Everybody will be happy that the first highway in Sri Lanka from Colombo to Matara is nearly completed and will be ready for use in July this year. It is time to draw up a landscape plan where by planting trees and shrubs the highway road system would be connected with the surrounding environment.
Ideally every road should be of aesthetic value. By location and design, a road should be a pleasure to drive on. Everybody likes to drive or walk along a road planted with flowering and shade trees interspersed with flowering or foliage shrubs. The Colombo - Matara highway is laid out in the low country wet zone so planting material should be selected to suit this climatic condition.
Trees should be hardy, long living and of medium size. A large tree, like the rain tree (Samanea saman), is not suitable for planting along highways as its crown spreads out over a large area blocking the view. Dense shade keeps the road and pavement wet for a considerable period. Trees should also not have frequent leaf fall, and breaking of twigs and branches. Fruit trees are not suitable for street planting in highways.
|Chiang-Mai, Thailand: Tree planting at access road to highway
On both sides of the road along soft shoulders, medium sized trees can be planted. Indigenous tree species should be selected as far as possible. Planting mixed avenues of trees is not recommended. One species of tree should be planted along both sides of the road for a considerable length for an avenue composed of a single species exhibits a closer harmony in form, texture, colour and perennial habit, such as the time of leaf fall and flowering season. Trees of a single species give a certain character to an avenue.
Groups or clusters of trees can be planted in wider shoulders to have a natural planting scheme. Some of the suitable trees for planting in soft shoulders are Adenanthera pavonina,Alstonia scholaris, Azadiracta indica, Barringtonia acutangula,Barringtonia racemosa, Bauhinia racemosa,Bauhinia blakeana, Cananga odorata, Cassia roxburghii, Cordia sebestana, Doona zeylanica, Humboldtia laurifolia & Lannea coromandelica, Lysidice rhodestegia, Mesua ferrea, Memusops elengi, Lagerstroemia speciosa, Myroxylon balsamum, Phyllanthus indicus, Pongamia pinnata, and thespesia populnea. In between trees, perennial flowering and foliage shrubs such as Allamanda species, free flowering Bougainvilleas, (Caesalpinia, Brunfelsia calycina (fragrant), Brunfelsia americana (fragrant), Cassia biflora, Cassia fruticosa, Hibiscus species, Hamelia, Memecylon umballatum, Jatropha panduraefolia, Kopsia fruticosa, Murraya paniculata, Pachystachys lutea, Tecoma varieties, Thunbergia erecta, Ixora (semi dwarf), Wrightia religiosa (fragrant) etc can be planted, also foliage shrubs such as Acalypha, Graptophllum, Polycias, Pseuderanthemum. Single variety of shrubs should be planted in two rows among trees.
Necessary action should be taken to protect the slopes with a suitable type of grass. Filled slopes should be levelled, firmed and grass sods fixed with the help of pegs. The best type of grass for the low country wet zone is Axonopus compressus (Buffalo grass). This grass will cover the earth and control erosion. Recently introduced Zoysia matrella and Paspalum vaginatum grass types also thrive well in this area. These two grass varieties grow as a carpet preventing erosion, and need less frequent mowing, unlike other grass types.
A large quantity of grass sods is required for these highway projects, and hence it is easy to establish turf by sowing suitable type of grass seeds. Permission should be obtained from the Department of Agriculture to import grass seeds. Lawns on slopes should be mowed by bush cutters at least once in 2 - 3 months depending on the weather condition.
During rains, cut slopes can wash away. If the existing soil is suitable, turfing can be done. If the slopes are hard and gravelly with rocks, creeping plants can be planted in soil filled pockets. Creeping plants like Thunbergia, creeping Lantana, Macfadyena unguiscati, Ficus pumula, Antigonon leptopus, Mansoa hymenae can be planted to cover rocky areas and retaining walls. Dwarf flowering shrubs like dwarf Ixora, Allamanda (yellow), Vinca rosea, Asystasia, Crossandra, Lantana camara, and Plumbago auriculata also can be planted in groups on rocky areas. They can be mixed with dwarf foliage shrubs to look like a rock garden.
There are 11 access roads to the main highway. Flowering trees of different colours can be planted at each entry point and at interchanges. One variety of flowering tree can be planted over one kilometre before and after the entry point, along both sides of the highway. Planting one variety of flowering tree at each entry point will make it a landmark to road users. Planting similar colour shrubs in between trees will show uniformity of landscape design and enhance the aesthetic value.
The toll plaza should present an open view for the drivers to see from a distance. Shade trees should be planted on both sides of the highway opening a vista towards the toll plaza. Flowering shrubs like Bougainvilleas can be planted in between trees.
Different colours of free flowering Bougainvilleas are available in nurseries. They can be trained like an umbrella after periodical pruning of water shoots. Travellers can identify the toll plaza from a distance with its mass of flowering Bougainvilleas.
The functional part of planting in the median strip is to provide a screen against glare at night caused by on coming traffic. The height of the screen depends on the degree of screening that is expected. On a flat road, in an express way 1.5 m is sufficent to cut off the light. Foliage shrubs like Acalypha godseffiana heterophylla, Duranta (yellow) Excoecaria, Hamelia, Malphighia, Phyllanthus myrtifoliu, Syzygium campanulatum,(young leaves red) are suitable for median strip planting. Small leaved plants are suitable for screening as they show a compact growth. In widths of less than one metre, planting shrubs should be avoided; instead turfing is recommended.
Along roads that permit high speeds like the Matara highway, trees should be planted at least 4 metres away from the edge of the carriage way. Along secondary roads planting should be at least 2 -3 metres away from the carriage way.
The best way is to grow plants in large containers such as large poly bags, plastic and wooden containers. These plants are stronger than root balled trees.
Maintenance of landscape work and trees should be by landscape supervisors attached to each Divisional Council along the highway project.
These supervisors can be selected among trainees of the Royal Botanic Gardens Peradeniya. Landscaping of highway projects of Singapore, Malaysia and Thailand are supervised by well trained landscape officers. This way the lawns and trees of highways can be maintained in an excellent manner throughout the year.