Conservation advisor Helen Buckingham said grassland farming, with its ploughing and re-seeding, fertilisers and herbicides, was killing off flower-rich meadows.
"The last detailed study across England and Wales indicates only 3 per cent of the enclosed unimproved grasslands of the 1930s survive," she said.
"The best examples are a colourful refuge for uncommon plants such as greater butterfly orchid and wood bitter vetch.
"In contrast to species-poor grasslands that dominate so much farmed landscape hay meadows provide a rich nectar source in early summer."
"The diversity of our meadows depends on a complex interaction between natural factors such as soils, water levels, geography and the idiosyncrasies of people and place.
"We ignore this at our peril if we want our meadows to remain."
She highlighted examples on trust land such as Westhay Meadows on Golden Cap Estate, Dorset with thousands of rare green-winged orchids and corky fruited water drop worts.
Berthlwyd Farm Hay Meadows in the Brecon Beacons brims with waving grass and tall herbs, and buzzes with bees, crickets and butterflies.
And Upper Wharfedale in the Yorkshire Dales National Park has aniseed-scented sweet cicely and pink splashes of bistort, blue meadow cranesbill and yellow rattle.
Other National Trust meadows include Runnymede in Berkshire, Hartland Moor and Middlebere, Dorset, Plas Newydd Country House and Gardens in Anglesey and Downhill Demesne in Co. Londonderry.
Subscribe to Horticulture Week for more news, more in-depth features and more technical and market info.