There is a poem "How to get grass and talk about the back of a tree" in the Book of Songs Feng Wei Bosi. According to the interpretation of this poem by scholars in the Han Dynasty, "Grass forgets; Back, Beitang also. " In other words, weeds are a kind of girl who has no worries. If planted in the place where her mother lives, it can make people forget their worries. Therefore, later generations referred to mother as "Xuantang" or "Xuantang" for short.
The "back" here refers to the North Hall. "Tree" is a verb, which means planting.
How to get grass (day lily), say the back of the tree (species). -"Shi Feng Wei Bo Xi"