watering 495 hectares.

The vast show boasts more than 1,000 varieties of Chinese flowers, 100 indoor and outdoor gardens, and pavilions hosted by more than 80 countries. Organisers expect 16 million visitors – more than the number of tickets sold at the 2012 Olympic Games in London – to pass through its doors.

At its centre is the sprawling China pavilion, a massive semi-circular dome in the shape of a traditional Chinese ornament. Embedded in artificial terraces topped with wheat (a gesture to northern China’s staple grain), it is meant to evoke nostalgia and awe. The roof collects rainwater to be recycled. A sunken courtyard is intended to remind visitors of being inside a traditional Chinese home.

More than a gardening show

The horticultural show, which organisers call “the expo at the foot of the Great Wall”, is the largest international event China has held since the 2008 Olympics. It has taken more than six years of planning and the endorsement of some of the party’s top officials.

But it is more than just an unusually lavish gardening show. It is one of Beijing’s most ambitious deployments of soft power, aimed both at its own citizens and the outside world.

“The government likes what we call ‘home-field diplomacy’ to show the strength and prosperity of China,” said Zhang Lifan, an independent historian. The event, which runs through the October anniversary of the country’s founding under the ruling Communist party in 1949, is a way of rallying the country while projecting an image of an environmentally minded China.

Like the China pavilion, the exhibits aim to impress with displays of Chinese advancements in green development. At the “horticultural life experience” pavilion, visitors will see 3D printing, robots and horticultural technology pioneered by China. A greenhouse covering about 3,000 square metres will house about 1,000 plant species.

A performance hall has been built in the shape of a giant butterfly. A billboard in one of the visitor lots describes the expo as “an outstanding example of ecological civilisation”.

China is in need of the publicity boost. A trade war with the US continues to dog Beijing, whose Belt and Road initiative, a campaign to expand Chinese investment and trade links across the globe, is facing more scrutiny than ever. The UN and human rights groups continue to criticise China over the detention of thousands of Muslim minorities in Xinjiang, the country’s far west. Internally, it is grappling with a slower economy and continued public health and environmental concerns.

‘A classroom for learning Xi Jinping’s ecological civilisation’

The expo also offers a distraction from a year of difficult anniversaries for the Chinese leadership, from the 60-year anniversary in March of the Tibetan uprising that led to the Dalai Lama’s exile from China, to the 30-year anniversary of the June Fourth incident, when Chinese troops violently put down pro-democracy demonstrators in Beijing.

Within days of the event’s opening is the 100-year anniversary of the May Fourth movement, a student uprising that planted the seeds for the birth of the Chinese communist party – but officials prefer not to encourage mass student demonstrations. “The expo is a way to scatter the people,” Zhang said.

The expo is a mass mobilisation, in its own way. Less than two weeks before the opening, gardeners and construction crews were shuttling in and out of the park. Workers were connecting CCTV cameras along the perimeter of the venue. Nearby, another group of workers had gathered and was being divided into groups before walking towards the park.

About 20,000 volunteers, mostly students, have been recruited to man the expo in shifts. Organisers describe the opportunity as “a classroom for learning Xi Jinping’s ecological civilisation”, a vague slogan popularised by the country’s leader, who has promised to clean up pollution.

The expo appears aimed at fostering a sense of national unity. The “China gardens” feature a Taiwan pavilion as among the country’s 34 provinces and administrative regions. (Beijing claims self-ruled Taiwan is still a province of China.) The gardens also include a pavilion for Xinjiang, with desert landscapes and camels.

China has framed the show as a diplomatic win, with more than 100 countries and international organisations attending, including Russia, France, Germany and the UK. The Vatican, which is repairing ties with Beijing severed in 1951, has accepted an invitation. The US has not announced its attendance.

“China is very good at organising these big spectacular events,” said Richard Burn, the UK’s trade commissioner for China. Burn said the event is a way for his country, which committed early to attending, to show its commitment to UK-China ties. The UK is hosting a pavilion that will include gardens, a “living lab” of British horticultural technology and a gin terrace.

In some ways, the event is similar to the more extreme staged events of countries like North Korea, according to the cultural diplomacy expert Alessandra Cappelletti, from Xi’an Jiaotong-Liverpool University. “These huge events and parades are really a way to show a very fake image of the country but, at the same time, a very ordered, well-functioning country. In the case of China, it is more mild, but the aim is attracting attention and moving attention from what is more problematic to what is to be praised,” she said.

But flower diplomacy has not worked on everyone. “My understanding is it’s extremely expensive, and what is the point?” said one diplomat from a western country, who asked not to be named because they had not been authorised to speak on the topic.

The expo also appears to contradict its theme of “ecological civilisation”. Two villages, or about 2,400 people, were relocated to make room for the expo, moved to nearby new apartment buildings. Residents of one of the villages were each given a book memorialising the village, and the homes and land some of them had had for generations.

“It’s worthwhile if the country needs it. We belong to the nation. If the country says it’s necessary, then it is,” said one of the relocated residents, who asked to only give his surname, Wu.

More than half a dozen highways and roads have been constructed for the expo, including a 5.9km tunnel bored through a mountain, crossing underneath the Great Wall seven times, according to China Railway Construction, which built it.

Over the course of the event, 42m cubic metres of water will be pumped into the Guishui River, which runs through the park. The river, which was once used by locals sneaking in to swim or fish, has now been completely fenced off. A dozen “ecological parking lots” have been set up to house the cars and buses transporting visitors.

“China has a craving for grandiosity … China is always very proud if we spent the most, created the largest event,” says Wang Yongchen, the deputy director of the Green Earth Volunteers NGO.

Not everyone expects millions of visitors to flock to the site on opening day. Local residents said they supported the event, which they hope will bring more tourism to the area, but most doubted they themselves would spend between 120 yuan and 500 yuan ($18 and $75) for a ticket to get in.

“Paying 150 yuan for a visit is not a very good deal,” said a security guard, standing watch by a side entrance to the park. “It’s a waste of labour and money, but the country has a lot of money, and they aren’t bothered by what we think.”