Two of the cameramen experienced altitude sickness－they even tied scarves around their heads to ease the pain.
Han vomited several times and could barely walk on his own on the last day of the trek.
But the arduous journey was worthwhile. They shot the meconopsis tibetica, one of the world's most exotic flowers, in the valley.
Han is now staying home because of the COVID-19 pandemic.
He says he has spare time to plant rhododendron seeds given to him by Chinese American horticulturalist Shen Yinchun, son of the late Shen Yuanru, known in China as the "king of orchids", who imported exotic breeds from Japan and Belgium to cultivate hybrid flowers in his garden in Wuxi, Jiangsu province, in the 1920s.
"Shen Yinchun is in his 80s. He lives alone in the United States, with flowers as his only company. Every day, he waters the plants and records the growth in his garden," says Han.
"Sometimes, he places a blanket on the ground, lies on it and takes photos of saplings, even those less than 10 centimeters tall."
The Signature of Flowers marks Han's first attempt at directing a documentary on plants, and it has changed his view of the world.
"Gardening is a way to understand time. You have to slow down to taste the beauty of life and nature," Han adds.
