G. densiflora was first described as a variety of G. conopsea by Wahlenberg in the 18th century and its subsequent taxonomic position has see-sawed between variety and full species classification. A study in 2007 has however confirmed its species status, having concluded that it is genetically distinct from the other four G. conopsea group species to be found in Europe.

Whilst all of these taxons share a family resemblance, G. densiflora shows a particularly close similarity to both G. borealis and G. conopsea. These morphological likenesses would be problematic from an identification point of view, but for the fact that there are few areas of distribution overlap, a situation which it is supposed arises from the differing habitat requirements of the three species. The above mentioned study of UK populations in 2007 discovered that there was little genetic crossover between the species, attributing this to habitat separation and also to slightly different flowering times.

G. densiflora can be found throughout much of Europe, where it favours fens, wet flushes or marshy ground on alkaline soils and often in montane conditions. These preferences give rise to its common name, the Marsh Fragrant Orchid. It is however tolerant of lower altitudes and can occasionally be found in drier environments such as sand dunes or even damp chalk grassland. Where this occurs it may come into contact with G. conopsea but in these situations, hybridization is not at all as common as was once thought, with the two species remaining quite distinct. G. densiflora is a robust plant up to a metre high with a long cylindrical inflorescence of up to 150 densely packed individual flowers. The broad, strongly keeled leaves form a sturdy rosette at the base and are easily distinguished from the rather weedy leaves of G. conopsea and G. borealis. The pictures are from Cumberland and Hampshire, UK, dating from July.