Deathweed

A blooming plant is worth several times a dormant one. Blooming stalks hand you seeds on top of the herb, and cutting them with a Staff of Regrowth or Axe of Regrowth roughly doubles the haul again. Harvest one that is merely grown and you get the herb with nothing to replant.

The window is night, and only on a full moon or a Blood Moon. There is an easy tell: the Zoologist becomes a werefox on exactly those nights, so if she has changed shape, every plant in the world is ready to cut.

In the wild it only grows on evil biome ground, which means hiking into the Corruption or Crimson every time the moon lines up. Seeds planted in Clay Pots or Planter Boxes grow anywhere, so a row of them beside your alchemy station removes the trip entirely and lets you cut the whole batch in one pass.