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