Complimentary 
NEW SERIES VOL. VII 
NO. 8 
ARNOLD ARBORETUM 
HARVARD UNIVERSITY 
BULLETIN 
OF 
POPULAR INFORMATION 
JAMAICA PLAIN. MASS. JUNE 2. 1921 
Hybrid Rhododendrons. It is to the hybrids and not to the species 
of Rhododendrons that our gardens are most indebted. The history of 
many of these hybrids is obscure, and the records of their breeding 
have been so badly kept that it seems practically impossible to obtain 
the information about them needed to continue intelligently the breed- 
ing of Rhododendrons with the view of obtaining hardier races for New 
England gardens. The plants which have been imported from Europe 
in the last seventy years in numbers running up into the hundreds of 
thousands are practically all the so-called Catawbiense Hybrids. These 
hybrids were obtained in the first place apparently by crossing Rhodo- 
dendron catawbiense with R. ponticum, a Caucasian species not hardy 
here, and with R. maximum. Later the red-flowered Himalayan R. ar- 
boreum was crossed either with R. catawbiense directly or with its 
hybrids. Probably other Indian species were used in these crosses, 
which appear further to have been more or less crossed among them- 
selves. Several hundreds of these, hybrids have received names, but 
only a comparatively small number have proved hardy in this country, 
those in which R. catawbiense and R. maximum preponderate being 
naturally the hardiest, although a few of the hybrids with red flowers 
showing the influence of R. arboreum are hardy here. 
Some of the Rhododendrons which have proved hardy here are evi- 
dently hybrids of the pale yellow-flowered Rhododendron caucasicum, 
a shrub which grows at high altitudes on the mountains of the Cauca- 
sus and of Asia Minor. These hybrids, or those of them which have 
been successfully grown in the Arboretum, are low shrubs with compact 
clusters of pink, white or red flowers which open from two to three 
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