Orchids are one of the most glamorous and diverse plant groups on earth, with 28,000 species worldwide. However, only 51 are known in Britain and Ireland – and just 22 grow in Hertfordshire, according Co-county Flora Recorder, Ian Denholm.

Giving the 2026 Gerald Salisbury Memorial Lecture on 22 January he provided an overview of all the county's extant species and seven others that have disappeared in the past 60 years. These ranged from those like Pyramidal Orchid (Anacamptis pyramidalis) that is thriving in a warming climate to the exceptionally rare Military Orchid (Orchis militaris) that flowered on a Herts. gravel extraction site for two years before being destroyed in 2020.

A video of Prof. Denholm's lecture, written with fellow orchid expert Prof Richard Bateman, can be viewed on the HNHS YouTube site.

He explained that difficulties predicting where and when some species will be found relate to their tiny seeds, which can be wind-blown over long distances, and their need for a symbiotic relationship with mycorrhizal fungi. Often germination requires one type of fungus, developing a leaf rosette requires another. It may be several years before flowering takes place.

An identification problem confronting botanists is a capacity across some species for hybridisation, resulting in flowers displaying untypical colour and marking variations. Prof. Denholm described how hybridisation had occurred between Common Spotted-orchid (Dactylorhiza fuchii) and Southern Marsh Orchids (Dactylorhiza praetermissa) at Roughdown Common, Boxmoor. After the site was split by the A41 dual carriageway, an even more confusing 'introgression' occurred where hybrid plants back-crossed with Common-Spotted-orchids.

One orchid species that appears to be extending its range into Herts. is the impressive Lizard Orchid (Himantoglossum hircinum). By contrast, the Burnt-lip Orchid (Neotinea ustulata), once found at Therfield Heath, and the Marsh Fragrant-orchid (Gymnadenia densiflora), once known at a site near Ardeley have not been found in the county for 30 to 40 years.

The lecture, jointly organised by HNHS and the Welwyn Natural History Society, took place at Welwyn Civic Centre in front of an audience of more than 50 members of the two societies and guests.

Photo: Ian Denholm giving his lecture © Tim Hill