Fireblossom

It grows wild on Ash Blocks and ash grass, which puts it in the Underworld and essentially nowhere else. You can also plant it yourself in a Clay Pot, a planter box, or straight into suitable soil, and a Herb Bag will occasionally hand you the herb or the seeds without a trip down there at all.

It only opens for a few hours around sunset, and the plant tells you plainly when it happens: it brightens and throws off fire particles. Harvest inside that window and you get seeds along with the herb, which is the only way a plot restocks itself. Harvest outside it and you get the herb alone, leaving an empty pot until you replant. Rain blocks the bloom, though only where rain actually reaches, which is one more reason to keep the farm deep.

Growth is far quicker in the Underworld than anywhere else, so a row of planter boxes on a safe platform down there beats any surface plot by a wide margin. Bring a Staff of Regrowth or an Axe of Regrowth for the harvest pass. Both multiply what a blooming plant gives you, herb and seed alike, and over a few cycles that is the difference between a farm that grows and one that slowly dies out.

In decent light it glows and shows the color you expect. In the dark it dims to a faint green that is easy to walk straight past, so sweep for wild plants during the bloom window rather than at midnight. Journey Mode players can skip researching every herb separately by researching one Garland, duplicating it, and breaking it back down in shimmer.