Biome spread

For a guide on how to halt biome spread, see Guide:Maintaining world purity.

This article is about natural biome spread. For means of manually spreading biomes, see Clentaminator and Terraformer.

The evil biomes and the Hallow, unlike all other biomes, can spread to other areas of a world. They slowly convert, or "infect", certain tiles which are near their own biome-exclusive tiles to their respective biome counterparts, increasing the size of the biome. The exact rules for biome spread are slightly different for each of the three infectious biomes, as described in depth below.

Collectively, these three biomes can be called the "infectious biomes". Blocks and walls which are vulnerable to infection are called "corruptible", and those which cannot be infected under any circumstances are "non-corruptible". Blocks and walls which could be infected, but have not been, are called "pure".

Biome spread starts immediately upon world generation, is greatly enhanced and sped up after defeating the Wall of Flesh for the first time (after initiating Hardmode), and is slowed down again after defeating Plantera for the first time. Once Hardmode begins, the world's evil biome and the Hallow have few restrictions on their spread, and without player effort, will eventually take over almost the entire world between them.

The biome spread process is part of the game's "tile update cycle": Every game tick, the game chooses a handful of tiles to update, which covers several processes including biome spread. The tiles above and below the surface level are selected in separate passes; as an approximation, "surface" tiles will each average about one update every 140 seconds, while "underground" tiles each average about one update every 830 seconds. (That is, above-ground tiles get updated about six times as often as below-ground tiles, so biome spread will be that much faster above ground.) Due to the random selection method, even the averages here are approximate, and the actual length of time between a given tile's updates can vary wildly. When an infected tile is updated, it is considered as a potential "source tile", which can infect multiple other tiles in a single update.

Pre-Hardmode spread

In pre-Hardmode, the three biomes can only spread in a very limited way: The only spreading tiles are Corrupt, Corrupt Jungle, Crimson, Crimson Jungle, Hallowed, and Mowed Hallowed grass(Desktop, Console and Mobile versions), and Corruption and Crimson thorny bushes. They can only spread to directly adjacent blocks. See below for the details of grass spread.

Even though the Hallow is normally not present in a pre-Hardmode world, Hallowed grass and Mowed Hallowed grass does spread if artificially planted with seeds brought over from a Hardmode world.

Hardmode spread

In Hardmode, the pre-Hardmode grass spreading remains unchanged. However, many more tiles become corruptible, and many more tiles will be able to convert other tiles.

The following blocks and plants of a biome are capable of converting corruptible tiles in their vicinity. Blocks and plants can convert blocks and walls, while walls can only convert other walls (see below for the latter case). None of the three biomes can convert each other's blocks, with the exception of grass (see below for details).

Spreading tiles

Corruption	Crimson	Hallow
Corrupt grassInternal Tile ID: 23	Crimson grassInternal Tile ID: 199	Hallowed grassInternal Tile ID: 109
Corrupt Jungle grassInternal Tile ID: 661	Crimson Jungle grassInternal Tile ID: 662	Mowed Hallowed grass(Desktop, Console and Mobile versions)Internal Tile ID: 492
Ebonstone BlockInternal Tile ID: 25	Crimstone BlockInternal Tile ID: 203	Pearlstone BlockInternal Tile ID: 117
	Wild Crimson plantsInternal Tile ID: 201	Wild Hallowed plantsInternal Tile ID: 110, 113
	Vicious Mushroom(Desktop, Console and Mobile versions)Internal Tile ID: 201
Corruption thorny bushesInternal Tile ID: 32	Crimson thorny bushesInternal Tile ID: 352
Corrupt vinesInternal Tile ID: 636	Crimson vinesInternal Tile ID: 205	Hallowed vinesInternal Tile ID: 115
Purple Ice BlockInternal Tile ID: 163	Red Ice BlockInternal Tile ID: 200	Pink Ice BlockInternal Tile ID: 164
Ebonsand BlockInternal Tile ID: 112	Crimsand BlockInternal Tile ID: 234	Pearlsand BlockInternal Tile ID: 116
Ebonsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 400	Crimsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 401	Pearlsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 403
Hardened Ebonsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 398	Hardened Crimsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 399	Hardened Pearlsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 402

Note the inconsistency: While wild Crimson plants and Vicious Mushrooms are able to act as source tiles, their Corruption equivalents, wild Corrupt plants and Vile Mushrooms, are not.

