Volume 24, Number 3
July 1970
A Survey of Rhododendron Species of Washington, D.C. and Vicinity
Richard T. JohnstonU. 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
Locations1Number of
Plants2Age or Size3 Performance4 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, xanthostephanumJudges 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.
Volume 24, Number 3
July 1970
