In the Canon:-== No such plant or vegetation exists in the Terry Pratchett canon.
In the Pessimal Discworld:-Edit
Like its close relative the Yaqui Tequila Cactus, this is one of those plants which is such a mind-bogglingly useful consolation to people living in a bleak and inhospitable desert that it is difficult for professional botanists to beleive it evolved naturally. Doctor Davinia Bellamy, Head of Botany at the Assassins' Guild School, maintains some very good specimens alongside many other professionally interesting plants she cultures for the Guild. In conjunction with colleague Professor Horace Pennysmart, Head of Unseen University's Department of Extreme Horticulture, she hs developed the theory that the original proto-cactus derived from Counting Pine seeds, which drifted on the high winds of the Disc to settle in remote Howandaland and promptly turbo-evolved into something suited for the local environment. For the genetic prerogative of the Counting Pine is to be useful to mankind, in such a way that it is never uprooted or killed, and can get on with the business of creating more Counting Pines.
The method by which the Yaqui Mescal Cactus makes itself useful is that it will periodically bloom with flowers and seed pods shaped like buttons. Caled peyotl in the local Indian languages, the flowers may be infused into an interesting hot drink, and the buttons chewed, in order for an initiated shaman to go on one mother of a Vision Quest. Dr Bellamy (in association with Inimical Alchemy master Mr Mericet) has isolated the active ingredient and called it mescaline: The new drug was tested out on a volunteer student*, who on awakening in the care of Matron Igorina some days later, reported going through a freaky desert world where coyotes spoke to him and jaguars imparted great wisdom whilst chewing his face off.
This tallies with the accounts of Yaqui shaman such as Don Frank and Carlos Castanet, who recommend the peyote button to young shamen as a sure-fire way of contacting and speaking to the Trickster God, Coyote. That's if you can prevent the top of your head unscrewing, drifting off, and floating away. Don Frank also said that it's one Mother of a trip, man, and the great wisdom sums up as Overall, there is a pervading smell of boiled cabbage.
* - well - not exactly a volunteer as such. Both Dr Bellamy and Mr Mericet had identified this senior student as being over-confident around poisons and potentially dangerous chemicals. Inviting him to experiment with a new and untried potent hallucinogen was intended to cure him of this otherwise terminal character flaw in Assassins. Look. Do not mistake the mumsy and gentle-seeming Davinia Bellamy for some sort of right-on hippy chick tree-hugger. She's an Assassin.