Though its seeds do resemble beans, do not confuse the castor bean--a common ornamental--with the edible garden vegetable. Castor bean seeds contain the extremely potent poison ricin and are so toxic that only a few beans could kill an adult human if swallowed. The seeds also provide a useful oil with many applications in industry and medicine.
The castor bean--ricinus communis--has no close relatives in the plant world but is distantly related to other spurges such as the rubber tree plant and the tung oil tree. Naturally occurring in Africa, the castor bean now grows wild in many parts of the world where it was introduced as an ornamental or field crop. Though the plant does not survive frost, seeds often overwinter in temperate growing zones. In one growing season, this vigorous plant may reach heights of more than 10 feet--in tropical regions, 40 feet of mature growth is possible.
The castor bean's industrial uses depend upon the thick yellow oil that forms about half the mass of the seeds. Most of the poison in the seed remains in the pulp when the seeds are pressed. The oil finds use in machine lubricants, waxes and even crayons. Many folk medicine cure-alls used castor oil as an active purgative and laxative, even though in large doses the oil can be poisonous. If boiled to remove poisons, the seed cake itself makes good animal feed.
Large star-shaped leaves and nearly explosive growth make the castor bean a well-liked planting choice in many modern gardens. When planted around the perimeter of a vegetable garden, the plants form a protective barrier against common garden pests. Though the seed holds concentrated poison, other parts of the plant, including the roots, contain enough toxin to repel burrowing animals that feed below ground.
Ricin, the toxin found in castor bean seeds, is 6,000 times more potent than cyanide. Ricin has been considered an active component of biological military weapons. Even particles of the pure toxin landing in open sores or on the eyes can kill. A lethal dose for an adult human being would be equal to one grain of table salt in size. No antidote exists, but those who survive the three- to five-day bout of pain, vomiting, diarrhea and convulsions associated with ricin poisoning often make full recoveries.
Despite the dangers, the castor bean remains a common sight in yards and gardens. Some varieties show ruddy purple or red foliage and stems, while other varieties yield dark green or golden leaves. The showy clusters of spiky seedpods can be harvested and saved for next year's planting. Pods left on the plant or on the ground could poison birds and animals as well as curious children.