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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Volume 5, Number 1
January 1951

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Ratings for Rhododendron Species
By J. Harold Clarke, Long Beach, Washington

        The first American Rhododendron Society variety ratings were published in the preceding issue of the A. R. S. Quarterly Bulletin, Vol. 4. No. 4, Oct., 1950. In that article both the British and American rating systems were explained and so will not be given in detail here.
        However, for the benefit of those who may be reading this without having the other at hand it should be stated briefly that the British hardiness ratings go from (A), hardiest, to (F), very tender, usually greenhouse subjects. Requirements as to shade enter into the R. H. S. ratings which make them a bit difficult to interpret for Americans not familiar with growing conditions in the British Isles. The A. R. S. hardiness ratings indicate the minimum temperature which the plant may be expected to stand with no appreciable injury to leaf, stem or bud. H-1 Hardy to -25 F; H-2 Hardy to -15 F; H-3 Hardy to -5 F; H-4 Hardy to +5 F; H-5 Hardy to +15 F; H-6 Hardy to +25 F; H-7 Hardy to +32 F.
        The R. H. S. and A. R. S. quality ratings are similar in that **** and xxxx respectively indicate the highest quality. However, a "-" in the A.R.S. system indicates that there is not sufficient information at hand to rate the variety, whereas in the R. H. S. system a "-" may mean either that it has not been given a rating or that it is not deserving of even one *. The A.R.S. uses "o" to indicate varieties or species not worthy of even one x.
        It is generally understood, of course, that any ratings are suggestive rather than definitive. The many factors which may affect hardiness were discussed in the last issue and it was pointed out that quality ratings simply represent the combined opinion of the individuals cooperating in establishing the rating. These two things are rather obvious, that the more cooperators there are the more valuable the rating, and that ratings are always subject to revision as more information becomes available.

Species More Difficult To Rate
        Because of the greater variability of the plant material group varieties are more difficult to rate than clonal varieties. Species are still more difficult because: 1. there are fewer observers familiar with most of the species; 2. the material is seed propagated, often unselected and hence quite variable; 3. there is greater likelihood of material being wrongly named; 4. the source of seed may be from the wild or from some collection; 5. an individual plant may be a natural hybrid but considered by its owner to represent a species.
        Because of this great variability the A. R. S. ratings are designed to be representative of the best and hardiest forms and not the average. Seedlings of some species would undoubtedly range from o to xxxx and an average of that would probably not be as useful to growers as would an indication of how good certain forms of the species can be. This great variability has been pointed out in augustinii, arboreum, obtusum, cerulean, campanulatum and others. The rating, of course, is not based on a single clone which might represent a natural but unsuspected hybrid or a distinct mutation, but on forms obtainable in some quantity from seed.

Sources Of Information
        When the questionnaires used in obtaining the variety ratings were examined it was found that 142 different species had been written in by various growers, some being rated by several individuals. It seemed at first that a great deal of work should be done before species ratings would be ready for publication. However, there have been several requests for such ratings and it seemed that publication of a tentative list might stimulate observation and discussion which might lead to the revisions necessary to achieve the greatest possible accuracy and usefulness. Accordingly a group was called together on Nov. 30, 1950 at the home of the Editor of the Bulletin. The following were present, John G. Bacher, Portland; Mr. and Mrs. D. W. James, Eugene; Rex Peffer, Salem; Mr. C. L. McDonald, Salem; Mr. Ollie Schendel, Salem; Mr. Walter A. Barkus, Salem; Mr. John Henny, Brooks; Mr. Rudolph Henny, Brooks; and the writer who acted as compiler of the observations and opinions of the group. All of these people were from one state, Oregon, except the writer. However, temperatures in the gardens represented were as low as -18 F last winter and those present had had ample opportunity to observe injury due to low temperature. The English ratings were taken from the 1947 R. H. S. Handbook. A tentative list was made up and later shown to Dr. Clement Bowers, Dr. Donald Wyman, and Mr. Henry Skinner each of whom offered some criticism and suggestions. The writer accepted some, but not all, of these suggestions, trying to arrive at conclusions which would represent as nearly as possible, the combined opinions of the eastern and western growers.

