The waxcaps were first spotted by Northumberland National Park resident and smallholder Jan Ashdown who responded to an appeal by biodiversity ranger, Shaun Hackett for information for a National Park survey.
She reported sightings of various types including the Crimson waxcap – a striking blood-red, waxy-topped toadstool and an indicator of rich fungal sites.
When Hackett visited the site he found dozens of Crimsons and other rare species The Ballerina, Citrine and Blushing which has only been recorded from a handful of sites in northern England.
He also spotted a different type of grassland fungi – the Dark Purple Earth Tongue - which is only known at a few sites in England and classed as a threatened species throughout Europe.
Hackett said: "Northumberland National Park is one of the most lightly populated areas of the country and the stock farming way of life hasn't changed for centuries. The farming community of Tarset already has wonderful ancient haymeadows so it is fitting that they also have one of the country's best Waxcap sites.
"This site shows what can be found with the help of farmers and local residents and adds to our understanding of the distribution of these species and their habitats. It is accepted by mycologists that Britain is the best place in Europe for this assemblage of fungi and finds like this help confirm the commonly-accepted idea that rural upland areas such as the farmed landscape of Northumberland National Park will prove to be a stronghold in Western Europe."
Waxcaps are jewel-like fungi that seem to appear overnight when conditions are mild and wet in autumn which like unimproved grassland that is often rich in moss and grazed by cattle or sheep.
Their presence indicates an ancient landscape untouched by plough or fertilizer and they can be recognised by their array of bright colours - red, pink, white, orange, yellow and green.
In December the Northumberland National Park and Kielder Water and Forest Park were together named as the largest area of protected night sky in Europe.
Named the Northumberland Dark Sky Park, it was granted Gold Tier Dark Sky Park status by The International Dark Skies Association.
The 1,500sq km area is the first of its kind in England and one of the largest in the world.