JARS v53n4 - Establishing Collections of Rhododendron in West Norway and East Denmark

Establishing Collections of Rhododendron in West Norway and East Denmark
Poul Søndergaard
Hørsholm, Denmark

West Norway
When appointed in 1971 by the University of Bergen, Norway, to establish the Norwegian Arboretum at Milde, I soon became aware of the potentials for growing Rhododendron in West Norway. The relatively mild Atlantic climate, abundant rain and the acidic, humic soils of West Norway offer ideal conditions for the genus. Rhododendron was introduced to Norway during the 19th century, and Bergen has been known as the city of rhododendrons since the beginning of the 20th century. Rhododendron was bound to become an important component of the collections of woody species in the Milde Arboretum, which was founded in 1971 and developed on 50 ha of mixed woodland and farmland, 20 km south of Bergen.

Parts of the woodland were direct descendants of the natural West Norwegian forests of scots pine, Pinus sylvestris , which in open stands provide the mixture of light and shade that is so favourable for the growth and development of Rhododendron in the under-story. Nor does scots pine exert a too strong competition from its relatively deep striking root system.

An area of about one ha on a gentle slope exposed to the west was chosen for a species collection of Rhododendron , arranged systematically according to the Bayley-Balfour system. This arrangement was later adapted to the revisions initiated by Cullen and Chamberlain (1980 and 1982), continued by Philipson & Philipson (1986), Chamberlain & Rae (1990), K. A. Kron (1993) and W. S. Judd & K. A. Kron (1995).

A special collection of Rhododendron cultivars, with a historical grouping, was planned and later developed about 800 m to the south of the species collection on earlier farmed and grazed land with no tree cover.

Norwegian Arboretum, Milde, Norway
From the species collection at the Norwegian Arboretum, Milde, June 15, 1986.
Photo by Poul Søndergaard

The Species Collection at Milde

The soils are acid and in principle well suited for growing Rhododendron , but they were in many places waterlogged, and a thorough drainage, tillage and aeration was necessary before planting. In addition the soils were often very shallow, which demanded a supplement of peaty soil to each planting place. This was, during the first fifteen years, available in considerable quantities from building projects within a radius of 10 km from the Arboretum (extension of the Bergen Airport and development of huge industrial areas triggered by the booming oil business in Bergen). A good way of preparing the planting places was to add a thick layer of peaty soil on top of the intact ground vegetation (mainly Vaccinium oxycoccus , V. myrtillus , Calluna vulgaris and Arctostaphylos uva ursi ). The abundant and evenly distributed rainfall (1600-2000 mm/year) assured sufficient water supply, and the raised planting beds provided good drainage to the roots, which gradually penetrated the underlying decomposing ground vegetation down into the original soil.

Norwegian Arboretum, Milde, Norway
From the cultivar collection at the Norwegian Arboretum, Milde, June 16, 1999.
Photo by Poul Søndergaard

In order to get a quick start, plant material was obtained from other collections or commercial nurseries, either as gifts or for cash. This material was often insufficiently documented, if at all, and very soon efforts were concentrated on material collected in the wild (c.w.). Scions and plants from well documented collections were also preferred, e.g., the Royal Botanic Gardens at Edinburgh and Kew, from which we collected and obtained material from 1972 onwards, and since 1973 from the Glendoick Gardens near Perth, where the Milde Arboretum has bought plants from three generations of the Cox family.

Propagation of Rhododendron at Milde

To establish and maintain a vital collection of Rhododendron one must know how to propagate the genus. The process of propagating your own plants also gives you a deeper understanding of the infinite variation within the genus and its capricious ways of combining genes. At the Milde Arboretum simple propagation facilities were established from the very beginning in borrowed greenhouses and in primitive plast-covered frames. A suitable sowing medium was composed based on finely triturated sphagnum peat, and sowing was under control after a couple of seasons. Propagation by cuttings was also mastered after a short time with the use of bottom heated benches covered by tents of plast and with mist aggregates. However, most important of all was a dedicated and skillful propagator, Magne Sandvik, the arboretum's head gardener during sixteen years and a member of the ARS Danish Chapter since the 1970s. With a new and modern greenhouse, designed by M. Sandvik and erected in 1981, facilities were markedly improved, and propagation could be made on a large scale. The arboretum was now able to produce plants to yearly plant sales under the auspices of the Friends of the Arboretum Association, which was founded in 1985.

