Before that Britain had only 1,500 native plants, according to the documentary's producers.

Huang, who is in his 40s and made the documentary over three years, says he believes Forrest's collection opened up British knowledge about plants in China, particularly rhododendrons, and helped British botanists to take the lead in research.

When Huang was shooting an episode of the series in Europe, his colleague, Han Zhen, director of the second episode, was trekking with his team to Karma Valley on the eastern slopes of Qomolangma, or Mount Everest, in 2018.

"It was in the middle of June, when most rhododendrons (in warmer areas) were bidding farewell to their flowering phase. The rhododendrons in the Tibetan highlands had just started to bloom," Han says about the trip.

It was the most expensive shoot. Han had 13 yaks transporting tents and supplies for the team's 10-plus members.

In nine days, they hiked nearly 120 kilometers in the valley, climbing several peaks every day, with the highest altitude reaching around 5,300 meters.

"Every morning after breakfast, we embarked on the trek, taking with us simple food like biscuits and eggs," Han recalls.