The rapeseed (canola) flowering season at Wuyuan, running from roughly late February through early April depending on altitude and year, is the event that put the region on China's domestic tourism map. The yellow flowers fill the valley floors between the villages and the surrounding hills, creating a landscape that photographs extraordinarily well and looks just as striking in person. The peak usually falls in mid-March at lower elevations and two to three weeks later in higher valleys.
The practical reality of visiting during peak season is that the crowds are enormous by Chinese standards. Tour buses fill the parking areas from 8am; some key viewpoints are so crowded that the experience requires patience and early starts. If you're coming specifically for the rapeseed, a weekday arrival and an early morning start each day significantly improves the experience. The off-season visiting option — with fewer crowds and the chance to see the villages at their daily rhythms — has its own appeal.