Fireblossom Seeds

The plant only returns seeds while it is in bloom, and that window is narrow: the hours around sunset, and only on a dry day. Rain shuts it down completely. Harvest at any other time and you get the flower with nothing to replant, so run a lap of your planter boxes at the end of the day. Using the Staff of Regrowth on wild ones pulls seeds too and leaves the plant standing.

You do not need the underworld for this. Ash blocks, an empty clay pot or a planter box all work at any depth, and the plants keep to their bloom schedule regardless. They also grow perfectly well submerged in lava, and neither the seeds nor the flowers burn up in it, so a lava-flooded planter row is a safe place to keep a crop where nothing will walk through it.