Seed samples taken home from other botanical gardens often contained hybridized seed, and most of the plants arising from these were discarded. However, in the beginning of the 1990s a couple were selected and named as new cultivars, i.e., R. 'Fritz C. Rieber'* in honour of the founder of the Milde Arboretum and R. 'Madam Felle'*, as a tribute to a well-known Bergen lady who has given name to things as different as a local restaurant and a huge tunnel boring machine. From the very beginning a guiding principle for the intake of seed and plants at the Milde Arboretum was a well documented origin, and preferably collected in the wild. This was obtained from collecting expeditions, either individual expeditions, e.g., Johannes Hedegaard to Nepal and Bhutan 1973, 1974 and 1975 (sponsored by the Milde Arboretum) and the author's own collections in the Pyrenees 1983, Nepal 1984 and northeast Turkey 1986, or arranged as joint Nordic expeditions (e.g., Japan and South Korea 1976) or received from British or US expeditions. Gradually a stock of well documented material from the wild was being established. When M. Sandvik and I left the Milde Arboretum in 1986/87, the species collection of Rhododendron (by then ten years old) was beginning to pay off for the efforts by profuse flowering and prosperous growth in the beautiful setting of open natural scots pine forest. The hardiest 200 species of the genus were within reach in the eu-atlantic climate with species such as R. bureavii , R. calophytum , R. campanulatum ssp. aeruginosum , R. oreodoxa v. fargesii and R. wardii were peak performers, while R. rex ssp. fictolacteum , R. barbatum and R. strigillosum were borderline species, surviving for many years without really thriving, and finally badly severed, if not killed, by cold winters (or detrimental combinations of winter and summer).

Bergen Museum Garden
R. calophytum (right) and R. aff. alutaceum (left)
in the Bergen Museum Garden, April 19, 1983.
Photo by Poul Søndergaard

Public Awareness

In order to establish and maintain a flourishing collection of Rhododendron a good interaction with the public is essential. Since the foundation of the Milde Arboretum in 1971 Rhododendron has become a favoured genus for gardeners in West Norway, and the number of species and cultivars available in commercial nurseries has increased from a couple of dozens to, if not several hundreds, then far more than a hundred. A Norwegian Rhododendron Society was founded in 1998 as an offshoot from the Friends Association and with strong links to the Milde Arboretum. This development was markedly accelerated by the second generation of managers of the Arboretum, who took over in the mid 1980s. During the period when C. C. Berg was director (1985-1998) the species collection was further developed and more or less saturated. The collection of cultivars was extended from a modest beginning to become the probably most comprehensive of its kind in Scandinavia. The driving force behind this latest development was the rhododendron aficionado Per Magnus Joergensen. As curator of the Bergen Botanical Garden from 1973 and later professor of botany at the University of Bergen, he developed what seems to become a lifelong devotion to the genus. He gradually became the fervent whip in a carpet-bombing of Bergen with Rhododendron , mainly cultivars. His efforts were supported by benefactors in Bergen, and particularly the Rieber & Son concern, which donated thousands of plants both to the Municipality of Bergen, the Milde Arboretum, the Bergen Botanical Garden and to a tongue-breaking "Rhododendrarium," a special collection of Rhododendron cultivars in the centre of Bergen. Now at the turn of our century, Rhododendron has become a prevailing feature along the main entrance roads to Bergen, particularly the "Fritz C. Riebers Vei" and in parks and private gardens of Bergen. A dream nurtured by Fritz. C. Rieber has come through to a degree he probably would never have imagined. However, he sometimes had a vision of the mountains around Bergen sprinkled with Rhododendron , inspired by the "Alpine rose," R. ferrugineum , which he had seen in flower, covering whole mountainsides in the Swiss Alps. Because of changed environmental attitudes this extravagant dream will probably never come through.

Forest Botanical Garden, Charlottenlund, Denmark.
Forest Botanical Garden, Charlottenlund, Denmark, May 27, 1998.
Photo by Poul Søndergaard

East Denmark
One of the first tasks after my return to Denmark in 1988 was to plan and direct the establishment of a species collection of Rhododendron at the Hørsholm Arboretum, which is part of the Royal Veterinary and Agricultural University, Copenhagen. Rhododendron was planted in the Arboretum from its foundation in 1936. In its satellite, the Forest Botanical Garden at Charlottenlund (from 1838), some Rhododendron are more than one hundred years old (e.g., R. luteum planted before 1890). So there was plenty of Rhododendron around and many interesting species, particularly at Charlottenlund, where the genus had become a prominent feature of the garden. However, the collections were mixtures of species, cultivars and unnamed types derived from hybrid seed. In 1976 the Nordic Arboretum Committee sponsored joint nordic plant collecting expeditions to Japan and South Korea. Much of the material collected were provenances of Rhododendron: albrechtii , amagianum , aureum , brachycarpum , dauricum, degronianum ssp. degronianum and ssp. heptamerum , dilatatum , japonicum , kaempferi , mucronulatum , nipponícum , pentaphyllum , quinquefolium , reticulatum , schlippenbachii , tschonoskii , weyrichii , yakushimanum ssp. makinoi , and yedoense var. poukhanense . Some of the species were represented by more than ten provenances from different mountain areas in Japan and South Korea, and hundreds of plants had a size that was still suitable for transplanting as a core of the new species collection at Hørsholm (a replica of the same material was propagated with much success at the Milde Arboretum, and in the early 1980s transplanted to the species collection at Milde, where it became a main feature together with provenances and hybrids of R. campanulatum , campylocarpum , thomsonii and wallichii from seed collected in Nepal and Bhutan by J. Hedegaard during the 1970s).

