Can rising Colombo temperatures be controlled by tree planting ?

Tuesday, 28 August 2018 00:00 -     - {{hitsCtrl.values.hits}}

A major contributing factor to rising temperatures is the destruction of trees in Colombo and suburbs



Temperatures in Colombo have been rising over the decades and air-conditioned offices and bedrooms are taken for granted. Colombo was cooler decades ago. When I took up residence in Colombo in 1963 as a university student and later being employed, all in boarding houses; one common factor was rooms were without fans. Then room temperature was around 25/26 degrees and fans were unnecessary. On occasional warmer nights in April we used to open the windows. 

In 1976 when I built my own house in Nugegoda, there were no fan-hooks in the floor-slab.  Unfortunately, today suburban temperatures reach late 20s in the evening and early 30s during noon, along with high moisture content, feels like high 30s as ‘Accuweather’ claims. How did this happen and is there a possibility of Colombo temperatures be brought down to levels 50 years ago? The article investigates into possibilities and the action required.

Destruction of trees

A major contributing factor to rising temperatures is the destruction of trees in Colombo and suburbs. When the British ruled Sri Lanka, most roads including in Colombo Fort were tree-lined. Houses had large gardens with trees. Increasing population with booming house construction subdivided lands and trees were removed. Road-side trees particularly ‘Pare Mara’, most already removed with road widening, balance under threat of falling off during winds with roots cut-off during excavations. Pare Mara (Albizia Saman) is not a local species, originated in Central America. 

Decades ago “Thimbiri-gas-yaya” was filled with Thimbiri trees? Thimbiri (Diospyros Malabarica) native to Sri Lanka could reach 45 feet. After a dry period tree throws out fresh leaves forecasting arrival of rains. Deep pink leaves cover the tree turning gradually to green over two to three weeks. I have not seen a single Thimbiri tree in the locality, closest is on Havelock Road. 

British popularised the pink flowering Murutha (Lagerstromia Speciosa), referred as “Queen of Flowers”. British took the tree out of the jungle and planted by the stream sides spread over the wet country. Murutha is historically mentioned in the ancient “Salalihini Sandesaya”. Today the tree is popularly planted by the walking-paths.

Tarred roads and paving paths

Today, cities and suburbs are filled with buildings, tarred roads and paving stones. Local by laws for building approval specify, one third of the land area be open to sky, but balance ground is either tarred or paved. Excessive large roofs, later additions to buildings reduce grounds exposed to sun, reducing the extents to plant trees or grass. Covering the grounds with paving stones reflect sun’s heat increasing temperatures and prevents absorption of rain into the ground, thereby water gushing into drains and creating floods.

Carbon fixing and heat absorption of trees

Trees perform three major climatic functions: Green leaves in the presence of sunlight absorb carbon dioxide from the atmosphere, producing oxygen in the process called photosynthesis, creating a cooling effect. Trees block sunlight and further cooling occurs when water evaporates from the leaf surface. (The conversion of water to vapour removes heat energy from air). Trees draw water from the soil, evaporates into the atmosphere, creating low clouds that reflect the sun’s rays leading to cooling. 

In a tropical country as Sri Lanka, forests have a significant, overall cooling effect. Tree species that grow quickly and live long are ideal carbon sinks. Forests do not just absorb and store large quantities of carbon; they also release large quantities of oxygen as a by-product. Thus, in planting trees best would be tree varieties that grow fast, with a large canopy and live long. Meaning old Pare Mara trees have stopped growing, gives only shade, have outlived their useful life. 

Fast growing trees absorb as much as 80% of sun’s energy. Thus planting large number of fast growing trees with large foliage would help in lowering the atmospheric temperature.

Tree planting  by the President

Addressing the 24th Session of the Committee on Forestry (COFO) of Food & Agriculture Organization (FAO) in Rome in July 2018, President Maithripala Sirisena delivering the inaugural address informed that his Government plans to plant five million trees by 2020 and increase the forest cover of the country to 32% from current 29.7%. 

President Sirisena pointed out that two centuries ago over 50% of the land area in Sri Lanka was covered by forests and now the cover is only 29.7%. The President further informed that the country is highly committed to conserve its existing forest cover and enhance by adopting better practices, improve land governance, as a key measure to achieve Sustainable Development Goals. 

Public would remember in October 2016, the President’s tree planting programme was directed at planting one million gliricidia saplings on the tree planting day. But the progress is unknown.

