Which are good avenue trees

Which trees are good for roadsides? Trees are planted beside roads for more than one reason. One very obvious reason, at least in tropical countries like India is to provide shade. The trees that are suitable for this purpose should have good crown cover during summer months when the shade is needed most. But beyond this utilitarian aspect, trees are used for landscaping creating beautiful visual effects. Most of us like to drive through an avenue that has large trees on both sides with their canopy almost touching, creating a green tunnel visual effect. Also, some of the colorful flowering trees bloom, the roads become a treat to our eyes. Lastly, the more trees we plant, the more birds and butterflies will visit even the busiest of our city roads.

Safety first

The most important criteria of choosing species is safely. A roadside tree should be sturdy enough to withstand the gales/storms in the area. Post storm, it should remain upright and the branches also need to be intact. The second safety requirement is a straight trunk and adequate ground clearance (say, at least 10-12 feet). Shallow root system can be problematic for trees near pavement. For obvious reason, thorny plants are also unsuitable.

Son of the soil

Of late, there has been a growing advocacy towards avoiding exotic species as much as possible and preferring local (native) trees. Unlike exotic species, local species harbors local fauna and do not have chance of being invasive or weed-like. However, there is another viewpoint on this. Some authority consider unban forestry, can be cosmopolitan, just like its people. Personally I am not a fan of exotic trees, but I feel accommodating a few exotic trees should be fine, keeping the big majority native.

Size and Shape matters

The space available (which excludes obstructions like overhead wires etc.) vs. space required for an adult tree should be a basic criteria to choose tree species. For example Mast Tree (Polyalthia longifolia) growth habit is columnar, Pakur/Pilkhan (Ficus virens) is umbrella-like, widespread but not that tall. Bakul/Maulsri (Mimusops elengi) has a beautiful roundish shape but does not grow very large. So by itself alone Bakul may not be a good candidate for wide roads. Baheda (Terminalia bellirica) on the other hand is a large tree and will not fit the smaller road. The width of the pavement should also be taken into account. Otherwise the tree will grow large towards the roadside and be stunted on the pavement side. 

For ease of discussion, we can categorize the city roads narrow (single lane, say 12 feet), mid-sized (double lane, say 24 feet), and wide (multi-lane, 48 feet or more). Another classification can be whether the road has a median (only wide roads can have medians).

Track Record

It is very useful to do some research/observation on earlier planted tree in the locality, typically those have been planted over last few years. Are they thriving or just barely surviving? What sort of shape they are taking? These could be valuable input to species selection.

With this as a background discussion, let us try to evaluate some the trees for road-side planting purpose. First we will take the ones that are already in use as avenue trees in Kolkata followed by the ones that are rarely used but has good potential. After the name the first figure is height in feet followed by shape indication. Lets us group them by shape and then size.

Trees with Roundish Crown

Amaltas (Cassia fistula) 35’ Crown roundish

A slow growing mid-sized deciduous tree that bears drooping spray of bright yellow flower in summer making in one of the best looking flowering trees in the world. The tree does not look good when not in flower. Its leafless phase coincide with early summer when shade is in demand. So it is not a good avenue tree but can be used in combination (between other trees) for brightening up the roadside in summer. A, aesthetic tip is Jarul or "pride of India" can be planted along with Amaltas as they flower around same time.

Bakul (Mimusops elengi) 40' crown roundish

An excellent tree for mid-sized avenue plantation and landscaping for its perfect dome-like crown. A beautifully scented flower is a bonus.  

Neem (Azadirachta indica) 40' crown roundish

Another excellent avenue tree. Looks good and lively due to light color of its leaves. Surprisingly rare in Kolkata as avenue tree (relatively more common in private gardens). An alternated arrangement of Neem and Bakul could look beautiful because of the contrast of their leaf color.

Spanish Mahogany (Swietinia mahogani) 50’ crown roundish

A large exotic non-invasive tree from West Indies that provides excellent shade. Good for wide avenues as it require a lot of space. 

Kadamb (Neolamarckia cadamba) 60’ Crown roundish

A fast growing large tree with a straight trunk and dense canopy. Excellent for avenue plantation provided adequate space is available. The fallen ripe fruits are sometimes a bit of nuisance in urban roads.

