'It's really, really rare. ...I don't know if there's one in the greenhouse at Hogwarts, even. I can't wait to show it to Professor Sprout. My Great Uncle Algie got it for me in Assyria. I'm going to see if I can breed from it.'
-- Neville Longbottom (OP10)
The Mimbulus mimbletonia is a very rare magical plant, native to Assyria, which resembles a grey cactus but with boils where the spines would have been. The boils are a defensive mechanism that spew Stinksap upon contact.
- This plant is a particular favorite of Neville Longbottom's, who received one as a present from his Great Uncle Algie for his 15th birthday. Like its owner, the plant grew a lot over that year (OP10, OP11, OP38).
- When Neville's plant spewed Stinksap all over the railway carriage and onto Harry and Luna, Ginny cleaned up with the Scouring Charm "Scourgify" (OP10).
- "Mimbulus mimbletonia" was the password to the Gryffindor Common Room at the beginning of fifth year (OP11).
- By the end of the year, Neville's Mimbulus plant would making "odd crooning noises" when stroked (OP38).
“The name probably comes from the genus of plants called mimulus. In the lore of flower remedies, as developed by British physician Dr Edward Bach in the 1920s, an essence made from the mimulus flower is said to counter anxiety and fear.” (The Lexicon p204).
The name of this plant sounds a bit like the incantation for the Tongue-Tying Spell, "Mimble wimble" (Pm), but there seems to be no magical connection.
Back in 2003 when Harry Potter and the Order of the Phoenix was released, the scene on the train with the Stinksap was analyzed on many fan forums. Some fans thought it sinister that Great Uncle Algie gave Neville both the troublesome Trevor the toad and the Mimbulus mimbletonia plant, which seemed to react to humans like a cactus from another planet. Let's recall that Uncle Algie was the one who wanted to test Neville's magical skills by tossing him off Blackpool Pier, as well as holding the boy out the window by his ankles and dropping him on his head (PS7). Neville bounced down the road, proving he wasn't a Squib, and the family rejoiced, but Algie seemed to almost want Neville dead. Was he up to no good?
A more positive theory held that Algie tracked down the Mimbulus in Assyria because he wanted Neville to use the sap to cure his poor parents of their Crucio-induced insanity. Unfortunately, that scenario never developed in the books. In hindsight, the Mimbulus mimbletonia was a red herring, an object that seemed really important to the story but was actually just an odd magical plant that made Neville happy. Great Uncle Algie gave Neville strange gifts simply because he was fond of him, in his own eccentric way.