If choosing not to cover any top sides of grass that may grow thorns, make sure to at least leave a barrier of 5-7 blocks sticking out from the tunnel's sides to prevent spread from thorns that may grow over the tunnel. Afterward, another critical action is placing non-corruptible blocks (such as wood, hay, dungeon bricks, etc.) over or replacing any exposed top sides of corrupt/crimson/jungle grass within 6 tiles of the tunnel's edges and along the floor of the tunnel, as well as any exposed bottom sides of any grass type along the ceilings of the tunnels to prevent thorns & vines. To do this, the simplest step, and often the first, is to dig tunnels 3-6 tiles wide completely surrounding the areas. Because of this, prior to entering Hardmode, it is recommended to physically contain the existing areas of the evil biomes. However, because they become less effective when the evil biomes begin to spread underground in Hardmode, they should be used mostly as a temporary barrier for holding back surface grass spread. Sunflowers are a viable option for preventing spread early, as they can limit surface spread of both evil biomes, even in Hardmode. An example of shafts able to contain a patch of Hallow from spreading.