De cómo los acocotli se fueron a Europa, regresaron como emperifolladas dalias y se convirtieron en la flor nacional - Este País
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Ensayo sobre una de las contribuciones de México al mundo: las dalias, un delicada belleza de nuestro catálogo vegetal

Also would recommend reading the whole story, it gets wild.

TL;DR - The aztecs used to eat the tubers, spanish took it to try that in europe, didn't work but they liked the flowers, named them after some swedish dead guy, went crazy for them (even having a small "Dahlia Craze" like with the tulips) and then they were brought back, now seen as a high-class flower in the americas because of its popularity in europe.