Conversions can occur as outlined in the table below. Read the table in the following way: Any of the spreading tiles can convert any of the pure tiles that are within a 7×7 square centered on the spreading tile (i.e. within 3 tiles of the spreading tile) to the respective biome's converted tile. For example, a Red Ice Block (spreading tile) can convert any Jungle thorny bush tile (pure tile) that is within 3 tiles to a Crimson thorny bush (converted tile).

Hardmode spread overview

Pure tile	Converted Corrupt tile	Converted Crimson tile	Converted Hallowed tile
GrassInternal Tile ID: 2	Corrupt grassInternal Tile ID: 23	Crimson grassInternal Tile ID: 199	Hallowed grassInternal Tile ID: 109
Mowed grass(Desktop, Console and Mobile versions)Internal Tile ID: 477	n/a	n/a	Mowed Hallowed grass(Desktop, Console and Mobile versions)Internal Tile ID: 492
Stone BlockInternal Tile ID: 1	Ebonstone BlockInternal Tile ID: 25	Crimstone BlockInternal Tile ID: 203	Pearlstone BlockInternal Tile ID: 117
Mud BlockInternal Tile ID: 59	Dirt BlockInternal Tile ID: 0 (Old-gen console and 3DS versions)	Dirt BlockInternal Tile ID: 0 (Old-gen console and 3DS versions)	n/a
Ice BlockInternal Tile ID: 161	Purple Ice BlockInternal Tile ID: 163	Red Ice BlockInternal Tile ID: 200	Pink Ice BlockInternal Tile ID: 164
Sand BlockInternal Tile ID: 53	Ebonsand BlockInternal Tile ID: 112	Crimsand BlockInternal Tile ID: 234	Pearlsand BlockInternal Tile ID: 116
Hardened Sand Block(Desktop, Console and Mobile versions)Internal Tile ID: 397	Hardened Ebonsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 398	Hardened Crimsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 399	Hardened Pearlsand Block(Desktop, Console and Mobile versions)Internal Tile ID: 402
Sandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 396	Ebonsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 400	Crimsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 401	Pearlsandstone Block(Desktop, Console and Mobile versions)Internal Tile ID: 403
Jungle grassInternal Tile ID: 60	Corrupt Jungle grassInternal Tile ID: 661 Corrupt grassInternal Tile ID: 32 (Old-gen console and 3DS versions)	Crimson Jungle grassInternal Tile ID: 662 Crimson grassInternal Tile ID: 199 (Old-gen console and 3DS versions)	n/a
Jungle thorny bushesInternal Tile ID: 69	Corruption thorny bushesInternal Tile ID: 32	Crimson thorny bushesInternal Tile ID: 352	n/a

(Old-gen console and 3DS versions) Jungle grass is converted to Corrupt/Crimson grass, drying the mud underneath.

(Desktop, Console and Mobile versions) Sunflowers block all conversions within 2 tiles, i.e. none of the pure tiles in the 6×8-tile margin around them can be converted. This does not apply to grass spread, so evil and Hallowed grass can still convert directly adjacent pure grass blocks and spread to Dirt/Mud Blocks in this 6×8-tile margin. The two tiles directly beneath a Sunflower cannot be converted under any circumstance, however.

Defeating Plantera for the first time reduces the speed of the biomes' spread to 1/2 (50%) of the default Hardmode speed.

Grass

All types of grass spread to adjacent soil blocks (see Grasses § Spread for details). However, Corrupt, Crimson, and Hallowed grass are also capable of converting other grass blocks directly adjacent to them. The mechanics described in this section do not change upon initiating Hardmode nor defeating Plantera, but instead remain the same for the entirety of the game.

Corrupt and Crimson thorny bushes can also spread the respective grass type to adjacent blocks, but only above the surface. For the purposes of this section, they are considered Corrupt/Crimson grass blocks, respectively.