Earlier too (in October 2015) the President announced the Wana Ropa (planting of forest), a three-year program to increase forest cover to a third of the country’s land area, would begin in January 2016. President informed that the forest resources have been destroyed during the past few decades by racketeers who made profits while destroying the environment. It is everyone’s responsibility to protect natural resources and strict laws will be enforced against those who destroy them.

Solar panels

In cities where trees have been replaced by buildings, part of sun’s energy could be converted into electricity with solar panels and extensive use of solar panels could contribute to reducing atmospheric heating. Already, some companies as Keels, MAS Holdings have installed solar panels on roofs of their buildings. Their move was not solely based on greening the country, as electricity generated through solar is cheaper. Solar electricity costs around Rs. 15 a unit, whereas CEB power costs Rs. 25. Thus installing solar panels would pay for themselves and reduce the need for much criticised coal-powered plants.

Most efficient commercially available solar panels have efficiency ratings of 22.5%, whereas majority of panels efficiency rating range 15% to 17%, indicate solar panels could reduce atmospheric heating, although their contribution is much lower than a tree. 

Thus the government should enact legislation forcing owners of large buildings (say over 2,000 sq. ft. roof area), to cover at least 50% with solar panels and offer low-interest loans. Also, house owners to plant trees to cover minimum one third of ground area with foliage. In both cases house owners failing to comply be charged a higher percentage of their municipal rates. Also households with over 50% foliage cover be given a rebate on rates.

My personal contribution

Around year 2000 I wished to build an elders’ move near Bolgoda Lake where I had purchased a three acre property long ago. The construction would require clearing and levelling two acres. I intended to fill the cleared land with indigenous plants, travelled countrywide, purchased young plants, mostly from the Forest Department nurseries in Anuradhapura and Habarana. 

Today, I have over 100 trees in wide variety, most over 40 feet tall and some have reached 60 feet. Our garden possibly has the most varied collection of indigenous trees in the Colombo district.

I also planted 15 Khaya trees; they were harvested when 12 years old. After seasoning and kiln-drying, I used the timber as flooring for the house, also for pantry cupboards. Any timber used as flooring would be subjected to most punitive action as attacked by white ants and moisture from the ground. I wrote number of articles on Khaya giving information on cultivation and usage. 

Planting of 5 million trees 

The President at the FAO conference promised to plant 5 million trees by 2020. Does the President realise the intricacies of planting such numbers? In October 2015, President announced a three-year program to increase forest cover to a third of land area and in October 2016 the President directed planting of one million gliricidia saplings. If the President inquired into the progress achieved, would realise the difficulties in achieving such targets.

Coordinating office

Achieving the President’s target would require a coordinating office with full-time officials to plan, allocate tasks to individuals, check progress and make necessary changes to ensure targets are met. Planting large number of trees would involve: 

1. Identifying plant varieties and their climatic suitability. 

2. Collecting seeds. 

3. Germinating seeds and raising them into plantable specimens. 

4. Identifying locations that could accept plants. 

5. Planting and looking after them until they could stand alone. 

6. Fertilising and pruning and 

7. Monitoring progress.

Identifying varieties

Sri Lanka has over 1,000 varieties of indigenous tree varieties and some are threatened of extinction. Number of government departments and university lecturers have produced books on indigenous trees and a list of plantable trees could be the starting point. The program could help to identify, locate and sustain threatened varieties also restoration of rare varieties in special locations. 

The programme would identify fast growing, indigenous but slow growing trees, heavy canopy trees that would absorb large quantum of sun’s energy, some of them may not be indigenous, trees with edible fruits, high timber value, etc. In selection, invasive plants need to be avoided, unfortunately number of such trees had been introduced to the country. The programme would not be complete without identifying invasive and destructive plants and a plan to free the country of same. 

Collecting seeds

Most dry-zone plants produce fruits during February to May. To enable effective seed collection of selected varieties, seed producing period and available regions need be noted in advance. Target of five million trees would require seven-eight million seeds, would be far above the capability of the forest department. Thus seed collectors need to be identified and their collection possibility be noted in advance to ascertain the dates and possible numbers. In addition, country-wide assistance of villagers and students be sought who could be remunerated for their efforts. Also private plant sales centres too could be drawn into the programme, but they will expect payments. 

