Cunningham's Encyclopedia of Magical Herbs - Scott Cunningham 1985

Cactus
The Herbs

Few are poisonous, ingest only those used for food.

Powers: Protection, Chastity

Magical Uses: Cacti of all kinds are protective, owing to their spines. Grown indoors, they protect against unwanted intrusions and burglaries, and also absorb negativity. Outside, one cactus should be placed facing each direction, next to the house, to further protect it.

Cactus spines are sometimes used in Witch bottles, and to mark symbols and words on candles and roots. These are then either carried or buried to release the power.