'Many departments are responsible for tree planting, but the effect is not good and there are conflicts between departments. [So] the government plans to implement an overall greening programme,' said the Secretary for Environment, Transport and Works.

The plan would target densely populated districts such as Central, Wan Chai, Tsim Sha Tsui, Mongkok and Kowloon East and West.

'[We] have planted trees in country parks for many years; we should now focus on urban areas,' Ms Liao said.

The districts would be assessed and planned in detail, since there were many underground pipes.

Conservancy Association director Ng Cho-nam, a member of the Advisory Council on the Environment, welcomed the idea. But he said planting would be difficult because the districts were densely crowded.

'Many efforts are needed to turn these areas into a garden city,' Dr Ng said. 'There isn't enough space or there are too many pipes and wires underground.'

He cited Lockhart Road in Wan Chai as an example: '[Many] trees are planted there but there isn't enough space for them to grow.'

The job was also difficult because much of the land in the districts was privately owned. He said there should be more collaboration between departments.

'The Leisure, Culture and Services Department is in charge of trees on the roadside, while adding green elements on bridges can be the work of another department,' he said.

Dr Ng suggested the government include more greenery when it was planning redevelopment of districts.

Democratic Alliance for Betterment of Hong Kong legislator Choy So-yuk also admitted there were challenges in implementing the greening plan in urban places.

'It starts a bit late but it's better than nothing,' she said. 'The government should require property developers to plant a certain number of trees in their new projects and give incentives to those who plant trees.

'For example, if a social group planted a tree, the government could place a tag with the name of the group on the plant.'