QBARS - v24n3 Survey of Rhododendron Species of Washington, D.C. and Vicinity

A Survey of Rhododendron Species of Washington, D.C. and Vicinity
Richard T. Johnston

U. S. National Arboretum, Washington, D.C. The object of this survey was to compile a list of rhododendron species growing in Washington, D.C. and vicinity, and to obtain some evaluation of success based on the experience of those persons growing them. Information was collected on 184 species in 24 collections by means of a questionnaire or interview. It is based on growers' experiences in private gardens and in the U.S. National Arboretum, the majority of gardens being found inside of the area encircled by the Capitol Beltway 1495.

Table 1 lists species together with the number of locations, the total number of plants found in these gardens, age and/or size of the plants, the growers' evaluation of performance, and general comments. These comments where given represent a summary or condensed statement.

Table 2 lists species being grown but as yet unevaluated due to a lack of sufficient exposure.

Table 1: Rhododendron species being grown in Washington D.C. and vicinity.


Species
Number of
Locations 1
Number of
Plants 2
Age or Size 3
Performance 4
Comments

aberconwayi
5
6
2'
S
best bloom '69

adenophorum
1
1
3yr / 1'
P
apparently hardy

adenopodum
6
8
l1yr
S
needs sun to bloom

alabamense
3
3
15yr
S
likes sun

albrechtii
5
12+
5'
S

ambiguum
2
3
1'
U
winter killed

arborescens
6
8
l5yr / 8'
S
fragrant

arboreum
3
40
3yr / 2'
P
terminal dieback each year

argyrophyllum

1
2'
P
hardy in foliage; no buds yet

arizelum
1
1
2yr
U
heat killed

artosquameum
1
2
2yr / 1'
U
winter killed, perhaps frost heaving

atlanticum
4
6
35yr
S
suckers forming colonies: very fragrant

augustinii
8
29
5'
S
not reliably hardy

auriculatum
4
10
10'
S
slow starter

auritum
2
3
2'
U
winter killed

austrinum
4
7
16yr / 8'
S

bakeri
6
8+
15yr
S
mountain plant; slow growth and color loss here

barbatum
3
3
4yr / 2'
P

brachycarpum
7
16+
5yr
P
interesting foliage

bureavii
2
32
2yr / 6"
U
heat killed

calendulaceum
12
48
15'
S
excellent. but few flowers in shade

calophytum
5
6
9yrs
P
very slow growth but good foliage

caloxanthum
1
1
1'
U
summer death

campanulatum
3
4
1'
P
appears hardy

campylocarpum
6
17
6'
S
likes shade

campylogynum
1
1
4yr / 6"
U
many branches die back

canadense
3
13
1'
P

canescens
2
2
2'
P

carolinianum
16
150+
17yr
S
good drainage vital, bronze winter leaves

catawbiense
13
300+
30yr / 15'
S
tolerant of soils and exposure; borers experienced

var. album
3
40
8yr / 4'
S
best in sun

caucasicum
5
8
5'
S
slow grower, long time to bloom: root rot and caterpillars experienced

