We mentioned the National Memorial Arboretum. The term arboretum was first used by John Claudius Loudon in 1833. Presumably he thought that since every plant carries a Latin name, their gardens might as well carry a Latin tag, too. However, the botanic garden was established long before Loudon hit on the name.
In addition to two national arboretums, there is also the Royal Botanic Gardens at Kew. That is quite the place.