A potato cactus tree can be grown in cactus tree patch with a Potato cactus seed. 3 Potato cacti can be harvested when fully grown, or 6 if you've completed the Hard desert achievements. This may be increased by an additional 50% by having a Giant ent familiar summoned (93 Summoning).
After completing atleast the Medium desert achievements, one may speak to the Weird Old Man to obtain an amount of Potato cacti from him daily. With a Desert amulet 2 you will obtain 20, with a Desert amulet 3 30, and with a Desert amulet 4 40.
Add Lantadyme and Potato cactus to a Vial of water to make a Magic potion.
Use 51 Spirit shards, a Blue charm, and Potato cactus to make a Spirit kalphite pouch. Eight (8) Potato cactus (noted) are dropped by the Fungal mage in the Polypore Dungeon.