How to grow street trees that don’t endanger pedestrians

How to grow street trees that don’t endanger pedestrians

2024-05-26

In Taiwan’s big cities, trees can be seen almost everywhere. These trees provide shade, purify the air, and even reduce noise pollution. But street trees can also be hazards to the people around them. They’ve caused a number of issues in recent years, whether it’s by buckling sidewalks with their roots or by falling onto roads. Is it possible to have all the benefits of street trees and none of the risks? We find out in our Sunday special report.

Here in Kaohsiung, next to Chung-cheng Industrial High School, a row of roadside trees has been cut down. This stump belonged to a towering Bodhi tree and is now covered with cloth.

Looking back with Google Street View, in 2022, this Bodhi tree stood five stories tall, and three people were needed to encircle its trunk. But as the tree grew, its roots cracked the sidewalk. Putting pedestrians first, officials cut the row of trees to create a safe space for walking.

Zhuang Jie-ren
Forest City Association, Taiwan head
So, when this tree was first planted, it was still very small, so they create a tree pit of 1 square meter. Then the tree grew into a giant, so of course it burst out from that space.

Confining big trees to small shoes not only ruins the shoes, but also makes the trees unsteady, so that they can easily topple over onto people and vehicles. And these aren’t the only problems that can be caused by street trees.

Some trees drop flowers that make surfaces slippery or that emit unpleasant odors. Other trees shed leaves or fruits that can cause injuries. The question officials grapple with is whether such trees should be removed.

This issue is at play here in Xindian District’s Meitan Borough. The borough warden, Chien Ming-feng, is concerned about a group of 60-year-old breadfruit trees. They stand unsteadily under the sidewalk, and are especially hazardous in the summer when their fruits ripen.

Chien Ming-feng
Meitan borough warden
The fruit of the breadfruit tree is about this big. You can imagine what would happen if it fell from a great height.


The borough warden furnishes a stack of information on the trees. Discussions on relocating the trees began 10 years ago. Opinions were sharply divided, and 10 years later, the trees remain, with locals still lodging complaints.

Chien Ming-feng
Meitan borough warden
The main problem is that the trees endanger that path by the school. It’s a pedestrian safety problem. Navigating that path is already challenging enough for able-bodied people, let alone for someone in a wheelchair.

Despite the presence of a sidewalk, locals are forced to walk on the road and compete with passing vehicles. Why has it been so hard to relocate six breadfruit trees? What’s the case for leaving them in place?

Hsu Mei-hui is an urban planner and a resident of Meitan Borough. Today she’s measuring the cooling effect of tree shade.

In winter, tree shade can provide cooling of 3.6 degrees Celsius. In summer, the cooling effect is as much as 13 degrees. Hsu is an advocate of retaining the trees, saying they’re needed to mitigate heat waves amid climate change.

Hsu Mei-hui
Urban planner
Summer is arriving earlier these days. I remember that in 2016, in 2014, temperatures were already as high as 38 degrees Celsius in May. It’s especially bad in greater Taipei, where there’s a significant urban heat island effect.

Trees lower temperatures, reduce noise pollution, absorb carbon dioxide, and improve air flow to disperse air pollution. They also play an important role in pedestrian and vehicle safety.

Hsu Mei-hui
Urban planner
Look at the median strips of roads. They are mainly composed of shrubs, and that’s a safety consideration. They reduce the headlight glare of oncoming vehicles. Trees can also serve as a divider. On sidewalks, because people walk slower, large trees that provide shade are used.

Street trees enhance safety by separating pedestrians from vehicles. Their green leaves can reduce eye fatigue for drivers and provide a calming oasis for urban dwellers.

Shen Shu-chiao
Forestry and Nature Conservation Agency official
The planting design is multilayered, for instance, there are large trees at the top, followed by shrubs, and then flowers or grass at the bottom. This creates a rich visual effect that is more pleasing to the eye. The other thing is that this design attracts insects, butterflies, bees, and birds, creating an unfiltered and accessible natural landscape for the public.

Zhuang Jie-ren
Forest City Association, Taiwan head
If you look at it from a variety of angles, you’ll find that these trees can help to lower health insurance costs. They can save us the cost of building power plants, of installing infrastructure like air purifiers and air conditioning. You’ll find that trees are very cost-effective utilities.

