Letchworth couple Anna Maria and John Atkinson are concerned their garden has been invaded by "very tall, threatening plants".
"We are very worried about these very tall, threatening plants," John said.
"They have invaded our Letchworth garden recently and we suspect are from another planet or galaxy far far away!
"They are over 10 feet tall and still growing."
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The Atkinsons suggested it was like The Day of the Triffids - a 1951 post-apocalyptic novel in which an aggressive species of plant starts killing people.
However, the grounds team at North Herts District Council has identified the giant plants as onopordum acanthium (cotton thistle). It is a vigorous plant, but "of no potential risk to people", reassured a spokesperson for the local authority.
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