Right trees for right seasons in Kathmandu

Every spring Kathmandu turns into a city of flowers blooming with jacaranda and bougainvillea. People wait year-round to escape the otherwise dull and dusty concrete landscape with a vibrant streets of purple, red and green. 

But very few people know that these beautiful roadside trees and their colourful seasonal flowers are ill-advised for our climate, and can increase road hazard. 

Trees shelter us from sudden downpours, and during hot summer days, we seek their refreshing shade. In fact, they reduce the temperature of the asphalt surfaces beneath them by as much as 15° Celsius.  

Trees purify the air by sequestering carbon dioxide and giving off oxygen, they remove harmful gases and particulate matter. They improve the soil by assisting in nutrient cycling and help regulate surface and subsurface water. And they provide food and shelter to insects, birds, animals, and plants.

However, not all trees are suitable for urban streets. We need to select them based on more than aesthetics and convenience. Ecological value and road hazards are equally if not more important considerations when choosing a species.

Unfortunately, past strategies and the recent National Urban Development Strategy 2017 and Nepal Urban Road Standard 2076 have failed to provide clear guidelines for tree selection, leading to haphazard planting along Kathmandu’s roads. Concerns raised by ecologists, conservationists and locals over the years have gone unheard.

History of street trees

Early records of roadside trees date back to the 14th century when Jayasthiti Malla (1380-1395) ordered plantations alongside streets and public wells. Later, Prime Minister Jung Bahadur Rana, brought back exotic species like Eucalyptus, bottlebrush (Callistemon), and Kaiyo ful (Grevillea robusta) from his Europe visit (1850-51) to decorate palaces and roadsides.

More systematic efforts started in the early 20th century. Prime Minister Chandra Shumsher Rana (1901-1929) expanded Kathmandu’s road network and planted trees, introducing new species like Monkey puzzle (Araucaria araucana). Juddha Shumsher Rana (1932-1945) continued planting trees to beautify the roads like Putali Sadak, particularly after the devastation of the 1934 earthquake.

Modern urban environmental planning began during the Panchayat in the 1960s and 1970s. The government renovated roads, mandating the planting of either a single line or double line of roadside trees. Part of the newly constructed Arniko Highway (Kathmandu to Kodari) and the Ring Road encircling the capital, had rows of trees on either side. 

In the 1980s, urban planners suggested three lines of green belt alongside the roads instead of previously mandated one-line roadside trees. As a result, more than a hundred thousand fast-growing trees were planted around the Ring Road.

Rana and Panchayat planners were wrong to pick roadside trees such as poplars and eucalyptus based primarily on aesthetics and their fast-growing nature. In recent decades, Nepal has been steadily expanding its road network while integrating green belts, but planners have neglected the inclusion of criteria on species selection.

Over time, attempts have been made to suggest a more scientific species selection. Researchers have recommended over 300 potential species for urban forestry in Kathmandu, including a comprehensive list of suitable species that could be planted alongside roads. The list includes native and exotic trees and ornamental plants. These suggestions have been largely ignored.

Mitra Pathak, research officer at the National Botanical Garden in Godavari says, “It is unscientific, we are planting whatever saplings are easily available.”

Ecologically, the majority of trees planted around Kathmandu are misfits. Swami (Ficus benjamina) for instance is a weed, whilst Grevillea robusta, a deciduous branchy species, and Raj Sallo (Cupressus torulosa), are climatically unsuitable for the city.

Urban environment expert Ramji Bogati also laments the lack of ecological criteria. “We should take into account factors like location, soil, nature of the species, in selecting roadside trees,” he says.

The Tinkune-Maitighar Road, for example, is one of Kathmandu’s better-maintained green belt stretches. But despite having high species diversity, proper tree species selection has been disregarded. Planners mainly opted for exotic species that grow fast, with minimum care, and with a high survival rate instead of evergreen and local species. 

The 3.2 km road with four rows of plants consists mostly of non-native ornamental trees: fast-growing deciduous poplars, eucalyptus which lowers the groundwater table due to fast moisture absorption, and Jacaranda mimosifolia, an invasive species. Poplar and eucalyptus can damage foundations and develop cracks in pavements because their roots grow close to the soil surface. 

Some native species planted, such as birch (Betula alnoides) and silk tree mimosa (Albizia julibrissin), both deciduous and fast-growing, were poor choices owing to their open crown and drooping branches that are potential road hazards. Willow (Salix babylonica), native to central Asia, on the other hand, is a better choice because it is elegant, robust and tolerant to air pollution and injuries.

Guides to roadside tree selection

Nepal Road Standard 2070 has three guidelines for roadside plantations: ensure good visibility by planting trees on sides and shrubs on the median, avoid wide-crowned trees, and ensure tree crowns do not cross the pavement's edge. Yet, even this fails to address important aspects of species selection. In particular, we need criteria to evaluate ecological, economic, and aesthetic values.

  Functional Criteria:Block noise, clean the air, control wind and provide safety and shade to vehicles and pedestrians, and habitat for urban wildlife.

  Ecological Criteria:Fitting for the soil and climate of the city, resistant to wind, pests, diseases and air pollution.

  Socio-Economic Criteria:Cost, care, longevity, and reflection of urban identity.

  Structural Criteria:Fast development, pruning-ability, strong root system, tree shadow, flowering and fruiting, future size, and diameter.

  Visual Criteria:Colour of leaves, flowers, deciduous vs. evergreen, tree dimension, road width, and planting arrays.

Selecting appropriate tree species is often complex but if we engage urban planners, ecologists, and specialists of the field, we can create sustainable roadside plantation guidelines. Analytic Hierarchy Process (AHP) is an effective tool to aid decision-makers to set priorities and decide the most suitable trees for particular roadside ecologically and aesthetically.

We have to be cautious with the types of trees to plant along our roads, beauty can often be misleading. Carefully selected roadside trees can help reduce negative environmental and social impacts of urbanisation, making Kathmandu more resilient in the face of changing climate and overexploitation of nature.

Meena Bohara, PhD, is a researcher of ecology and environment. She is the President of Nature Conservation and Study Centre.