Of the large-leaved rhododendrons, yellow-flowered rosebay (Rhododendron aureum) has spread the furthest north, its range including eastern Siberia, the coast of the Sea of Okhotsk, Kamchatka, the Sakhalin and Kurile Islands, Japan, Manchuria, and Korea, usually at and above the tree-line. Its winter hardiness is excellent though it is considered difficult to grow for various reasons. In the Amur area and on the coasts of Russia’s Far East a forest form of the yellow-flowered rosebay is found, which the Russians sometimes consider a species in its own right, R. hypopitys. This form grows in moist spruce forests, taller and generally larger than the usual mountain form.
This forest form is, according to the literature, completely unknown to Western collections. At Mustila, however, it has been grown since the early 1980s, a Finnish-Soviet expedition having collected seed along the lower reaches of the River Amur in autumn 1976. The resulting shrubs were planted in Tuijalaakso (White Cedar Valley) in a moist depression where they have thrived, with a beautiful dense habit, reaching about half a metre high. The flowers are pale yellow, opening in early May as soon as the snow has melted. Flowering has been sparse at Mustila, probably due to lack of light and an over-extended growing season. The foliage appears early, which means it sometimes suffers damage from late spring frosts.
The same provenance has grown well and flowered as far north as Oulu and Kuusamo. It would seem that this variety is best suited to eastern and northern Finland, as it prefers a more continental, less changeable climate, with long snowy winters.
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