Ash Grass Seeds

The Dryad will only offer these while she is standing in the Underworld, so you either build her a hell house and move her in or walk her down for a single shopping trip. You can also strip the loose ash foliage that already grows down there and collect seeds that way, which costs nothing but time.

Planted ash grass spreads across ash blocks and lets ash trees grow, which is the only supply of Ash Wood in the game. The grass and the plants on it glow faintly, enough that a hell base built on a planted strip needs fewer torches. Remix worlds skip the whole errand since you start down there already.