Green rice balls are available all year round, but their demand spikes a few weeks ahead of the annual Qingming Festival. photo by gao erqiang / china daily

According to Liu, an average of 50,000 rice balls, priced at 4 yuan ($0.6) a piece, are sold every day at Wang JiaSha before the peak period that falls on the week of Qingming. Sales numbers often triple during the peak. Liu expects to sell up to 2.4 million balls this year, a 10 percent increase from last year.

“When you can have tomatoes and celeries all year round, it is only natural that people flock toward something that is seasonal,” said Liu.

Indeed, it is only during this two-month period every year that the most authentic and best-tasting green rice balls are made. Apart from sticky rice and red bean paste, the most important ingredient used is the mugwort, an aromatic herb that has been used to treat asthma, inflammation and viral infections.

The leaves and buds of the mugwort plant, which are best picked right after the arrival of spring, are smashed and turned into juice that is used to give the rice balls their distinctive color.

While the rice ball filled with sweet red bean paste is the most traditional and popular one, other types have emerged over the years and people can now also buy alternatives filled with Chinese herbs and tofu, salted egg yolk as well as sesame.

In 2015, the skill of making the snack was listed as one of the 41 new Shanghai Intangible Cultural Heritages by the municipal government, alongside other crafts such as paper cutting and comic painting.