The different types of grass blocks can convert other grass blocks as follows:

Spreading grass tile	Converts grass
	Pure	Mowed pure	Corrupt	Corrupt Jungle	Crimson	Crimson Jungle	(Mowed) Hallowed
Corrupt			n/a		❌	❌
Corrupt Jungle					❌	❌
Crimson			❌	❌	n/a
Crimson Jungle			❌	❌
Hallowed				❌		❌	n/a
Mowed Hallowed				❌		❌

All grass variants convert blocks in an equal manner, hence there is none that is "faster" or "stronger", for example.

In Hardmode, Corrupt, Crimson, and Hallowed grass will still spread to adjacent soil blocks and infect other grass as described above. However, they additionally gain the ability to convert any corruptible tile within three tiles (just like the other spreading tiles). Converting a tile that way is handled differently and is not considered grass spread.

Mushroom and Ash grass cannot be converted at any stage of the game. Jungle grass can be converted to Corrupt/Crimson Jungle grass, but only in Hardmode. This conversion is not considered grass spread, as it is part of the general Hardmode conversion triggered by any of the spreading tiles, including non-grass tiles.

Evil and Hallowed grass will never convert any tile underneath a Sunflower.

Background Walls

For a technical explanation and the complete algorithm in detail, see the talk page.

Background walls cannot infect blocks, but they can be infected by blocks, and they can infect certain other walls within certain restrictions. Walls can also be converted by the initial Hardmode transition, or manually by the player.

• Natural or placed Grass Walls, Jungle Walls, and Flower Walls are all converted to infected Grass walls. Only pure walls are infected, and afterwards cannot be re-infected by a different biome.
• Natural or placed Sandstone Walls or Hardened Sand Walls corrupt to the corresponding biome wall. Even if infected, these can be re-infected and converted to a different biome.
• Stone walls cannot be infected this way, but Ebonstone and Crimstone walls can infect the other wall types listed above.
• In all cases, any wall undergoing biome conversion (including from Thrown Water or the Clentaminator) thereafter qualifies as a natural wall. This may affect enemy spawning, and perhaps the presence of the various Underground Desert biomes.

Pure Wall	Corruption	Crimson	Hallow
Grass Wall Flower Wall	Corrupt Grass Wall	Crimson Grass Wall	Hallowed Grass Wall
Jungle Wall	Corrupt Grass Wall	Crimson Grass Wall	n/a
Stone Wall	Ebonstone Wall	Crimstone Wall	Pearlstone Wall
Hardened Sand Wall(Desktop, Console and Mobile versions)	Hardened Ebonsand Wall(Desktop, Console and Mobile versions)	Hardened Crimsand Wall(Desktop, Console and Mobile versions)	Hardened Pearlsand Wall(Desktop, Console and Mobile versions)
Sandstone Wall(Desktop, Console and Mobile versions)	Ebonsandstone Wall(Desktop, Console and Mobile versions)	Crimsandstone Wall(Desktop, Console and Mobile versions)	Pearlsandstone Wall(Desktop, Console and Mobile versions)

There are two overlapping patterns ("grassy" and "sandy") for infection of walls, depending on the contents of the source tile. Both patterns are considered for each tile update, and can separately infect different walls. The behavior can also vary according to whether the tile is above or below surface level, and which infection is being considered.

Common Rules

• In all cases, wall infection range is 2 tiles (a 5×5 square centered on the source tile), compared to 3 tiles for most blocks.
  • As with blocks, Hallow cannot infect Jungle Walls
• For a source tile to infect a wall, it must find a qualifying foreground block or plant with the infection in question (collectively, "biome blocks") either in the source tile, or within 20 tiles of it.
  • The qualifying biome blocks vary with context, but the following blocks always qualify: Grass, Ice, Stone, Sand, and Vines.
  • For Crimson, or to infect any grassy wall below surface level, the biome block must be in the source tile.
  • Otherwise (infecting a grassy wall above surface level with Corruption or Hallow or any sandy wall), the biome block can be found anywhere in a 20-tile range (a 41×41 square centered on the source tile).

Grassy Wall Infection

If the source tile includes any natural infected Grass Wall, natural infected Stone Wall (except Pearlstone Wall) or infected grass block, then it can attempt to infect a pure Grass, Flower, or Jungle Wall. Both natural and player-placed walls of these types can be infected.

