Q: Dear Uncle Wang,

I can always smell a sweet scent when I walk past certain trees on the street recently now it has got cooler. What is this smell? Does it just happen this year?

A: The smell comes from osmanthus fragrans, which is sweet osmanthus or sweet olive, an evergreen tree native to southern China. A unique fragrance comes from the blossoms of the trees in autumn. The fragrance is so strong that if there is a sweet osmanthus tree nearby your apartment building, chances are you will be able to smell the sweet aroma even if you live on the lower floors of a building.

Osmanthus trees can be found everywhere in Shanghai. When you can smell the fragrance it tells you that autumn has arrived. This year the fragrance is especially strong as it is the third blossoming of the osmanthus after the National Day holiday.

The first blossoming happened at the end of August and the second in late September. It is very rare for the trees to have such a long period of blossoming and this has already lasted more than two months in Shanghai.

Experts said the third blossoming occurred this year because different conditions, including a cool summer, ample moisture and moderate temperatures in autumn, have provided extra nutrients so the osmanthus trees are growing more.

The experts also said it would be hard to predict how long the blossoms would last this year because global warming has generally affected the blossoming of plants.

Because of the full and fragrant blossoming, the Shanghai Botanical Garden has used the sweet osmanthus as its theme for this year's autumn flower show. There are over 500 examples of sweet osmanthus in its osmanthus garden, which has 40 different species.

During the National Day holiday, the Shanghai Botanical Garden outlined different paths for visitors to take to learn about the trees.

The sweet osmanthus is closely connected to the Chinese Mid-Autumn Festival because of the time it blossoms. Legend has it that Wu Gang, a figure in Han Dynasty (206BC-AD220) mythology, was sentenced to cut down an osmanthus tree on the moon - a never-ending task because the tree keeps regrowing every day.

In traditional Chinese culture, the sweet osmanthus is a symbol of fortune, auspiciousness and prosperity. Some drinks and food are made using the sweet osmanthus as an ingredient.

For example, osmanthus wine is enjoyed during family reunions in the Mid-Autumn Festival. Sweet cakes made of osmanthus are also a popular traditional autumn snack in Shanghai.