Why were peonies sought after in the Tang Dynasty?

Wu Zetian once made a poem urging flowers to bloom, ordering a hundred flowers to bloom.

When I visit the upper garden tomorrow, I will report the spring at the speed of fire. The flowers must be sent out overnight, do not wait for the dawn wind to blow.

After Wu Zetian ascended to the throne, she wrote this poem on a cold winter's day when she was suddenly in the mood to enjoy flowers. The next day, all the flowers bloomed. Only the peony was slow to bloom. Wu Zetian was furious and ordered her palace staff to roast the flower branches with charcoal, and only then did the peony blossoms open. Wu Zetian was still angry. She ordered all the peonies in the imperial garden to be moved out of Chang'an to Luoyang as a punishment. Later, Luoyang was called the home of peonies.