THE CULTURE OF RHODODENDRONS 
The following notes are intended primarily for the general gardener in the hope that he, or she, might be encouraged 
to grow a wider variety of Rhododendrons. Perhaps there may be some observations of interest to the specialist who is 
seeking to improve his cultural methods and certainly I would be delighted to receive exchanges of ideas and techniques 
from all of you who treasure this genus as one of the most beautiful of evergreen flowering shrubs. 
Since 1921, when first I began my apprenticeship as a nurseryman, I have had the good fortune to be associated 
with growers whose skill I have noted and from whom I have gained much valuable information. The old time nursery- 
men with Dobbie & Co. of Edinburgh, Scotland; the members of the staff at the Royal Botanic Gardens, Edinburgh; the 
instructors and professors at the University of California in Berkeley; and the many friends and growers on the Pacific 
Coast have all contributed, subconsciously perhaps, to these notes. 
BOTANICAL 
The genus Rhododendron is a member of the Heath family 
and contains over 800 species which occur mainly in the tem- 
perate regions of the northern hemisphere. A few outlying 
species are native to Sumatra, Java, New Guinea, Philippine 
Islands, with the most southcriy of all in the northern part of 
Australia. In the United States there are seventeen native 
species, including the Azaleas, which are Rhododendrons, bo- 
tanically speaking. So far as garden hybrids are concerned, 
the most important native species in this country are the Great 
Laurel (Kk. maximum), found growing wild in New England 
and extending southwards through the Appalachians to north- 
ern Georgia, and the Mountain Rose Bay (ix. catawiense) which 
is found on the highcr elevations of the Allegheny and Appa- 
lachian Mountains. As ornamentals, these two species are not 
to be compared with our present day hybrids, but their influ- 
ence on these hybrids has been and will continue to be most 
extensive. 
It is interesting to note that the first Rhododendron species 
to be introduced into England was R. maximum whose flower- 
ing was recorded in 1756. I. ponticum arrived in England in 
1763, R. caucasicum in 1803 and R. catawbiense in 1809. The 
famous scarlet tree species, Fk. arboreum was introduced from 
China in 1802 and once seen in flower was quickly combined 
with the earlier arrivals to give us a combination from which 
many of our finest hybrids have been derived. About the middle 
of the nineteenth century, botanical explorations to the Indian 
Himalayas, Tibet, and Western China became more frequent 
and a vast array of new species began to find their way into 
the gardens of Europe to be cultivated and constantly improved 
so that today we are beginning to reap a harvest of beauty from 
a genus whose variations in color, floral shape, and leaf pattern 
are unexcelled. In the provinces of Southwestern China and 
on the slopes of the Himalayas, Rhododendrons exhibit a 
wide range of growth characters. Dwarf species, barely a foot 
in height, extend for acres, clothing the alpine meadows above 
14,000 feet altitude and resembling their cousins the heathers, 
only more colorful with shades of yellow, purple, and rosy pink 
flowers. At lower altitudes, the larger shrubby species domi- 
nate the vegetation and seas of yellow, crimson, and white 
paint the hillsides in spring. In the forested areas, tree species 
such as Fk. grande are known to attain a height of 60 feet and 
specimens have been seen whose trunks measured 4 feet in 
circumference. In the more humid zones and at lower altitudes, 
species with the fragrance of spice are found and oftentimes 
occur as epiphytes, growing in the crotches of Oak and Mag- 
nolia trees and lighting the forest gloom with their pristine 
loveliness. 
Consider the contrasts and variations in this aggregation 
of wild types and think of the re-combination of their characters 
which is possible to the plant breeder! Each year, many new 
hybrids are being created by enthusiastic growers who are striv- 
ing to make tender ones more hardy and hardy ones more 
beautiful. It is safe to predict that before many years have 
passed, the tough characters of our native species will be com- 
bined with the clear colors and interesting leaf forms of their 
Chinese relatives so that the gardens in Maine might be graced 
with the loveliness which is now limited to the sheltered gardens 
of Long Island and the Pacific Northwest. 
SOIL 
During a recent trip to the slopes of Mt. Hood where the 
blossoms of our native Rhododendrons (R. macrophyllum) were 
enjoyed, it was noted that the soil in which they were growing 
was almost pure coarse grit and apparently devoid of humus 
although there was ample moisture present. This type of soil 
is in sharp contrast to the forest loam in which the same species 
is found growing in the coastal areas and where, incidentally, 
the plants are more lush. The accounts of botanical travelers 
tell us that in the alpine meadows of Yunnan, some species are 
found along the margins of boggy meadows and then again 
others are found thriving on thin layers of humus overlying 
rocky stratas and irrigated by the melting snows from above. 
These variations in soil types provide a clue to the requirements 
of Rhododenérons in cultivation. An abundance of moisture at 
the roots is essential, particularly during spring when the plants 
are putting out their greatest growth. 
Sharp drainage along with moisture is also essential for with- 
out air, the roots will not function properly and the plants show 
signs of distress. Their roots are exceedingly fine and hair like. 
They are confined to the upper 9 to 12 inches of soil and because 
of their fineness are unable to penetrate a heavy clay nor are 
they able to survive in a wet soggy medium. It has been demon- 
strated by soil scientists that Rhododendrons succeed best in an 
acid medium preferably with pH reading of between 5 and 6. 
This may not mean too much to the home gardener whose con- 
cern is chiefly with the physical structure rather than the 
chemistry of the soil. /f roses, pansies, and liies are thriving 
and if the temperatures are not extreme nor humidity too low, 
then Rhododendrons may be grown with reasonable success. A 
good soil should contain an abundance of humus. If it runs 
together when wet, or packs and becomes cracked when dry, 
then add generous amounts of peat moss or decaying leaves. 
MIXTURE OF PEAT, 
LEAF SOIL & LOAM 
HEAVY CLAY SOIL 
Dd FE Nd at calla AAD ta 
ae 
In the case of sticky clay soils, it would be wise to build a bed 
of loose humusy material on top of the clay and then plant the 
Rhododendrons, rather than attempt to make the original soil 
friable by digging in the humus. If this is not feasible, then 
apply a dressing of gypsum (calcium sulphate) at the rate of 
four pounds per 100 square feet and cultivate it in. This will 
flocculate the clay and make the soil easier to work. Bulky 
material in the form of peat or leaf mold or decayed pine 
needles may be incorporated to provide the porosity which is 
so essential. In sandy soils, the addition of humus is also a 
necessity, not so much to keep it open but to help retain mois- 
ture during the dry months. Black vegetable muck soils or loams 
which have been heavily enriched with nitrogenous fertilizers 
are not too desirable. When Rhododendrons are planted in 
this type of medium the root mass remains quite small, since 
they are not gross feeders, and in the event of a long dry period, 
the plants are more susceptible to damage. In such soils they 
should be planted in pure peat moss or leaf mold. Better still, 
a combination of the two. The cardinal points to remember are 
(1) Lots of humus, (2) Sharp drainage, (3) Plenty of moisture. 
PLANTING 
As was mentioned in the pre- 
vious paragraph, the roots of 
Rhododendrons are confined 
to the top 9 or 12 inches of the 
soil, It therefore follows that 
they should be planted so that 
the top of the root ball, as it 
arrives from the nursery, is 
planted not more than one 
inch beneath the surface. If the 
roots are placed too deep, it 
is not long before the foliage 
turns a sickly yellowish green, 
/ DIP ROOT 
fi BALL IN 
i! WATER 
BEFORE PLANTING 
e Copyright, 1951, by P. H. Brydon. All rights reserved. Printed in U.S.A. 
