What do you mean by the loss of begonia in the wind at dusk?

At dusk in spring or early summer, the begonia flowers swaying in the wind gradually dim in the afterglow of sunset.

"buildings" and "dusk" in this sentence are used to describe time and weather. "buildings" refer to criss-crossing fields, and "dusk" refers to a short time before the sun goes down. And "the loss of begonia" refers to the way that begonia flowers gradually wither in the wind.