Cactus is tied with Dragoneye's rose for the least utilized flower in the Herblore skill, it is a required ingredient in 1 potion. It is used when crafting the Heat resistance potion, and gives 16 XP when harvested.
You can find Cactus throughout the Kharidian Desert, and a couple other spots scattered around the map.
The best spot to gather large amounts of Cactus is outside of the Shantay Pass bank, on the other side of a wall there are 3 large Cactus plants.
Alternatively there are 3 Cactus plants just north of Al Kharid bank