The plants – now titled Rhododendron Hybrids Bred at Bodnant Garden 1927-1983 – were raised at the garden near Conwy in North Wales from parent plants brought back from Asia by explorers.
The first rhododendron came to Bodnant Garden in 1910. The garden's owner, Henry Duncan McLaren (second Lord Aberconway) and head gardener Frederick Puddle developed a successful breeding programme, extending the flowering season, producing stronger plants and breeding smaller plants for the domestic market. The most famous of the latter, Rhododendron 'Elizabeth', is still one of the UK's most popular rhododendrons.
More than 300 hybrids were raised and registered to the garden, but Bodnant now hosts only 115 varieties, Bodnant's nursery specialist Alison Clarke said.
Those threatened with extinction - including some that exist only as a single specimen - are being propagated through grafting, layering and micropropagation. Staff are searching for the others both at Bodnant and at nurseries and other gardens which may have them.
Clarke added: "Many of the 'missing' plants are thought to be still in the garden but have lost their labels so we are working with metal detectorists from Mold Historical Society. In future DNA analysis may also help identify plants thought to be missing hybrids."
Head gardener John Rippin said the achievement will help safeguard the rhododendrons for the future.
"The history of the Bodnant Hybrids is so closely intertwined with the social history of the early twentieth century; the search for new and ever more beautiful plants by the intrepid plant hunters, the obsession of wealthy garden owners in amassing great collections of exotic plants and the skills of their gardeners who sought every way possible to give their employers the edge over their ‘show bench’ rivals in creating better forms and colours of flowers through extensive hybridisation programmes."