Cactus

An axe or a chainsaw harvests it exactly like a tree, and the sand block underneath is permanent: it will not break and it will not fall. Where cactus differs from wood is regrowth. Chop a plant all the way to the ground and that spot is finished, but leave the bottom segment standing and the plant grows back out of the stump. A patch harvested carefully keeps producing indefinitely while new plants seed themselves in the gaps, so the farm gets more productive over time rather than less.

Side arms can be turned into whole new plants. Put a sand block directly beneath a stem, cut away the trunk it was attached to, and that branch is now its own cactus standing on its own base.

You cannot plant one. Cactus seeds itself onto sand, slowly and at random, and only where the conditions allow. It wants to be above the surface line, away from the far left and right edges of the world where the oceans sit, with clear air overhead and no liquid directly above. It also checks how wet and how crowded the neighborhood is: too much standing liquid in the surrounding area, or too many cactus tiles already present, and nothing new appears. Existing plants creep taller under similar rules, up to a height limit that is rerolled slightly differently on every check.

The practical version is simple. A dry stretch of open desert with plenty of exposed sand and few plants on it fills in by itself. Flood the same stretch and the plants standing in the water start breaking apart, so a water project next to a farm costs you the farm.

This is the cheapest bulk material in the game, which makes it the cheapest thing in the world to gamble on modifiers. Crafting cactus swords over and over until one lands the top prefix costs nothing but walking time, and an early character carrying a legendary weapon is well ahead of schedule. The cactus pickaxe is worth the trip for a different reason: it skips the reach penalty that makes the copper pickaxe so irritating to dig with, and you can have one on the first morning.

Corruption, crimson and hallowed deserts all drop the same plain item, so there is no need to hunt down a specific biome variant for a build. Placed cactus takes on the paint and coating of whatever it grew from, which matters if you are theming a desert, and the rubblemaker turns it into scattered props for dressing a scene. Prickly pears grow on the tops of tall plants without any help from you.

Touching one is harmless and has been for a very long time, with a single exception: in a Constant world the plants hurt on contact, the way they did in the earliest builds. The design is a saguaro, the tall armed cactus of the North American deserts, which is where the arms and the height come from.