Do You Love or Hate the Scent of Kinmokusei? Why Japan's Autumn Osmanthus Keeps Some Insects Away
URLスラッグ: kinmokusei-osmanthus-scent-insects
At this time of year in Japan, the scent of kinmokusei drifts toward you from far away. Kinmokusei is fragrant orange osmanthus, Osmanthus fragrans. Do you love that scent, or do you hate it?
It is a popular fragrance, and I often meet people who say they adore kinmokusei. I also meet people who say they can't stand the smell and that it makes them feel sick.
When you notice that sweet scent on the wind, you think, ah, it's kinmokusei season, and the short season touches your heart. Yet a certain number of people react against how sugary the scent is, and how it arrives everywhere with no mercy.
For people who dislike it, the smell can be very strong. They say it smells like air freshener or like school paste, that it gives them a headache, or that it is hard to bear as perfume in a small room.
In nature, too, the scent has something unusual about it. One of its components, gamma-decalactone, is said to keep certain insects away.
We tend to think plants always want to attract insects for pollination. But this compound repels the small white butterfly. It is said that only a few kinds of flies visit the flowers. In other words, kinmokusei may be choosing its insects through its scent.
The cabbage white butterfly never came to the flowers
The relationship between this scent and insects was studied by Hisashi Omura of Hiroshima University (Comparative Physiology and Biochemistry, vol. 23, 2006).
In a preliminary experiment, cabbage white butterflies often visited yellow artificial flowers scented with flower odors they like. But they never once visited real kinmokusei flowers.
So the scent was extracted from the flowers, separated into its components, and each was tested one by one. A component that suppressed the butterflies' attempts to feed on nectar was found. It was gamma-decalactone. When it was applied to artificial flowers, the number of butterfly visits clearly dropped.
The paper, however, says the ecological meaning of this scent is still unclear. Is it keeping certain insects away so the plant can choose its pollinators? Or, in its native range, is there a pollinator strongly drawn to this scent? The answer is not known yet.
Kinmokusei in Japan does not bear fruit
The same paper contains another sentence that is easy to miss. Kinmokusei has separate male and female trees, and in Japan only male trees are planted.
A plant page from Tokyo Gakugei University says that the trees brought from China in the Edo period were male trees that flowered well, and that since then they have been propagated by layering, not from seed. There may be traces of a pistil in the flower, but it does not go as far as setting fruit.
Even if the scent is choosing its pollinators, in Japanese gardens there is no partner for it to set fruit with.
NHK Publishing's gardening guide also notes that when people say their kinmokusei bore fruit, it is usually ginmokusei, the white-flowered osmanthus.
To enjoy the scent, thin the branches instead of shearing
With kinmokusei, it is hard to tell which branches will flower.
An old pruning book, Jumoku no Sentei to Seishi (Pruning and Training Trees), says that because the flower buds form before blooming, it is difficult to prune while keeping the flowers. The NHK gardening guide also says that hard pruning results in losing the branches that would flower.
On our job sites, we do not shear the surface into a round shape. Shearing cuts off the flower buds that are already prepared. Instead, we remove crowded branches at their base and let wind and light pass through.
In gardens where we look after the hedge only, we thin the kinmokusei lightly so that it holds for three years. This puts less strain on the tree and has less effect on the flowers than pruning hard every year.
Sunlight also matters for flowering. The NHK guide says the tree tolerates some shade, but the sunnier the spot, the better it flowers. When thinning lets light into the inside of the tree, more branches can carry flowers.
Kinmokusei at a glance
Kinmokusei (fragrant orange osmanthus): family Oleaceae, genus Osmanthus. Native to China.
Flowers from late September to mid October in Japan. A tall tree of about 5 to 6 meters. An evergreen that grows even in shade and suits hedges. Popular as an easy garden tree for beginners. It bears fragrant orange flowers.
Frequently asked questions
The kinmokusei in my garden never bears fruit.
The kinmokusei planted in Japan are male trees, so they do not bear fruit. If you see a tree with fruit, it is often ginmokusei, the white-flowered osmanthus.
There were fewer flowers this year.
Think back to whether the surface was sheared from summer to autumn of the previous year. The flower buds form before blooming, and shearing removes them. Flowering also drops in places without enough sun.
Does kinmokusei keep insects away?
For the cabbage white butterfly, it has been confirmed that a component of the scent makes the butterflies avoid the flowers. But it does not work on every insect. Please welcome it not as an insect repellent, but as a tree for enjoying the scent of autumn.
