Cactus - Terraria Wiki

Step 1

The target Sand Block is above 0 ft and is not actuated or sloped in any way.The target block is more than 378 tiles away from the lateral edges of the world, i.e. not in the zone where the Ocean biome is usually located.The tiles above and to both sides of the target Sand Block (only the directly adjacent ones) are open space.The tile above the target block is not filled with any liquid.There are fewer than 4 Cactus tiles and more than 10 Sand Blocks (or Ebon-, Crim-, or Pearlsand Blocks; all variants are added together) within a 13×5-tile rectangle centered on the tile above the target Sand Block.

Step 2

↑ Information taken from the PC 1.3.5.3 source code, method UpdateWorld() in Terraria.WorldGen.cs. There may be inaccuracies, as the current PC version is 1.4.4.5.