Jungle Grass Seeds

Mud plus these gives you jungle grass, and enough jungle grass gives you the biome, which is what people are usually after: a safe farm for jungle enemies, a place to grow bulbs, or a supply of plants that grow nowhere else. Depth decides which version you get. Planted near the surface it stays a surface jungle no matter how much you cover, and the cutoff for the underground version sits deeper on bigger worlds.

The grass creeps outward slowly, closer to mushroom grass than to ordinary grass, so plant more starting points than feels necessary and go do something else rather than watching it.

Farming these off wild plants at their drop rate is miserable. Two ways around it. Buy mushroom grass seeds from the Dryad inside a glowing mushroom biome and Shimmer them, which is by far the fastest. Or skip the seeds entirely: any grass growing on mud converts with Green Solution, so cheap Dryad seeds plus a Clentaminator reach the same end. If you take the corrupt or crimson route for that, purify every last tile afterward or you have started a spread you did not want.

Flower Boots are the third option. They sprout wild plants as you walk, so if you already own a pair, running laps across jungle grass produces seeds faster than cutting them by hand.