var. hidakanum
1
2
1'
P
O.K. winter '69

chaetomallum
1
1
6"
U
heat killed

chapmanii
6
14
2'
S
hard to establish

chrysanthum
5
8
4yr / 8"
P
slow growth

cinnabarinum
1
2
4'
S

concatenans
2
3
1'
U
winter killed

concinnum
3
5+
2'
P
appears hardy

crassum
2
2
1'
U
heat killed

dauricum
5
25+
30yr
S
white forms excellent

davidsonianum
3
39
3yr / 2'
P

decorum
9
50+
12'
S
very adaptable

degronianum
5
15+
5'
S

desquamatum
2
15+
4yr / 2'
P
grows well, appears hardy

diaprepes
2
2+
2'
U
never healthy

dichroanthum
1
1
4'
P

dictyotum
1
1
6"
U
winter killed

discolor
9
75+
5'
S

erubescens
1
1
4yr / 2'
P

fargesii
4
5
5'
S
a heavy bloomer

farrerae
1
1
2'
U
winter killed

fastigiatum
2
3+
1'
U
winter killed, perhaps frost heaving

fauriei
4
12+
2'
P

ferrugineum
4
5
1'
P

flavidum
1
1
6"
P

flavum
5
9
15yr / 8'
S
needs sun to bloom

forrestii
2
3
6yr
S

fortunei
14
600+
20'
S
vigorous, adaptable, profuse bloom

galactinum
l
1
2'
P

griersonianum
3
18+
4'
U
winter killed

haemaleum
1
2
14yr
S

haematodes
3
4
1'
U
winter kill and root rot

hanceanum
2
2
4yr / 1'
P
survives winter only with protection

hemsleyanum
2
2+
4yr / 3'
P

hippophaeoides
4
6
13yr
S
quite hardy, blooms in spring and fall

hirsutum
4
3+
1'
U
winter killed

houlstonii
3
12
7yr / 3'
S

hyperythrum
3
5+
1'
P
plastic winter protection

impeditum
10
120+
1'
S
excellent in full sun and dry soil

indicum
3
10
15yr
S
good here

intricatum
1
1
1'
U
winter killed

irroratum
2
5
3yr / 2'
P

japonicum
6
5
l0yr / 4'
S
best on clay loam

kaempferi
3
12
15yr
S

keiskei
12
153
14yr
S
needs shade and wind break; slow starter

keleticum
3
3+
1'
P
survived with plastic windbreak, others winter killed

kiusianum
4
5+
3yr / 2'
S

kotschyi
2
4
8yr
S
slow starter

laciniatum
2
3
2yr / 18"
S

lepidotum
1
1
1'
U
heat killed

leucaspis
1
1
6"
U
not hardy

litiense
4
27+
5yr / 18"
P
hardy with protection

lutescens
4
4
4yr / 4'
P

makinoi
9
15+
13'
S
very hardy, likes sun to bloom

mallotum
2
2+
1'
P

maximum
10
1000+
20yr
S
withstands neglect

var. rubrum
1
1
6yr / 3'
S

metternichii
11
17+
2 -1 /2'
S
likes sun to bloom

micranthum
5
6
14yr
S
spiraea-like flowers

minus
9
38
5'
S
hardy, but root rot and lacefly experienced

molle
5
13
15yr
S
powdery mildew experienced

mucronatum
4
15+
20yr / 8'
S
whitefly in sun

muctonulatum
14
200+
30yr
S
early flowers frost 2 years of 3

nipponicum
1
2
1'
U
heat killed

nudiflorum
6
41
25yr / 6'
S
likes sun and drainage; native here

obtusum
1
3
10yr
S
O.K. but slow

occidentals
3
11
3yr / 2'
P
powdery mildew experienced

oldhamii
1
1
10yr
S

orbiculare
4
12
1'
P
foliage turns red in fall

oreotrephes
4
5
3'
P
red winter foliage

ovatum
1
1
1'
P

pentaphyllum
2
3
1'
P

polylepis
1
5
7yr / 2'
P
one flower '69

pomense
1
2
2yr / 8"
P

ponticum
2
13
5'
S

poukhanense
4
12
15yr / 3'
S

prunifolium
7
10+
15yr
S
beautiful, blooms

pseudochrysanthum
2
1+
1'
P

pubescens
3
4+
4yr / 2'
S

pulchrum
2
4
12yr
S
does well

puralbum
3
7+
6'
S
profuse bloom

racemosum
12
150+
4'
S
variable hardiness

reticulatum
5
6
15yr / 5'
S
poor flower color

rex
4
2+
18"
P
needs winter protection

roseum
5
6
4'
S
root rot experienced

rubiginosum
4
6
6'
S
profuse bloom

rude
1
1
l'
U
heat killed

russatum
3
2+
15"
S

sanctum
1
l
3yr / 15"
P
slight winter damage

sargentianum
1
1
6"
U
heat killed

schlippenbachii
10
127
15yr / 12'
S
slow starter, leaves burn in bright sun

scintillans
1
1
4yr / 2'
P

scyphocalyx
1
1
9"
U
winter killed

searsiae
1
1
3'
S

serpyllifolium
3
3
3yr / 18"
S

serrulatum
4
5
6"
S

simsii
3
3
2'
S

var. eriocarpum
1
12
10yr
S
outstanding dwarf, does well here

smirnowii
7
13
12yr
S

souliei
4
3+
1'
U
winter killed

spiciferum
2
2
4yr / 2'
S

stewartianum
3
4
4'
P

strigillosum
3
2+
4yr / 18"
P

sutchuenense
6
27
5yr / 2'
P

taliense
2
3
2yr / 8"
P

tapetiforme
1
1
2yr
U
heat killed

thayerianum
3
2+
6yr
P

trichophorum
1
1
4yr / 1'
P

triflorum
2
2
2yr / 2'
P
leggy

vaseyi
7
12
30yr
S
needs sun to bloom

vernicosum
8
17
14yr
S

verruculosum
1
1
2yr / 1'
P

viscosum
7
19+
4yr / 3'
S
needs sun to bloom

wardii
8
25+
4yr / 3'
P
not reliably hardy

weyrichii
4
50+
5'
S
profuse bloom

wightii
4
5+
1'
U
winter killed

williamsianum
4
15
5yr / 2’
P
winters only with protection; root rot experienced

wilsonae
2
22
5yr
S
likes sun, very hardy

xanthocodon
2
4
3'
P

yakushimanum
11
64+
8yr / 1'
S
very slow grower, but performs well even exposed to N.W. sun and wind

yunnanense
7
23
5yr / 2'
S
winter kill and root rot reported but a few doing O.K.

1 number of separate gardens or collections in which species is found.
2 total number of plants found; + indicates young seedlings or cuttings
3 age in years of oldest plant if known, otherwise largest size in feet or inches.
4 performance: S - satisfactory plant and flower; P - satisfactory plant, no flowers as yet;
U - unsatisfactory (poor development or lack of tolerance).

Table 2: Species being grown but as yet unevaluated due to a lack of sufficient exposure.
amagianum, anthopogon, balfourianum, brachyanthum, bullatum, callimorphum, camtschaticum, chartophyllum, chlorops, chryseum, ciliatum, clementinae, crinigerum, decandrum, elliottii, fictolacteum, fulgens, fulvum, glaucophyllum, glomerulatum, grande, gymnocarpum, habrotrichum, hemitrichotum, linearifolium, macabeanum, maddenii, magorianum, manipurense, nakaharai, neriiflorum, oleifolium, pemakoense, planetum, praevernum, prattii, quinquefolium, sinogrande, sinogrande var. boreale, tephropeplum, thomsonii, titeum, tschonoskii, ungernii, xanthostephanum

Judges were Walter Eickhorst and Roy Nordine of Morton Arboretum. Most of the members visited the plantings in the arboretum. These are now coming into good bloom and are creating quite a spectacle. The plantings are on a gravelly hill under tall pines, The slope of the hill is to a road and the plants have developed to sizes where they are becoming quite showy.

While the show was better this year than previous ones it would have been much better except for the weather which was extremely hot and dry and turned wet with severe thunderstorms. The heat and rain between them injured many trusses that otherwise would have been in the show.