It consists of a spring-fed marshy fen or mire with reeds and sedge, and drier calcareous grassland containing a diverse range of flora.

It has a history of poaching and fox hunting, but since the late 19th century, the attention of botanists has been drawn to its large variety of flowering plants.

[10] The SSSI was first notified in 1954, the notification being revised on 13 January 1984, because it retains "the original grassland and marsh communities of Farnham Mires."

[11] This site is "one of a very few remaining sedge and rush dominated marshland communities in the Vale of York," the others being Upper Dunsforth and Kirkby Wharfe SSSIs.

[9] The special wetland plants noted in Natural England's citation for the SSSI are: Oenanthe lachenalii or parsley water dropwort, brookweed and bog pimpernel.

Therefore the underground aquifers should not be compromised by commercial water extraction, nor should they be contaminated by heavy use of agricultural fertilisers or by industrial pollution.

[16] The calcareous grassland potentially supports a large diversity of plants, but only if maintained to prevent it being overgrown with rank grasses and scrub.

Light winter grazing is recommended, to promote vegetation of varying height, and that in turn will support invertebrates and other wildlife.

[16] When the site was assessed in 2015, the 8.366 hectares (0.0837 km2; 0.0323 sq mi) of lowland calcareous grassland was judged to be in favourable condition, but under medium threat risk.

In the same year, the 1.9681 hectares (0.01968 km2; 0.007599 sq mi) of lowland fen, marsh and swamp (the mire) was judged to be in unfavourable condition and recovering, but to be under a high threat risk.

[19] In 2018 an application to the Environment Agency regarding the development of a poultry or pig unit was accepted for a site within 5,000 metres of Farnham Mires.

[21] Other nearby SSSIs are:[22] Bishop Monkton Ings,[23] Cow Myers,[24] Hack Fall Wood,[25] Hay-a-Park,[26] Kirk Deighton[27] Mar Field Fen,[28] Quarry Moor[29] and Ripon Parks.