Datura stramonium, also known as devil's snare, is normally found in South and Central America, but was discovered earlier this year by 79-year-old Phyllis Abbott from Newmarket.
The plant is highly poisonous and is used by South American Indians for their hunting spears and fishing hooks and in sacred ceremonies by Hindu monks for its hallucinogenic qualities.
Devil's snare also features in the first of JK Rowling's Harry Potter books, where it is used as an obstacle to would-be thieves.
Abbott discovered it in spring while tending polyanthus in the same bed.
It was only a few inches tall but has since topped five feet and, according to the RHS, could reach over 12.
Abbott said: "My biggest concern was for people with little children in case it spread. We have neighbours with young ones at the bottom of the garden."
A keen gardener, she thought the plant was a weed and considered pulling it. It was not until months later when it had grown shoulder height that their son identified it as devil's snare using the internet.
Abbott and her husband George, who have nine grandchildren, have been stunned by the media attention the find has generated.
Its poison causes dry mouth, blurred vision, heart irregularities, hallucinations, and eventually coma and death in severe cases.
Datura stramonium has large, pale, trumpet-shaped flowers and spiny pods.
Identification was eventually verified by the RHS, which warned the couple that it was highly venomous and speculated it may have arrived in Suffolk via bird droppings.
Abbott said: "I have no idea how it got there, it really is a bit of a mystery."
The RHS has advised the couple to dig up the plant due to the danger it poses to humans and pets.
But Abbott said she was happy to leave it for a while.
She said: "I don't mind leaving it there until people who want to have seen it but then we will dig it up. We have already had a couple of people call who think they may have the same thing in their gardens."
The plant belongs to the same family as deadly nightshade, though its poison acts more strongly on the brain.
The leaves give off a pungent, nauseating odour and the flowers smell sweet, but both are narcotic and can induce hallucinations or stupor if breathed in for too long. Its seeds are particularly poisonous if eaten.
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