Corrupt Seeds

The usual reason is that your world rolled the other evil and you want the drops and materials this one gates. Planting a controlled patch is far less work than travelling to another world for every stack. The Dryad will sell them even in a Crimson world if you talk to her while a Graveyard is active and you have entered Hardmode.

This is the mistake that ruins worlds. The grass spreads on its own, and thorny bushes sprout from it and reach several tiles in every direction, converting whatever they touch on the way. A patch you meant to keep small will crawl into your jungle if you let it. Box the whole area in with something the spread cannot convert, floor and ceiling included, before a single seed goes in.

• Mud takes them too, which gives you the corrupt version of jungle grass for the materials that only appear there.
• If you only need the grass and not the spread, converting ordinary grass with Purple Solution or Vile Powder is easier to control.