Species collection at Hørsholm, Denmark.
The species collection at Hørsholm, Denmark, April 13, 1991.
Photo by Poul Søndergaard

Species Collection at Hørsholm

In 1986 the Agricultural University bought a farm adjacent to the Hørsholm Arboretum. Ten hectares of undulating farmland was made available for an extension of the Arboretum. About one ha of this area forms a valley cut down from west to east, bounded to the south by a relatively high and steep north-facing slope, fairly open to the north and embracing a woodland tarn at its eastern lowest end; a gift from heaven for the making of a Rhododendron dell. The upper part of the valley is stocked with scattered trees, i.e., a few majestic oaks. The oaks were maintained as shade trees supplemented with scots pines, which were among the first plantings in 1988. A master plan was elaborated for a species collection in the valley. The area was divided in three main parts: the lepidotes, the elepidotes and the azaleas s.l. The Japan and South Korea collections from 1976 made a very good start. More t han a hundred plants from 1-1½ m were transplanted from the old arboretum area to the Rhododendron Valley. Many plants were given to the Arboretum from amateur growers of Rhododendron , and many plants were purchased from nurseries, particularly the Glendoick Gardens in Scotland, famous both for their exquisite selection of Rhododendron , many collected in the wild by Peter and Kenneth Cox, and for a number of handbooks on the genus Rhododendron , written by three generations of the Cox family. The latest, The Encyclopedia of Rhododendron Species , was published in 1997. Plants were also obtained as a gift from the Rhododendron Species Foundation, Tacoma, Washington. Wild collected seed has during the last ten years been obtained from the ARS Seed Exchange, from Kew expeditions to Sechuan, China, Taiwan and Turkey and from a Swedish/Scots/Chinese collecting expedition to Yunnan and from Danish members of the ARS Danish Chapter collecting in China and elsewhere.

R. ponticum mixing with R. caucasicum at Ikizdere, Turkey.
R. ponticum mixing with R. caucasicum at 2.100 m elevation at Ikizdere,
30 km south of Rize, northeast Turkey. Sept. 25, 1986.
Photo by Poul Søndergaard

Propagation at Hørsholm

When the work with the species collection was initiated in 1988, there was no tradition for propagating Rhododendron at the Hørsholm Arboretum. Now, ten years later thousands of seedlings are under way in a propagation greenhouse, net-covered wooden frames and in a special nursery for Rhododendron . And here, as well as at Milde, it was a question of dedication. A skilled gardener, Thomas Poulsen, has taken a vivid interest in the genus and has been experimenting with different sowing media, sowing times, light and shade and watering, and proper use of fungicides and insecticides. Propagation both from seed and from cuttings is now made with a most satisfactory rate of success at the Arboretum.

Soils and Climate at Hørsholm

In contrast to Milde both climate and soils are far from ideal for Rhododendron at Hørsholm. The yearly rainfall is only 600 mm (compared to nearly 1700 mm at Milde) and long dry periods during spring and summer often cause considerable damage to the plants. Winters can be very cold both in Bergen and in Copenhagen. The table shows mean monthly temperatures, December-March, during the two coldest winters recorded at Bergen and Denmark since 1975.

Table 1. Mean monthly temperatures, December-March, in Bergen (Søndergaard 1989) and Denmark (country mean, Rosenørn 1986) in the coldest winters recorded since 1975 (in degrees centigrade).
Dec. Jan. Febr. March
1978/79 -1,5 -3,8 -3,4 4,1
1985/86 0,6 -3,1 -4,8 3,5
1981/82 -4,0 -3,6 -0,7 3,3
1984/85 2,7 -5,0 -4,3 1,0

Winters can be quite as cold in Bergen as in Denmark and the soils can be frozen to a depth of more than one meter in both places. In Denmark the eternal winds are adding to the harmful effects of freezing, while in Bergen instant freezing of soaked soils can kill plants right away, and the damage caused by freezing can be strongly enhanced by a wet summer and autumn (I have experienced autumns in Bergen with rainfall up to 1500 mm in four months).

