Some of the most common exotic plants in Britain have evolved rapidly to better suit the environment – whether that means becoming larger or smaller



Bristol City Council is attempting to use vinegar to kill weeds as an alternative to glyphosate herbicide. Whether the vinegar actually makes a good weedkiller is debatable, but it does make streets smell like a chip shop.

More worrying is how some of our commonest weeds are evolving rapidly to become tougher, larger and better adapted to their environment. An ingenious study looked at how old herbarium specimens of foreign plants brought to Britain long ago have changed by comparing them with today’s living plants of the same species, and the results were astonishing.

Glyphosate is a 'probably carcinogenic' herbicide. Why do cities still use it? Read more

The Oxford ragwort is a daisy-like yellow flower that was first brought to Britain from Mount Etna in Sicily and escaped into the wild from Oxford University’s gardens in 1794. Since then it’s grown taller and increased its leaf area by a fifth. It is now widespread and poses a big threat to horses, cattle and even chickens because it’s poisonous to them.

Winter speedwell is a pretty little blue flower that originally came from Eurasia. This plant has grown about 14% taller and its leaves have shrunk and become rounder since discovered here in 1826.

The American willowherb is a straggly plant with purply-pink flowers, a native of North America that was first discovered growing wild in the UK in 1891. It grows in clumps with tough underground roots, often by the sides of streams and ditches and it has rapidly evolved with the leaves becoming 50% smaller.

This is grim news as weeds are evolving hundreds of years after their introduction and could spread to more diverse environments. Perhaps they will never be beaten – even with vinegar.