The beach walkers and their playful dogs running in arcs on the rippled sand seemed blind to the casual alien in their midst. Anchored above the tideline, it wobbled slightly in the breeze, spiny green pods raised like medieval weapons. “Casual aliens” are non-native species that occasionally escape from cultivation, grow for a while and then die off without establishing a population. Botanical jargon aside, the term’s otherworldly weirdness particularly suited this plant. Its thick, branching stems, jagged leaves and menacing seed cases reminded me of the man-eating specimen in Little Shop of Horrors. Though not carnivorous like the one in the musical, it could still kill. Thornapple (Datura stramonium) is one of the most poisonous plants in Britain. It contains three potent alkaloid toxins that block the body’s neurotransmitters, causing intense nausea, delirium, coma and death. Originating in Central America but now widespread round the world, it’s part of the Solanaceae family, which includes potatoes, tomatoes and deadly nightshade. It’s relatively common in North America, where it’s known as jimsonweed. Despite its dangers, thornapple was grown in medieval herb gardens for medicinal use. Boiled with lard, it made an ointment for treating scalds and haemorrhoids. But it had a host of darker applications, such as turning people into werewolves. Witches were said to include it in flying ointment, the hallucinogenic grease they reputedly rubbed on their naked bodies when communing with the devil. Today, it mostly crops up on dry waste ground, often from seeds in bird droppings. The plant beside the Bristol Channel might have come from a racing pigeon that had eaten a contaminated seed mix. It was lucky to fall in a crevice between rocks, giving some shelter on this perpetually windblown shoreline. Even so, the exposed, salty conditions stunted its growth to a modest 30cm rather than the average spread of 1.5 metres. It poses no threat if left alone and will perish naturally over the winter. New plants are unlikely to germinate in such a tough environment. I was sorry to have missed its summer flowers. The white, trumpet-shaped, slightly twisted blooms would have opened at night, glimmering faintly in the moonlight, their scent attracting moths. Country diary is on Twitter/X at @gdncountrydiary Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 20% discount