When you look at the trees in the street, your attention is often only focused on the tree forms, whether they have beautiful flowers or cause any obstruction to the traffic and pedestrian flow, etc. In fact, you may not realise that the selection of tree species for street planting involves a lot of knowledge as every tree species has its unique “character”, and on top of that, they have to grow in the dense and compact city of Hong Kong. Thus, there are a series of considerations behind the decision.

Capability to withstand different roadside conditions

The Government has been striving to create a quality environment for urban planting to enrich vegetation diversity and enhance the outdoor environs, so as to provide outdoor spaces for public enjoyment. In this connection, the Greening, Landscape, and Tree Management Section (GLTMS) of the Development Bureau (DEVB) commissioned a consultancy study to provide a reference for selecting suitable tree species for different types of streets in Hong Kong. The GLTMS has recently completed and issued the Street Tree Selection Guide (the Guide). By reviewing and screening more than 500 tree species with a set of criteria, the consultant team eventually recommended 80 less commonly used tree species, which are however suitable for planting in Hong Kong. Of these, half are native species. Regarding these species, it is important for us to consider their essential of resilience to the street environment, including the tolerance to wind, heat, drought, waterlogging, and pests and diseases. This time, I have invited a landscape architect of the DEVB to introduce the details of the Guide and talk about the considerations in selecting tree species for the streets of Hong Kong.

Recommendation on vegetation diversity in tree planting

Currently, there are only around 20 species of trees planted at roadsides in the urban areas of Hong Kong, which account for about half of our urban trees. However, low diversity in the species planted is making our urban forests more vulnerable to outbreaks of pests and diseases and diminishes soil quality, consequently leading to higher maintenance pressure. The DEVB’s Landscape Architect (Greening and Landscape), Mr CHEUNG Ka-wai, Allen, says that the purpose of the Guide is to explore the possibility of planting a greater diversity of species and encourage the planting of suitable native species to improve the resilience of Hong Kong's urban forests through promotion of vegetation diversity under the "right tree, right place" principle, with a view to improving ecological health and minimising tree risks. Simply put, the width of pedestrian paths, soil volume, distribution of underground utilities and pipes, upper ground space, air flow, sunlight, etc. may affect the planting and growth of trees. Also, different tree characteristics such as buttress roots, forms and sizes may affect the selection of suitable planting locations. Therefore, the Guide provides a reference for government departments and industry stakeholders in selecting street tree species.

To enable the public to have a better understanding of the tree species introduced in the Guide and the importance of selecting trees with essential attributes suitable for street environments, Mr Allen CHEUNG talks to us about some tree species suitable for street planting. One example is Xanthostemon chrysanthus, also known as Golden Penda. When Golden Penda blooms, the stamens will form a ball shape and the flowers are in bright colours. Its nectar provides food for wild animals. Most importantly, it has essential attributes suitable for the street environments in Hong Kong. For example, it can tolerate roadside pollution, less prone to pests and diseases, and is wind and drought tolerance.

Preparing for more frequent extreme weather

To address the more frequent occurrence of extreme weather conditions due to climate change, and to meet the challenges brought by ageing street trees, we need to make our urban forest more resilient and adaptable to enable sustainable development. Super Typhoon Mangkhut has caused extensive damage to our tree stock when it hit Hong Kong in September this year, but it also opens up opportunity for planting at the same time. After clearing the tree debris, various departments will carefully inspect the extent of damage of the planting sites and related areas. Trees will be replanted only if it is feasible and suitable to do so. To provide a better growing environment for the new trees, the number of trees to be replanted will depend on the sites’ condition. For example, trees will not be replanted on steep slopes (35 degrees or steeper) to safeguard public safety. Besides, departments will not indiscriminately pursue quantity or blindly follow the compensatory planting ratio of 1:1. Furthermore, departments will examine the feasibility of expanding the size of existing tree pits and study the potential of linking the soil volume below the pedestrian paths before replanting trees.

Dedicated tree care and maintenance

After selecting suitable tree species, we have to make sure that other aspects of planting are also done properly. For example, tree stock quality, planting standards, and the implementation of associated planting works have to be appropriate. Therefore, we encourage landscape designers and departments to widely apply the Guide when replacing and planting street trees.

As proper selection of tree species is only the first step and the work that comes afterwards is very important, departments will pay great attention to the maintenance of trees. Through routine maintenance and management, including regular yearly tree risk assessment, timely inspections and appropriate pruning, we can minimise the risks posed by trees to the public and their property. Moreover, our urban forest will grow healthily and sustainably.