Suggestions For Revision Are Welcome
        It is hoped that those disagreeing with any of the ratings will write the author of this article. Those species questioned will be listed and made the subject of further study, probably by means of another questionnaire at a later date. Revisions would then be published if the replies so indicated.
        It should be pointed out that failure of a particular species to survive in a given garden may be due to over-all lack of hardiness, but it may also be due to a particularly tender strain, or to drought, lack of sufficient acidity, too dense a soil, lack of organic matter, lack of shade or too much shade, etc' Instances of failure to. survive, therefore, may not be as valuable evidence in rating a species as the more positive evidence of survival under known conditions.
        Those rhododendron species of the Azalea Series have been grouped together for the convenience of the reader.


SPECIES Hardiness Quality Hardiness Quality
achroanthum A - H-2 o
adenogynum A * H-2 x
aeruginosum A * H-2 -
agastum D - H-4 -
agetum - - H-4 -
agglutinatum C - H-2 x
ambiguum A * H-4 o
amesiae A - H-4 -
aperantum B ** H-3 -
apodectum A *** H-4 -
arboreum D **** H-4 xxxx
argyrophyllum A ** H-3 o
arizelum C ** H-4 -
astrocalyx C ** H-4 xxx
augustinii C **** H-4 xxxx
auriculatum B ** H-2 xxxx
auritum C * H-5 xx
baileyi C ** H-4 -
balfourianum B - H-2 o
barbatum B **** H-4 xxxx
basilicum B ** H-3 xx
bauhiniiflorum C -- H-1 -
beanianum B *** H-4 xxx
beesianum C ** H-2 o
brachycarpum A - H-2 x
brevistylum B * H-4 o
bullatum D **** H-5 xxxx
bureavii B * H-2 xx
burmanicum F ** H-5 xx
cerulean - *** H-3 xxxx
callimorphum B *** H-4 xxx
calophytum B *** H-3 xxxx
calostrotum A *** H-3 xx
caloxanthum B *** H-2 xxx
campanulatum B ** H-2 xxx
campylocarpum B **** H-4 xxx
campylogynum A ** H-2 xx
camtschaticum A - H-2 o
cardiobasis - - H-4 xx
carneum F ** H-7 xxx
carolinianum A - H-2 xx
catawbiense A - H-2 x
caucasicum A * H-4 x
chaetomallum C *** H-4 xxxx
charianthum B ** H-4 x
chartophyllum B ** H-3 xx
chasmanthum C **** H-4 xxx
chrysanthum B - H-3 o
chryseum A ** H-2 xx
ciliatum C *** H-4 xxx
ciliicalyx F *** H-6 xxx
cinnabarinum B **** H-4 xxx
cinnamomeum C *** H-4 xxx
citriniflorum C ** H-4 xx
concatenans C *** H-4 xxxx
coriaceum C - H-4 xx
crassum D *** H-4 xx
crinigerum B ** H-2 o
croceum C *** H-4 xxxx
cuneatum A * H-2 x
cyanocarpum C ** H-4 xx
dalhousiae F *** H-6 xxxx
dauricum B * H-2 xx
davidsonianum B *** H-4 xxx
decorum C ** H-4 xxx
degronianum B * H-3 x
delavayi E *** H-5 xxx
deleiense C *** H-4 xxx
desquamatum D ** H-3 xx
diacritum A - H-3 o
diaprepes D ** H-4 xxx
dichroanthum B *** H-4 xx
didymum B ** H-3 xxx
discolor B ** H-3 xxx
drumonium B ** H-3 xx
eclecteum C ** -3 -
edgeworthii F ** H-6 xx
elegantulum C - H-2 o
elliottii D **** H-4 xxxx
eriogynum D **** H-4 xxxx
euanthum C * H-4 -
euchaites C **** H-4 xxxx
exasperatum C - H-4 o
eximium C *** H-4 xxxx
exquisitum - *** H-4 xxx
falconeri C **** H-4 xxxx
fargesii B *** H-4 xxx
fastigiatum A *** H-2 xxxx
fauriei B - H-3 x
ferrugineum A * H-2 o
fictolacteum B ** H-3 xxx
flavidum A *** H-2 x
floccigerum C * H-3 x
fortunei B *** H-3 xxx
fulvoides B ** H-4 x
fulvum B ** H-4 x
fumidum C - H-3 o