• Each of the infectious biomes is checked in turn: Crimson, then Corruption, then Hallow. If the tile has a qualifying wall and a grass tile from different biomes, it can attempt to spread both biomes.
  • For each infection attempt, a single tile within range is chosen, and if it is a pure grassy wall, it will be infected.
  • Again, infection needs a "biome block" as above:
    • For Crimson or if below surface level, then the biome block must be in the source tile, otherwise within 20 tiles.
    • In addition to the list from above (Grass, Ice, Stone, Sand, and Vines), Demonite Ore qualifies for Corruption. It only qualifies as a biome block in this step, and Crimtane Ore does not qualify for Crimson.
    • Most biome-specific plants (if infected) also qualify as biome blocks here. This includes thorny bushes, tall grass/flowers (that is, growing from proper grass blocks), Vile/Vicious Mushrooms, and (again) vines.
  • As above, Hallow cannot infect Jungle Walls, and Pearlstone Walls cannot infect grassy walls at all.

Sandy Wall Infection

If the source tile contains any Sandstone Wall, any Hardened Sand Wall, and/or any Sandstone Block, then it can attempt to infect a Hardened Sand or Sandstone Wall.

• Only Hardened Sand or Sandstone Walls, pure or with any infection, are converted this way.
• Unlike with grassy walls, only one biome is chosen, and then a single nearby tile is chosen for an infection attempt.
  • To attempt infection, the source tile must have an infected wall and/or an infected biome block.
    • Qualifying walls include Grass, Stone, Sandstone, or Hardened Sand. (Note that the first two imply a Sandstone Block from the first step)
    • Qualifying biome blocks include the list from above (Grass, Ice, Stone, Sand, and Vines), or Sandstone Blocks, but not other plants.
    • If the tile has different infections for wall and block/vine, Crimson takes priority, then Hallow. Corruption applies only if there is no competing infection.

Notes

• Spreading tiles can spread their biome even if there are non-corruptible blocks "in the way". For example, a one-block-wide wall of Wood between a Corrupt grass block and a pure grass block in Hardmode will not prevent the grass from being converted because the Corrupt grass can convert any tile within three tiles. Thus, to effectively stop the spread of a biome, there must be at least a three-tile-wide obstacle of either open air or non-corruptible blocks.
• The evil biomes and the Hallow grow vines from beneath their respective grass blocks. These vines can spread their biomes to nearby blocks (in Hardmode) and walls as above. Vines themselves cannot be converted except by converting the block they are connected to.
  • There are no Corrupt vines.
• Stone Blocks with moss on them will be treated like regular Stone Blocks. The moss will be destroyed upon conversion.
• Thorny bushes grow from Corrupt and Crimson grass blocks, can extend a significant distance in any direction, and can furthermore spread the Corruption and Crimson, even over non-corruptible blocks. Therefore, it is possible for an evil biome to spread over a gap in pre-Hardmode, if its thorny bushes happen to grow across it and touch dirt, mud (), or pure grass on the other side.
• Chlorophyte Ore and Chlorophyte Bricks limit the spread of the evil biomes:
  • If the Corruption attempts to convert either a Mud Block or a Jungle grass block, or if at least one of the four tiles directly surrounding the target tile is a Mud or Jungle grass block, then it will check for the existence of nearby Chlorophyte: If there are at least 5 Chlorophyte tiles (regardless of whether Ore or Brick, both are added) within a radius of 10 tiles, the conversion will fail entirely. If there are fewer than 5, but more than 0 Chlorophyte tiles, there is a chance of <tiles>/5 (e.g. 3/5 if there are 3 Chlorophyte tiles) for the conversion to fail.
  • The Crimson will always check for nearby Chlorophyte, regardless of the target or adjacent blocks.
  • Chlorophyte Ore can also turn nearby dirt into mud, partly countering conversion damage to the Jungle.
  • On the  Desktop version,  Console version, and  Mobile version, within a 13x13 radius, Chlorophyte ore and Chlorophyte brick can convert grass, Corrupt grass, Crimson grass, and Hallowed grass to Jungle grass, convert Corrupt or Crimson stone, sand, hardened sand, and sandstone to their ordinary variants, and kill Corrupt or Crimson thorns, grass, and vines.