'Boule de Neige' is one of the old cultivars of rhododendron that, because of its exceptionally good frost hardiness, has been classed as an ironclad variety in the Northwest US states. It was hybridised by the French nurseryman Oudin in the latter half of the 19th century. Its maternal parent was the Siberian Rhododendron (R. caucasicum) and the pollinator an unknown hybrid of the resilient Catawba Rhododendron (R. catawbiense). 'Boule de Neige' (meaning Snowball) blossoms at Mustila at the beginning of June in pure-white, beautiful round trusses.
'Boule de Neige' grows into a thick round shrub and reaches a height of about 1.2 metres in 10 years. It has thrived at Mustila ever since the 1930s and blossoms abundantly each year, but it has never grown to more than just over a metre over the decades.