Fire-Resistant Trees in the Japanese Garden: What 1923 and Leaf Science Tell Us
As autumn turns to winter, the leaves come down, the air dries, and fallen leaves pile up under hedges and against the walls of houses. In Japan, trees planted partly to slow a fire are called boukaju, "fire-defence trees". An older name is hibuse no ki, "trees that hold back fire".
We are a small tree care and landscape business in Mie Prefecture, central Japan. This page explains where the idea came from, what research says about why some leaves resist flame, where the tree lists disagree, and what a home gardener can actually do.
Two refuges in 1923
On 1 September 1923, the Great Kanto Earthquake struck Tokyo and Yokohama. According to the National Archives of Japan, about 105,000 people died, and about 90% of them died in the fires that followed.
The worst single loss was at the Hifukusho site in Honjo, a cleared plot where an army clothing depot had stood. Crowds fled there with their belongings. The Cabinet Office records that sparks set the belongings alight, and strong winds and a fire whirl turned the open ground into a sea of fire. About 38,000 people died there.
Across the district, in Fukagawa, people also fled into the garden of the Iwasaki family estate, now Kiyosumi Garden. Most of them survived. The Tokyo Metropolitan Park Association puts the number at about 10,000; other accounts, including the ecologist Tsukasa Fukushima, say about 20,000. Fukushima describes what stood between the people and the fire: a large pond, a brick wall, and an earth bank planted with sudajii (Castanopsis sieboldii) and tabunoki (Machilus thunbergii), both evergreen, and ginkgo.
The garden did not come through untouched. The park association writes that most of its buildings and garden trees burned. Fukushima quotes, from an earlier book, the garden historian Matsunosuke Tatsui. The shii trees along the boundary walls were burned in places, yet a ginkgo among them was already putting out new shoots by late September. Inside, the garden was red as far as he could see, and the pines and maples had suffered worst. The trees that took the heat were the ones that held it back.
The idea is older than 1923. Fukushima notes that Japan's Yoro Code, in force from 757, already required about 150 metres around storehouses to be kept free of other buildings, so that fire could not spread to them.
Why a living leaf resists flame
Most of the answer is water. In 2016, Hisanori Hayashi of the IGES Japanese Center for International Studies in Ecology gathered leaf moisture measurements from earlier studies and his own work: 561 records for 213 species of trees and bamboo. Leaf moisture ranged from 42.9% to 88.0%, with a mean of 62.4%.
Hayashi cites experiments in which leaves began to catch fire from a touching flame once their moisture fell below about 20%, and almost all species caught fire below 10%. Fallen leaves on the forest floor behave the same way. So a healthy green leaf normally holds about three times the water at which it becomes dangerous. A dead leaf does not.
Hayashi also weighed whole plants while exposing them to fire. Japanese red pine (Pinus densiflora) began to lose weight at once and lost it quickly. Sangoju (Viburnum odoratissimum, sweet viburnum) and aoki (Aucuba japonica) lost weight slowly and kept losing it for longer. He notes that not all of the weight lost is water. His conclusion is that the best trees for fire defence hold a lot of water and keep it for a long time under heat.
Moisture is not the whole story
Hayashi is careful here. Sudajii and shirakashi (Quercus myrsinifolia), the trees credited with stopping fire in 1923, are not especially high in leaf moisture. Measurements for the same species also varied widely from study to study. He writes that leaf moisture alone cannot rate a tree as fire-resistant. The shape and density of the crown, the oils in the leaf and how it burns all matter too.
He adds one point that every gardener should read twice. Many urban trees are in poor health, or have been pruned so hard that they have far fewer leaves than they should. Such trees hold much less water overall, whatever their species.
The lists do not agree
Japanese garden books and websites give tables of fire-resistant trees. Common names include sangoju, ginkgo, aoki, masaki (Euonymus japonicus), sasanqua and other camellias, and inumaki (Podocarpus macrophyllus), a conifer often listed as an exception to the rule that conifers burn easily.
A 2025 study by researchers at Shimane University and the University of the Ryukyus tested twelve trees used in Okinawan homestead windbreaks and street plantings, looking at the moisture of branches and leaves, the heat they give off when burned, leaf volume and crown shape. It found that Diospyros ferrea, fukugi (Garcinia subelliptica), isunoki (Distylium racemosum), Calophyllum inophyllum, a Cinnamomum and sangoju showed effective fire prevention properties. Tabunoki, inumaki and three others did not, because of low moisture or high heat of combustion.
Both tabunoki and inumaki appear on many mainland lists. We do not think this makes the lists wrong; the trees, the climate and the test methods differ. But it is a good reason to treat any single table with caution, and to choose trees that are healthy and dense where you live.
A screen, or a fuse
The strongest evidence on houses comes from Australia. After the 2009 bushfires in Victoria, a team led by Philip Gibbons studied which houses were lost. The more tree and shrub cover there was within 40 metres of a house, the more likely it was to burn; this was the strongest of the fuel factors they measured. In severe fire weather, their model predicted that reducing that cover from 90% to 5% could lower house loss by an average of 43%. Planted vegetation also did better than leftover native bush: changing the vegetation near a house from mostly remnant to mostly planted could reduce losses by 38%.
That seems to point the other way from Kiyosumi, where trees stood between people and the fire. We think both are true. At Kiyosumi, a broad belt of trees on an earth bank, with a pond behind it, faced a fire coming across the town. In the Australian bush, the fire travels through the vegetation itself, and a shrub against the wall carries it to the house. Japanese guidance for parks is written for the first case: Osaka Prefecture's guide to fire-defence planting in parks says that the denser the trees, the safer each zone becomes.
A garden is small, so the lesson we take from both is placement. A healthy tree with open space between it and the house can act as a screen. A tree pressed against the eaves, with dry leaves heaped at its foot, is more like a fuse.
In the garden
What we do in practice is simple.
Rake up fallen leaves and dead twigs from under hedges and along walls, especially from late autumn into winter. This is the fuel.
Remove dead branches from inside the crown at least once a year. A dead branch is a dry leaf on a larger scale.
Do not burn prunings on site. Chip them and take them away, or compost them well away from the house.
Keep trees healthy and fully leaved rather than clipped back to bare stubs. A tree with a thin, over-pruned crown holds less water and screens less heat. Thinning pruning, which opens the crown without stripping it, keeps both the leaves and the air flow.
Where you want a green screen, use an evergreen hedge that suits your region. Keep it healthy and full, keep the base clean, and leave space between it and the walls.
No tree is a fire wall. A healthy one, with nothing dry beneath it, can buy time. In 1923, that time was the difference.
Our separate article on sasanqua, one of the trees Japanese gardeners plant for this reason, is listed in the sources below.
