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...