Watts With the Species: Rhododendron brachycarpum
Sometimes referred to as "the hardiest rhododendron in the genus," Rhododendron brachycarpum has been successfully grown in Iceland and is included in the collection of rhododendrons in Mustila Arboretum in southern Finland where, according to Peter Cox, it has survived a low of -49°F.
Rhododendron brachycarpum is native to north and central Japan and is widely distributed in the mountains of this region often growing above timber-line. Leach reported R. brachycarpum as "particularly evident above the tree line on Mt. Fujiyama. It is found near sea level in Hokkaido, as far north as the Kuril Islands, into mid and south Korea. The variety known as R. brachycarpum ssp. brachycarpum Tigerstedtii Group is evidently native to Korea and the Korean island of Dagelet (Ullung Do).
Rhododendron brachycarpum is generally considered to have a rounded, compact habit, but seed grown plants vary considerably in their growth habit, some being very compact and dense while others from the same seed capsule will assume a much more open, upright growth habit. Plants grown in the shade will be much more open than plants grown in a more sunny location. The shade grown plants generally have leaves with a much darker green color, while the sun grown plants leaves often have a lighter yellow-green appearance.
This rhododendron is particularly valuable as a landscape plant because of its late flowering. The attractive small compact trusses are composed of from up to 20 flowers cream to pink in color with brownish-green spots in the throat and generally appear in late May to late June. An attractive feature of this plant is the new foliage. Often appearing as early as April, the new leaves are held in an upright manner proudly displaying their attractive light fawn colored indumentum.
R. brachycarpum flower
Photo by Lynn Watts
New foliage of R. brachycarpum
Photo by Lynn Watts
When we acquired the Caperci collection in 1981 we inherited a large number of R. brachycarpum which Jim had grown from seed sent to him by several of his Japanese friends. The variability of these plants was evident at a very early stage. The Tigerstedtii Group was much more robust and larger in all of its parts, the leaves being up to 7 inches in length. The most compact form Jim called the Sado Island form, much denser and more compact in its growth habit.
In addition to being an attractive landscape plant, Rhododendron brachycarpum will extend the blooming season of your collection. If you have not already included R. brachycarpum in your species collection you will be amply rewarded when you do.
Lynn Watts is the ARS Western Vice President.