In 1931 Mustila Arboretum acquired some seeds of Rhododendron aureum from the Pungsan Mountains of North Korea. But the seedlings turned out to be something quite different: a previously poorly-known Korean form of the Japanese rhododendron (R. brachycarpum), a sturdy-growth variety with large leaves and flowers that in 1970 was named Rhododendron brachycarpum subsp. tigerstedtii and subsequently known in Finland as the Mustila Rhododendron.
The Mustila Rhododendron was the only rhododendron variety to survive the exceptionally cold winter of 1939–1940. Being unusually hardy, it provided a basis for the hybridisation programme begun at the University of Helsinki in 1973 using the hardy varieties already growing at Mustila and the Mustila Rhododendron as its main maternal parent. The resulting varieties have been on sale since 1990, but the Mustila Rhododendron itself is hardly produced anywhere commercially.
The original old Mustila Rhododendrons at the southern end of Rhododendron Valley are easy to spot by virtue of their size alone. The leaves are dark green, shiny and large, and they protect themselves from cold and drought by winding themselves into tight rolls even at temperatures only just below zero. The Mustila Rhododendron blossoms abundantly roughly once in three years. The flowers are like open bells, white with brown dotting, and break in the first half of June.