You can build cactuses on planets you own. To do so, send a message

cactus: build NNN

to yourself, where NNN is the planet Id. There may be requirements for the planet (ClansRequired, NeedBase, RebuildCactus), and there may be an overall limit for the number of cactuses you can build (CactusLimit), but other than that you can build as many as you want, whenever you want.

Building a cactus may cost score points (CostAdditive etc.). Building cactuses may cause the score to go negative.

As long as you own the cactus, it will produce TurnScore points per turn.

Instead of build, you can also write defhw.