The National Trust has been carrying out a survey of its gardens to establish what species they contain.
According to the Royal Horticultural Society, the research has yielded some surprising results, with numerous rare cultivars being discovered.
Some of the specimens – such as the Weinmannia trichosperma found in Northern Irelands Mount Stewart – are believed to be over 100 years old.
Many are said to be of historical importance or not found anywhere else, such as the Creeks Cross Rhododendron, which was originally created by Alfred Creek – head gardener at Trengwainton in the 1930s.
According to Mike Buffin, the man in charge of the study, the National Trust only knew of a small proportion of the plants it had in its possession.
"Were not a botanical institution and historically we were more interested in whether a landscape had an interesting feel and historical presence rather than what was in it," he said.
Other specimens to be discovered as part of the research include the Quercus robur Purpurascens purple oak at Belton House in Lincolnshire.
There are more than 300 houses and gardens on the National Trust register, with 52,000 volunteers helping to look after them.