Grasses

Grass is a surface layer that sits on top of dirt, mud, sand, snow or stone depending on the type, and it is the gate on almost every plant in the game. An acorn only becomes a tree if the block under it is grass of a type that accepts one, wild flowers and weeds only appear on grass, vines only hang from grass, and the tree you get is chosen by the grass rather than by the biome you stand in. Building any farm starts with laying the right grass in the right place.

Every natural type has a matching seed you can plant: Grass Seeds, Jungle Grass Seeds, Corrupt Seeds, Crimson Seeds, Hallowed Seeds, Mushroom Grass Seeds and Ash Grass Seeds. Plain grass has a shortcut: the Staff of Regrowth and later the Axe of Regrowth place it directly, which is far quicker than seeding a long strip by hand.

Grass only moves one tile at a time, into any of the eight blocks touching an existing grass tile, and only if the target is the right soil. It also needs an opening: a block sealed on all eight sides by solid blocks, platforms or planter boxes will never take grass. Lava in any adjacent tile blocks it outright.

Depth matters for some types. Plain grass, and both mowed varieties, will not spread below the surface at all. You need to be at 4 feet or higher on the Depth Meter for it to move underneath you. Jungle, mushroom and ash grass have no such limit, which is why an underground jungle keeps creeping and an underground lawn does not.

Both Dirt Blocks and Mud Blocks accept the evil grasses, and the result depends on which one it landed on rather than which one it came from. Corrupt jungle grass reaching a dirt block produces plain corrupt grass; plain corrupt grass reaching a mud block produces corrupt jungle grass. The infection moves between the two soils in both directions without ever stalling, which is exactly why a Jungle sitting next to a Hardmode stripe disappears so fast.

Thorny bushes are the part people forget when they dig a quarantine trench. A Corruption or Crimson bush grows six tiles in every direction and creates its own biome grass on adjacent dirt and mud, so a bush at the lip of a narrow trench can seed the far wall without any block ever touching another. Make trenches wider than you think you need, or line them with something the spread cannot use.

Grass absorbs the first pickaxe hit on any block it covers, so mining a grassy tile always takes one extra swing. That inverts the usual rule about tools: through grass, speed beats power outright, and a fast weak pickaxe out-digs a slow strong one every time. Pre-Hardmode that makes the Bone Pickaxe the fastest thing you can hold, and in Hardmode the Shroomite Digging Claw. It matters most in the Underground Jungle, where practically every block you touch is covered in Jungle grass.

The Lawn Mower converts grass into a mowed variant that keeps the biome but refuses to grow wild plants, which is the point of it. Mowed grass still spreads to adjacent dirt above the surface, but whatever it creates comes out unmowed, so a lawn quietly grows a fringe you have to go back over.

Painted grass passes its color to anything growing on it, and it passes the paint along as it spreads. Paint the block under an acorn and the tree comes up painted too.

A background wall behind the soil tile or the tile above it stops most plants from growing. The grass, jungle, flower and mushroom walls are exempt, and so is every fence, which is the loophole that makes indoor farms work. Put a Wooden Fence across the whole back of a room and it counts as full wall coverage for spawn suppression while plants carry on as normal.

If you want the flowers but not the critters bursting out of them, the Flower Boots sharply cut the rate at which broken flowers spawn one.