Peepal (Ficus religiosa) 60' - crown roundish

An excellent large avenue tree that needs lot of space to grow. Good for large avenues with ample space beside the road. An alternative to Peepal, where space is a constraint is Golden Rumph's Fig (Ficus rumphii). It resembles Peepal fairly closely but is a relatively smaller tree.

Arjun (Terminalia arjuna) 60' crown roundish

A massive tree from Indian forest which is good for large avenues due to its size. Also, good for canal/water side plantation as in its natural habitat it tends to grow along watercourses.

Trees with Umbrella-Like Crown

Jarul (Lagerstroemia speciosa) 30' crown (slightly) umbrella-like 

Not a very good looking tree when it is not flowering (spectacular while flowering). Not suitable for wide roads for shade purpose due to its small size. Ideal for mid-sized avenues and medians

Indian Tulip Tree (Thespesia populnea) 35' (slightly) umbrella-like

This smallish tree is good for small to mid-sized avenues (but still requires good space)

Gulmohar (Delonix regia) 40' (slightly) umbrella-like

Though very popular avenue tree due to its beautiful scarlet flowers that grow abundantly in summer, this tree is highly susceptible to breaking/uprooting in strong winds - both branches as well as the whole tree. So it is better for planting in parks and private spaces where people are less likely to be present during stormy weather. Also the root system is shallow and hardly anything grows under this tree. When planted, it can be used alternated with Copper pod tree which bears yellow flowers almost at the same time.

Saptaparni / Devils's Tree (Alstonia scholaris) 45' crown slightly umbrella-like

An evergreen tree with fairly dense shade. Good for avenue planting. The tree is somewhat susceptible to breaking in storm. The strongly scented flowers are good from far but can be nauseating for many people from close. So pure stand should be avoided.

Wild almond Tree (Sterculia foetida) 50' crown slightly umbrella-like

This is a tree of straight growth habit and tend to start branching from quite a height. Good shade tree but flowers have unpleasant odor. So planting here and there makes sense.

Banyan (Ficus benghalensis) 50' crown umbrella-like

Not good for avenue planting mostly because it needs its aerial roots to touch the ground. It is more suitable for planing in park and around habitation.

Rain Tree (Albizzia saman) 50’ Crown umbrella-Like

A fast growing exotic tree (from tropical America) of enormous spread. Pretty good for wide avenues provided ample space is available all around. Of late a number of these tress are dying in Kolkata due to some disease or infestation. So, for now it is better to avoid planting this tree, at least in pure strand.

Tamarind (Tamarindus indica) 80' crown umbrella-like

Though originally from Africa, it has been naturalized in India for many centuries and probably millenniums. An ideal tree for large avenues that has amble of space. Two things to keep in mind, one it is a slow growing species and hardly anything grows under it (since its leaves are acidic).

Trees with Columnar Crown

Putranjiba (Putranjiva roxburghii) 45' Crown slightly columnar

An evergreen tree of handsome drooping branches with dark dense glossy leaves. Excellent roadside tree.

African Tulip Tree (Spathodea campanulata) 50' Crown slightly columnar

An exotic tree from Africa, famous for its scarlet bell like showy flower. Tall, slender and usually light canopied tree. Not that good for shade purpose but can be used with other species for ornamental purpose. Older trees stem are susceptible to rot and might fall abruptly. 

Copper Pod (Peltophorum pterocarpum) 50' slightly columnar

 Reasonably good roadside tree. Branches slightly susceptible to very strong winds. Looks lovely in flower in April-May. Fallen flowers create a lovely carpet of yellow (especially when there are many trees one after other). Looks rather somber when not in flower because of its stiff leaves and dark green foliage.

India Almond Tree (Terminalia catappa) 50' Crown slightly columnar

A tree of straight growth habit and whorled branches. Good avenue tree but has susceptibility to getting uprooted in storm. One of the very few tropical tree that shows a reddish fall color. 

Semal / Silk Cotton (Bombax ceiba) 60' Crown slightly columnar

A tall deciduous tree with whorled branches. Look stunning in early spring in large red bell like flowers. Except for the flowers, not a great avenue tree. The shade is usually rather scanty. The fallen flowers do not "gel" with vehicular traffic.

Large leaved or Honduras Mahogany (Swietinia macrophylla) 60’ Crown slightly columnar 

Another tropical American species that does not spread so much like the previous and is usually branch-less up to quite a height. A darker color leaf make this tree a bit somber looking