Biome spread

Only three biomes grow: the Corruption, the Crimson and the Hallow. Everything else stays where world generation put it. The three work by converting tiles near their own blocks, one at a time, forever. Blocks fall into three groups worth keeping straight: ones that can be converted, ones that never can, and ones that could be but have not been yet. Wood, bricks, clay, ash grass and mushroom grass are in the never group, and that is the entire basis of every containment strategy in the game.

Spread rides on the game's tile update cycle. Each tick the game picks a handful of tiles to process, and tiles above and below the surface line are drawn from separate passes. On average a surface tile comes up about once every 140 seconds while an underground tile waits closer to 830, so the surface converts roughly six times faster than the caverns do. Those are averages over a random draw, and any individual tile can go far longer than that untouched. When an infected tile does come up, it can convert several neighbors in that one update.

Before the Wall of Flesh dies, the only tiles capable of spreading anything are infected grass and the thorny bushes growing out of it, and they reach only directly adjacent blocks. That is slow enough that a normal playthrough never notices. A Sunflower blocks conversion in the tiles around it, and the two tiles directly beneath one can never be converted under any circumstances, so a row along a farm edge is a cheap early fence.

Once Hardmode starts, infected stone, sand, hardened sand, sandstone, ice, vines, thorns and grass all become sources, and each reaches three tiles in every direction, a 7 by 7 square centered on every single infected tile. Non-corruptible blocks in between do nothing, because the conversion jumps over them. Beating Plantera once halves the rate for all three biomes for the rest of the world's life, which is the largest single thing you can do about spread short of buying a Clentaminator.

Three tiles of open air or non-corruptible block is the floor, because three tiles is the reach of a source. Near the surface that is not enough. Thorny bushes grow out of infected grass and extend well past the block they are rooted in, and vines hang down from grass the same way, so either can carry the infection over a gap that looked safe. Six tiles is the working figure for surface trenches, and lining the walls with something non-corruptible beats widening the gap.

• Dig the trench before the biome reaches anything you care about. Cleanup afterward costs far more time than fencing early does.
• Run it from the Underworld ceiling to the sky. A barrier that stops at the Cavern layer only delays the problem.
• Actuated tiles cannot spread anything, but normal tiles can still convert them, so an actuated wall is not a barrier.

Background walls have their own quieter system. Blocks can infect walls, walls can infect other walls, and a wall can never infect a block. Wall range is two tiles rather than three. Grass, Flower and Jungle walls flip to their infected version once and cannot afterward be claimed by a different biome, while Sandstone and Hardened Sand walls can be taken over again and again by whichever biome reaches them next. Any wall that has been converted, including by a Clentaminator, counts as natural from then on, which changes what is allowed to spawn in front of it.

Chlorophyte Ore is the one material that pushes the evil biomes backward. Before the Corruption or Crimson converts a block it checks for chlorophyte within ten tiles, counting ore and bricks together. Five or more and the conversion fails outright. Fewer than five gives a proportional chance to fail, so three nearby tiles means a three in five chance nothing happens.

It also works in the other direction. Chlorophyte within a short radius reverts infected grass, stone, sand and sandstone to their pure forms, strips out evil thorns and vines, and turns nearby dirt into mud. A ring of chlorophyte brick around a jungle base is a permanent barrier that needs no maintenance.

Left alone, the evil biome and the Hallow will eventually split the whole world between them. That sounds worse than it plays. Two places matter: your NPC town, because displaced or unhappy NPCs cost you pylons and prices, and the Jungle, because Life Fruit and your entire chlorophyte supply come out of it. The rest can rot. Keep a fenced pocket of the evil biome on purpose too, since its enemies drop materials you still need well into the endgame.