Waterleaf

Rain is the whole story. It only reaches its blooming state during a storm, and it takes longer to get there than any other herb, so a patch produces nothing for long stretches and then everything at once. Harvest while the weather holds or you get the plant and no seeds at all.

Sand is the other requirement, so planter boxes and clay pots need to be filled with sand rather than dirt. Anywhere works except the Ocean, where it flatly refuses. A garden at home beats wandering the desert, since you can be standing in it the moment the sky turns.

Harvest with the Staff of Regrowth or the Axe of Regrowth. The seed yield jumps enough that it is the difference between a patch that sustains itself and one you keep replanting by hand.

Obsidian Skin Potions are the main draw, which is why most players notice how slow this herb is at exactly the moment they want to go mine Hellstone. Crate Potions and Luck Potions want it too. In Journey Mode you can skip researching every herb one at a time by researching a Garland, duplicating it, and shimmering the copies apart.