Cactus Farm

Cactus grows the same way sugarcane and bamboo do: straight up, harvested with an observer and a piston knocking the top off once it's tall enough. The catch is spacing. Cactus can't touch any block on any of its four sides, including another cactus, or it pops instantly and drops nothing.

That rule shapes the whole layout. Every column needs its own sand tile with a full block of clearance around it, usually solved by alternating cactus columns with a solid block in between, checkerboard style, so nothing ever touches. Skip this and pack the columns too tight, and you'll get a farm that produces nothing while you try to figure out why.

Each column gets its own observer watching the growth point and a piston to knock it over, same wiring as sugarcane, just spread out. Drops land on a hopper floor and feed into a chest.

Cactus is more useful than people give it credit for. It's the only renewable green dye source (via smelting), and a standing cactus deals contact damage, which some players use as a passive mob deterrent around a base instead of building actual walls.

Since it smelts, running a cactus farm straight into a hopper-fed furnace line is a genuinely efficient XP loop. No mob spawner, no combat, no upkeep beyond emptying the dye chest now and then.