Germinating seeds

Seeds kept in seeding trays or poly-bags with compost or rich soil, kept in a shade and watered will show signs of germination between 10 days to one month depending on variety. These seedlings need to be well looked after until ready for planting in a suitable locations.  

Identifying locations 

Most difficult part of the program would be finding five million locations to accept young trees. To lower Colombo temperatures, majority of plant locations need to be around Colombo. To identify locations, staff need to travel in every road, government properties, schools and discuss with landowners the possibility accepting trees in their gardens. 

Best locations would be open gardens currently devoid of trees or insufficient trees. Large trees would require 20 feet by 20 feet or 15 foot centres as a single row; but smaller trees would require closer spacings. Currently most walking paths edges are planted with small trees and gaps could be filled.


Young plant requires a proper void in the ground for the plant to get established. In a ground with good fertile topsoil, a hole of 1 foot cube is sufficient, but in harder grounds say on excavated soil would require 3x3x3 foot hole filled with fertile soil. Plants usually come in poly-bags, when carefully removed roots adhered to base is visible; carefully de-attach the roots, add little urea or NPK fertiliser to the hole and mix with soil prior to planting. With watering loosened roots will be drawn towards fertiliser and the plant will take root early. Plants require regular watering for nearly three months until plant takes root, also occasionally during a drought.  

Most important factor in tree planting is the weather at the time of planting. Tree planting in public locations get minimum care, hence need to coincide with favourable weather. Thus in Colombo trees could be planted in May or October after commencement of rains, but in the dry-zone only during early October, expecting three months of rain.

Fertilising and pruning

Young trees and trees in confined locations need regular fertilising for vigorous growth and chemical fertiliser or decomposed vegetable matter/leaves could be used. Plants require regular pruning for guided growth.

In addition, parasitic plants growing on planted trees need to be removed. Galle road centre in Katubedda – Ratmalana sector was host to large Tabubia Rosa trees that bear pink flowers; today most trees are covered with parasitic plants as Bo and Nuga, which have not been removed for years. 

Invasive plants

Meanwhile, spread of invasive plants especially in dry-zone are depriving animals of feeding matter is partly responsible for the human-elephant conflict. Visitors to South would notice low bushy grounds, devoid of large trees and leaving mostly thorny plants, a result cut-and-burn chena cultivation. The widespread encroachment of mostly thorny, fast-spreading invasive plants into shrubs has resulted elephants and other animals being deprived of food. Even the elephants in Uda Walawa National Park has not been spared, a fact raised by environmental organisations. 

Amongst the invaders are giant mimosa (maha nidikumba) brought to the country on the backs of goats as food for IPKF senior staff, Gandapana “Lantana camara” from W. Indies, Podisingho-maran or “Eupatorium” (Japan Lantana) native to Mexico, Andara (Dichrostachys cinerea) and Giant eraminiya are wide spread over the dry low country. A common factor in all above are being thorny and are not consumed by animals (except the fruit) and are fast-spreading. 

These fast expanding thorny plants are depriving food for elephants and wild animals, forcing them to encroach into farmlands. The issue is so massive could controlled only by the engagement by the armed forces. The lands so cleared could be planted with suitable local trees or developed as grasslands. 


Khaya Senegalensis, originating from West Africa (Senegal) was introduced to Sri Lanka decades ago by the Forest Department (FD) has proved to be the most effective tree for reducing atmospheric heating. The fast-growing, non-invasive tree with a large canopy, yielding excellent quality timber can be harvested for timber in mere 12 years. Currently, Khaya is planted by FD replacing harvested teak. As for seeds, the group of Khaya trees in a school garden in Ibbagamuwa can yield at least one million seeds, if all seeds are collected, could become the backbone on the tree planting programme. Established Khaya tree is drought resistant and can replace removed invasive plants in the dry-zone, if well looked after during early days. I have given details of Khaya in number of my past articles.


The project planting five million trees as envisaged by the President would require a well-coordinated effort from identification of varieties, collection of seeds, germinating and converting to young plants, selecting locations to accept young plants, proper planting and aftercare would require a coordinated effort. Planting five million trees and looking after them, would result a greener, pleasant country to live in. 

In addition, bringing by-laws encouraging house owners to plant trees in their gardens and installation of roof-top solar panels, would help to prevent further rise of Colombo temperature. But with industries and increasing numbers of vehicles emanating carbon dioxide and vehicle fumes, expecting temperatures of 50 years ago would be near impossible.

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