Wild flowers are being driven off Britain's roadside verges by air pollution and poor management, the charity Plantlife claims.
It says emissions from vehicle exhausts are acting as a fertiliser for a group of nitrogen-loving plants like nettles, which outcompete traditional flowers.
The group reports an almost 20% loss in the diversity of plants.
And this has led to a decline in the insects that depend on many of these plants, the charity says.
The government says nitrogen emissions from exhausts will steadily fall as vehicles get cleaner over coming decades.
The Botanical Society of Britain and Ireland, BSBI, agrees that roadside plant diversity is falling, and nitrogen deposition near roads had increased.
But it said the interplay between air pollution and poor management impacts on roadside verges by local councils was not well understood.
Its spokesman, Kevin Walker, told BBC News: "More frequent cutting with the (mowings) left in place can reduce plant diversity as well. This is due to suppression and disturbance rather than elevation of nutrients."
Plantlife has been campaigning for local authorities to follow the lead of Highways England and positively reduce nitrogen levels by removing hay from verges.
It admits this will not be easy in areas where motorists have treated the verges as linear rubbish dumps.
Among the nitrogen-lovers thriving on exhaust fumes are those that Plantlife calls the "thugs of the plant world". They include stinging nettle, bramble, rough meadow-grass, cow parsley, Yorkshire fog and creeping buttercup.
The losers are plants that cannot cash in on the surfeit of nitrogen, so are crowded out.
These include tufted vetch, bugle, tormentil, red clover, lady's bedstraw, white campion and greater knapweed.
The charity says roadside verges are especially valuable as a habitat for wild flowers because so many traditional meadows have been lost since the 1930s. It says often verges are the main contact with the plant world for many motorists.
Plantlife's botanist, Trevor Dines, said: "Our once colourful and botanically diverse road verges are becoming mean, green thickets where only thuggish species can thrive.
"After the froth of cow parsley in May, many verges no longer enjoy a bountiful summer. For the 23 million people who commute to work by road, the verge can be their only daily contact with nature."
The charity says road verges are home to over 700 species of wild flower - nearly 45% of our total flora – including 29 of 52 species of wild orchid.
Dines continued: "The impact of air pollution on human health is well documented but how pollution affects plantlife remains under-appreciated.
"Poor management has combined with pollution to create a perfect storm. Councils have adopted an over-eager regime that sees flowers cut down before they can set seed and the mowings left on the verge simply add to the soil richness."
The charity calls for councils to cut less – and later in the year. It says councils following this advice save money as well as flowers.
The Plantlife analysis was done by homing in on road verges that have been monitored as part of the Countryside Survey.
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