galactinum A - H-3 o
glaucum B ** H-4 x
glischrum C * H-4 -
glomerulatum A - H-3 x
grande E **** H-4 xxxx
griersonianum D **** H-4 xxxx
griffithianum E **** H-4 xxxx
gymnocarpum A ** H-3 xxxx
haemaleum A ** H-3 xx
haematodes B **** H-4 xxxx
heliolepis C * H-3 x
hippophaeoides A **** H-3 xxx
hodgsonii B ** H-4 -
impeditum A **** H-2 xxx
insigne A *** H-4 xxx
intricatum A *** H-2 xx
irroratum C ** H-4 xx
johnstoneanum C *** H-4 xxx
keiskei A ** H-2 xx
keleticum A ** H-3 xxx
keysii C ** H-4 xx
kyawii E **** H-6 xxxx
lapponicum A - H-2 -
ledoides C *** H-4 xxxx
lepidotum A-C * H-3 x
leucaspis C *** H-4 xxxx
lindleyi E *** H-6 xxxx
litiense C ** H-4 xxxx
lutescens C *** H-4 xxx
macabeanum C **** H-4 xxxx
macrophyllum A - H-2 o
maddenii E *** H-6 xxx
makinoi B ** H-3 x
mallotum C ** H-4 xxx
martinianum C ** H-4 -
maximum A - B-2 x
megacalyx E *** H-6 -
megeratum D ** H-4 xx
micranthum A - H-1 o
minus C - H-2 x
moupinense B **** H-4 xxxx
mucronulatum B **** H-2 xxx
myrtilloides A *** H-3 xxx
neriiflorum C **** H-5 xxxx
niphargum B ** H-3 o
nuttallii F **** H-6 xxxx
orbiculare B **** H-3 xxxx
oreodoxa B ** H-3 xx
oreotrephes B *** H-3 xxx
orthocladum A ** H-2 xxx
pachytrichum C - H3 xxx
pagophilum C - H-4 x
parryae E - H-6 -
pemakoense A ** H-3 xxx
phoenicodum C ** H-4 x
polyandrum D *** H-6 -
polylepis B - H-3 o
ponticum A - H-4 o
praevernum C ** H-4 xx
prostratum A * H-3 xx
pruniflorum C ** H-3 x
pubescens B *** H-3 xx
racemosum A **** H-2 xxxx
radicans B **** H-3 xxxx
ravum A - H-2 x
repens A **** H-3 xxxx
rhabdotum E **** H-6 xxxx
ririei C ** H-4 x
rubiginosum A *** H-2 xx
rufescens C - H-4 -
russatum A **** H-2 xxxx
saluenense A *** H-3 xxx
sanguineum B *** H-3 xxx
sargentianum C ** H-3 xxx
scabrifolium D *** H-3 o
scintillans A **** H-3 xxxx
scyphocalyx B - H-4 o
semilunatum C - H-4 -
semnoides - - H-4 xx
shweliense C - H-3 -
silvaticum - - H-5 x
sinogrande D **** H-4 xxxx
smirnowii A * H-2 x
souliei B **** H-4 xxxx
sperabile C ** H-4 xx
sperabiloides - *** H-4 xx
sphaeranthum C *** H-4 xxxx
spinuliferum C *** H-4 xxx
stewartianum C *** H-4 xxx
strigillosum D **** H-4 xxxx
sutchuenense B *** H-4 xxx
taggianum E ** H-6 xxxx
telmateium A ** H-3 xx
tephropeplum C *** H-4 xxx
thomsonii B **** H-4 xx
timeteum C *** H-3 xxx
traillianum B - H-4 -
triflorum D * H-3 o
ungernii A ** H-4 xx
valentinianum D *** H-5 xx
venator C *** H-4 xxx
vilmorinianum B ** H-4 xx
wardii C *** H-4 xxxx
williamsianum C **** H-3 xxxx
xanthocodon C *** H-4 xxxx
yungningense A - H-3 xxx
yunnanense B **** H-3 xxxx

Azalea Series

SPECIES Hardiness Quality Hardiness Quality
alabamense C * H-4 x
albrechtii B **** H-4 -
arborescens A ** H-2 xx
atlanticum B ** H-2 xx
calendulaceum A ** H-1 xxx
canadense A * H-1 x
cumberlandense - - H-1 xxx
japonicum A *** H-2 xxx
linearifolium B - H-3 o
luteum A ** H-3 xx
molle C **** H-2 xx
mucronatum C **** H-3 xxx
nudiflorum A * H-1 x
obtusum A **** H-3 xxx
occidentale A *** H-3 xxx
pentaphyllum C **** H-3 xxx
prunifolium C *** H-4 -
quinquefolium B *** H-3 xx
reticulatum A *** H-3 xx
roseum A * H-1 xx
schlippenbachii C **** H-1 xxxx
vaseyi B **** H-1 xxx
viscosum B *** H-1 xx
yedoense B ** H-2 x

Volume 5, Number 1
January 1951

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals