Marijuana and hemp are different varieties of the same species of plant, Cannabis sativa L. There are different varieties of Cannabis, just as Chihuahuas and wolves are different breeds of Canis lupus. They are scientifically different and cultivated in different ways.
Marijuana is the flowering tops and leaves of psychoactive varieties of Cannabis that are grown for their high THC content.
It is used legally in some states, excluding Wisconsin, for medical reasons, and is used for recreational purposes as an illegal drug.
Unlike hemp, marijuana has a high THC (delta-9 tetrahydrocannabinol) content and a low CBD (cannabidiol) content. CBD blocks the psychoactive effect of THC in the nervous system. That means in all plants in the Cannabis family, there is a chemical that will induce a psychoactive effect and another that will block it. The illegal drug is cultivated to have a THC content of anywhere from 3 percent to 22 percent. The ratio of CBD to THC is less than one.
Hemp, or industrial hemp, is a cultivated, low-THC variety of Cannabis. It is grown for its seeds, oil and fiber.
Industrial hemp has a low THC content compared to its CBD content. THC is typically less than 1 percent. Certified varieties in Canada and Europe are bred with the THC level purposefully decreased to less than .3 percent, the same THC level under recommendation if hemp farming is legalized in Wisconsin.
Ditch weed is hemp found growing in the wild, usually near places where it was once cultivated. It has a low THC level.
Sources: www.votehemp.com and “Hemp and Marijuana: Myths and Realities” By David West, Ph.D, for the North American Industrial Hemp Council