Gardeners at the university-owned venue had to remove a pane of glass from the roof of the cactus house to accommodate the rapidly-ascending dasylirion lieophyllum last month.
But the flower stalk has defied all expectations by continuing to grow and is now stretching several feet out of its home, taking aback even experienced garden director James Hearsum.
Mr Hearsum said: “The plant hit the glass ceiling a month ago so we took the pane out and let it grow.
“All of a sudden it has shot up 15 feet through the glass ceiling, and it’s done that in basically four weeks.
“It’s pretty astonishing because it’s far, far bigger than we expected.
“The literature we have said it could have a 15-foot flower spike but this is 25-feet and growing.
“To be honest, we haven’t managed to get up a ladder to measure the thing because it is very spiky.”
Dasylirion lieophyllum - officially classified a “succulent” - is a native of Chihuahua in Mexico, and is a member of the asparagus family.
More often known by its common name of green sotol, it has not flowered in 40 years but staff expect it will do so over the next few weeks.
The stalk should fall over after it has flowered, with flowers set to emerge near the tip.
Until then, Mr Hearsum says it will continue to rise.
He said: “Apparently, Kew Gardens in London are making a big fuss because their one has just gone through their roof, but we think we’re doing rather better here.”