"Although it has obviously thrived in the warm summer, you had better destroy it.
Her answer read: "It looks like hemp (Cannabis sativa).
She advised Barry to destroy it immediately – but warned against burning it on a bonfire.
Bird seed can cause illegal cannabis plants to grow in your garden, a BBC Gardeners’ World expert has warned readers of the programme’s magazine.
"It is safe to put on the compost heap but I would advise against a bonfire."
The September 2019 issue contains an email from Barry Thorp who sent a picture of the plant, and asked: "Can you identify this mystery plant from my garden?".
Responding in the Gardeners’ Question Time section, Anne Swithinbank identified it as hemp and pointed out it is illegal to grow cannabis at home.
A reader who sent in an image asking the experts to identify a mysterious shrub in his garden was warned to immediately destroy it.
Guy Barter, RHS Chief Horticulturist, agreed with the BBC specialist, and told The Telegraph: “Yes this is a cannabis satvia plant. It is illegal to possess/cultivate it in the UK without a special licence. Some legal cultivation is allowed under licence both for pharmaceutical use and, using low cannaboid content cultivars, for seed, oil and fibre.
"It's quite common for bird seed to contain a huge variety of plant seed and cannabis has been known to come up from it."
Patricia Hewitson, from Exmouth, contacted BBC Radio Devon's gardening programme asking for help identifying "a weed".
After discovering it was an illegal cannabis plant the police were involved.
Mrs Hewitson said: "I sent a picture in via email and I got a couple of interesting emails back, including one from the police.
Cannabis is illegal to grow without a licence so Mrs Hewitson and her husband John were advised by Devon and Cornwall Police about what to do with the specimen.
BBC Radio Devon's gardening expert, Sarah Chesters, said: "I think the seed of this came from the bird feeder which hangs just above their door.
Officers said Mrs Hewitson was growing the plant illegally but in good faith.