杨柳絮
willow and poplar catkins

4月5日，北京市园林绿化局和北京市气象局联合发布了2021年第一期北京杨柳飞絮预报。

As the week begins, the capital's downtown and southern parts will see willow and poplar catkins wafting through the air, and the city's northeastern areas will start to face the problem a bit later, between Wednesday and Saturday, according to the forecast.
预报显示，自4月5日起，北京中心城区和南部城区率先进入杨柳飞絮期，城区东北部稍晚，于4月7日至10日开始飞絮。