Described as “nitrogen guzzlers”, the marauding plants thrive in nitrogen-rich soils caused in part by pollution from transport which settles on roadsides.
They include brambles, nettles, buttercups, blackthorn, sycamore and cow parsley.
But experts at the charity Plantlife say these “thuggish” plants are forcing out other wildlife-friendly and threatened species that prefer poorer soil.
Research shows the variety of wild flowers growing on roadsides has fallen by almost a fifth since 1990.
Species such as red clover and lady’s bedstraw, which support high numbers of insects, have seen the most rapid declines.
And rare wildflowers such as fen ragwort and wood calamint are clinging on in just a handful of verges, their last remaining habitat.
Pollution from traffic, which sees 90 per cent of nitrogen of car emissions deposited within 15 metres of the road edge, combines with poor management of verges to create a “perfect storm” for wild flowers.
Plantlife is calling on councils to manage verges better for wild flowers and the creatures they support, while keeping them safe for motorists.
Changes could include cutting less and later in the year so flowers can set seed, allowing semi-parasitic plant yellow rattle to act as a natural lawnmower and not leaving cuttings on the verge where they increase the nutrient richness.
If all road verges in the UK were managed for nature there could be almost 420 billion more flowers – 6,300 per person – the charity estimates.
Victims of the changing verge include tufted vetch, bugle, tormentil, white campion and greater knapweed.
While nettles and other species that thrive on nitrogen are important for certain creatures, road verges are losing the diversity that support a range of wildlife, Plantlife’s Dr Trevor Dines warned.
“It’s playground thuggery. It’s these bigger things which produce big leaves and lots of vegetation and they literally shoulder these small plants aside and out-compete with them,” he said.
More than 700 wild flower species grow on verges, making them increasingly important as meadows disappear.
Dr Dines added: “You’re driving past field after field without any flowers. The only places flowers are in much of our countryside are road verges.
“They’ve been eradicated from one side of the hedge in farmers’ fields and we’re now doing our best to eradicate them from the other side of the hedge.”