Under what circumstances is folding willows a custom?

The ancients had the custom of parting and giving gifts. "Liu" and "Liu" are homophonic and presented to Liu as a souvenir. In the Northern Dynasty Yuefu's "Blowing the Drum Horn Across", there is "Folding Yang Liuzhi", and the lyrics say: "If you don't catch the whip when you get on the horse, you will hit Yang Liuzhi instead. Dismount and play the flute, worrying about killing travelers. " It is mentioned in the song that pedestrians break willows when they walk. This kind of farewell wind was very popular in the Tang Dynasty. Thus, there is a close relationship between willow and parting.