Retoñar means 'to sprout again' or 'to re-sprout', specifically referring to plants that grow new shoots or leaves after being cut back or after a period of dormancy.
In "Flor Sin Retoño", this word is used metaphorically to describe a love that has withered and died. The singer laments that his 'flower' (representing his love) "ya no quiso retoñar" (no longer wanted to sprout again), signifying that the love is beyond revival. It's a poignant and evocative word that perfectly captures the song's theme of lost love.
Pedro Infante's "Flor sin retoño" is a beautiful, melancholic song about a love that just wouldn't grow. He tells us he "Sembré una flor sin querer" [I planted a flower without wanting to], almost by accident, just to see if it was serious. But after only three days of not watering it, he returned to find it "seca, ya no quiso retoñar" [dry, it didn't want to sprout anymore]. It's a sad metaphor for a relationship that withered away so quickly.
He really tried to keep that flower alive, saying "Yo la regaba con agua que cae del cielo y la regaba con lágrimas de mis ojos" [I watered it with water that falls from the sky and I watered it with tears from my eyes]. His friends, seeing his heartbreak, told him, "Ya no riegues esa flor, esa flor ya no retoña, tiene muerto el corazón" [Don't water that flower anymore, that flower won't sprout anymore, its heart is dead]. It's a blunt but honest assessment of a lost cause, and you can really feel the pain in his voice as he sings about this love that, like the flower, just couldn't come back to life.