A parent's sacrifice is exactly 'fallen petals are not heartless things — they become spring soil to shelter the flowers.'
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history
The closing line of Gong Zizhen's (龚自珍) late-Qing poem 《己亥杂诗·其五》 (Miscellaneous Poems of 1839, no. 5): 浩荡离愁白日斜,吟鞭东指即天涯。落红不是无情物,化作春泥更护花。 — 'Vast parting sorrow under the slanting sun; my whip points east toward the edge of the sky. Fallen petals are not without feeling — they turn into spring soil and shelter still more flowers.' Written when Gong resigned from office and left Beijing.
usage
Always paired with the preceding line 落红不是无情物. The standard poetic compliment to teachers, parents, and retired officials who 'nurture the next generation.' Extremely common on Teachers' Day cards.