How to let cacti grow even if no player is near?

Currently, the random ticks only happen within the 16x16 chunk area around the player. That means even keeping the chunks loaded doesn't help if there's nobody within the radius.

The common vanilla approach is to move the farm near frequently visited areas, where players' presence will keep the farm active. Alternatively, just afk near the farm to keep it running.

The somewhat less vanilla approach are server-side bots (e.g. provided by the Carpet mod) - they act like players for the purposes of random ticks and mob spawning.

And the sneaky approach is to build a zero-tick based cactus farm, then afk by it for a couple hours. It will produce more cactus than you'd know what to do with. I'm not sure if it works on Craftbukkit though.