But while street trees offer immense value, they can also endanger the people around them, by falling over, destroying sidewalks, or otherwise causing injuries. Is it possible to have all the benefit of trees without any of the risk?

Deep in the mountains of Yilan, a government-run nursery focuses on trees most suited for the urban environment.

These verdant saplings are trimmed routinely so that they thrive. They are all species indigenous to Taiwan.

Shen Shu-chiao
Forestry and Nature Conservation Agency official
These trees are naturally from Taiwan. They have some unique characteristics and are already well-adapted to the environment here.

The Forestry and Nature Conservation Agency selected 110 native species that are most suited for urban planting. The goal is to preserve the many benefits of street trees bring, while eliminating issues like shallow roots or unpleasant flower odors.

Shen Shu-chiao
Forestry and Nature Conservation Agency official
Currently we’re focused on promoting trees with more deeply rooted systems, such as the Formosan gum, Subcostate crape myrtle, Taiwanese cheesewood, and Formosan ash. These species have been well-received.

But the plain look of native trees can’t compete with more eye-catching, trendier species such as the cherry tree, pink trumpet tree, or the giant crepe-myrtle.

Pan Han-chiang
Association of Taiwan Tree-Huggers secretary-general
These trees are used mainly due to public preference, and due to PR from the horticultural industry. Take cherry trees, today’s most popular tree. They are more suited for viewing in gardens or parks, as they’re in bloom only two to four weeks out of a year. As street trees, they’re very poor at providing shade.

Zhuang Jie-ren
Forest City Association, Taiwan head
When it comes to the street trees we have today, there’s the bald cypress, which damages sidewalks with its roots. The pink trumpet tree’s branches snap easily and it has an unstable root system, which leads to toppling. Twenty years after planting these trees, we realized that they had problems. And we are not assessing the trees we plant today, so we’re forever going to be discovering problems 20 years too late.

The Forestry and Nature Conservation Agency provides native saplings free of charge for public use, or to charitable organizations. But good urban planting design isn’t only about using native species. It’s also about planting the right tree in the right place.

Zhuang Jie-ren
Forest City Association, Taiwan head
For example, if a sidewalk is not very wide, you should plant a medium-sized tree. If there’s quite a bit of room on a sidewalk and you can have a large tree pit, go for a big tree. These considerations must be included in the planning stage.

After planting the right tree, proper pruning is essential. Completely removing the top of a tree can weaken its structure, making it prone to toppling. As the tree grows, more room must be made under a sidewalk to allow roots to expand.

Pan Han-chiang
Association of Taiwan Tree-Huggers secretary-general
New Taipei City’s tree protection ordinance had stipulated that tree pits should measure 1 square meter. The interior ministry increased that to 1.5 square meters. This allows trees to grow properly. If there’s a sidewalk, you could consider using an elongated nature strip. An elongated nature strip provides enough space, enough soil so that the roots can absorb water and nutrients.

Zhuang Jie-ren
Forest City Association, Taiwan head
The other thing is, it’s good for the roots to grow deeper. Actually the best typhoon defense is improving the trees’ habitat, not pruning the trees. If a tree is stable, you don’t need to fear high winds from a typhoon. A tree topples because of issues with its roots, not its branches.

In Xindian’s Meitan Borough, the New Taipei City Maintenance Office offered two plans for dealing with the six breadfruit trees. The first was tree relocation. The second was working around the trees to create more space for pedestrians. This plan earned the city rare applause from conservationists.

Hsu Mei-hui
Urban planner
The city government’s plan is very good. It will slightly reduce the size of these scooter parking spots, moving them away from the six old trees. But the parking spots will be preserved. The scooter spot will measure 1.9 meters and it will allow the sidewalk to be widened, so that wheelchairs and strollers can be used on it, too.

Chien Ming-feng
Meitan borough warden
The trees are not being removed. The tree pits are being expanded, ensuring the trees’ right to live and thrive. As a borough warden, I have fulfilled my duty to public safety because they’re going to widen the sidewalk. When it comes to sidewalk accessibility for locals, students going to school, and people with disabilities, the Maintenance Office has done what it should.

With every tree, flowering and fruiting are part of the seasons of life. The expansion of the root system is integral to its growth. Even so, humans need not live in conflict with urban trees, but can learn how to coexist in peace.

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