'Kullervo' is a small round shrub. The red buds open in June as small pink bell-shaped flowers, changing within a few days to almost white.
'Kullervo' is the result of crossing the Mustila Rhododendron (R. brachycarpum var. tigerstedtii) with the Yak Rhododendron (R. degronianum subsp. yakushimanum), in habit, flower and hardiness closely resembling the latter parent. At Mustila it has wintered reasonably well, though without snow cover the buds have been damaged by -23°C frosts; the leaves are hardier.
'Kullervo' grows to less than a metre in 10 years.