The soils at Hørsholm are loamy and slightly acid, actually too rich for Rhododendron . The ground vegetation is dominated by a strongly competing sward of herbs and grasses, couch grass ( Agropyron repens ), cocks foot ( Dacty-lusglomerata ), raspberry ( Rubus idaeus ), and broadleaved dock ( Rumex crispus ). Before planting it is absolutely necessary to add a good measure of peat to the planting hole. Afterwards a heavy cover of chipped conifer branches and twigs is placed around the plants. The chip layer restrains the growth of weeds, retains humidity and enhances the acidity of the growth medium. For several years the competing vegetation must be cut two or three times per year, if the rhododendrons shall have a chance to survive and eventually prevail. During dry periods irrigation is possible with water from the small lake. However, the water contains lime and irrigation should be limited to an absolute minimum in order to avoid stunted growth.

Growing Rhododendron is but one of the many activities in the Arboretum and both time and resources are limited. The species collection at Milde was beginning to show off after the first fifteen years. The same is expected to happen at Hørsholm, which means that patience and perseverance are essential qualities, when one wants to establish unique collections of Rhododendron . Luckily there are good conditions for Rhododendron in the old Forest Botanical Garden at Charlottenlund and a well established and old collection of species and cultivars (e.g., R. auriculatum , campanulatum , lutescens , oreodoxa var. fargesii , orbiculare , schlippenbachii , sutchuenense , wardi , 'Hugo deVries', 'Jacksonii', 'Pink Pearl', 'Temple Belle'). The garden has a well established forest climate and the soils are favourable for growing many species and cultivars. Parallel to the establishment of the species collection at Hørsholm, the collection at Charlottenlund is being extended with some of the most interesting and tender species that have a better chance for survival at Charlottenlund than at Hørsholm, where the climate is slightly colder.

We are most grateful for a grant of DKK 55.000 - from "Direktør, dr. techn. A.N. Neergaards og Hustrus Fond" - to extend the collections at Hørsholm. It gave a welcome push to the azalea collection with plant material brought in from Scotland, Holland and the United States.

A replica of the Hørsholm species collection of Rhododendron is being established in the gardens of the Jægerspris Castle in northern Seeland. The Foundation of the Danish King Frederik VII (1848-1863) owns the castle and donated money to purchase plants. A special area of about 2 ha ash forest ( (Fraxinus excelsior ) on moist, humic soils has been thinned and made ready for the collection, and hundreds of plants delivered from the Arboretum at Hørsholm are gaining size in the gardens propagation department.

Cultivar Collections

Rhododendron cultivars have been - and will be - planted in areas detached from the species collections, both at Hørsholm and Jægerspris. An effort will be made to choose the best performing cultivars. In this respect much useful information can be obtained from the ARS Danish Chapter's collection at GI Kjøgegaard, 40 km south of Copenhagen. There are also good collections of Rhododendron cultivars in the Royal and cultivars in the Copenhagen Botanical Garden.

Map of Scandinavia.
Figure 1. Scandinavia including Denmark (DK) and the Faeroes (FO), Finland (SF), Iceland (IS),
Norway (N) and Sweden (S). 1 = Bergen/Milde; 2 = Copenhagen (Charlottenlund, Hørsholm).


There is a general and steady growing interest for Rhododendron in both Denmark and Norway. The main content of this article was recently presented at a meeting of the Danish Chapter of the ARS. As usual the meeting was attended by more than one hundred persons, which is a much higher number than similar societies normally can bring together. The collections now being established and extended at Hørsholm, Charlottenlund, and Jægerspris will become part of an existing network of Rhododendron collections in Denmark. We hope to be able to establish good and reliable reference collections and to introduce new and interesting material, i.e., new and better provenances to the species collections and new showy and dependable selections to the collections of cultivars.

Bibliography and Recommended Reading
Bean, W. J. 1976. Trees and Shrubs Hardy in the British Isles, Vol. III , 8th Revised Edition. London.
Chamberlain, D. F. 1982. A Revision of Rhododendron II. Subgenus Hymenanthes . Edinburgh Journal of Botany . Vol. 39, No.2
Chamberlain, D. F. & S.J. Rae. 1990. A Revision of Rhododendron IV Sub-genus Tsutsusi . Edinburgh Journal of Botany . Vol. Vol 47, No. 2
Cox, P. A. 1985. The The Smaller Rhododendrons . B. T. Batsford: London.
Cox, P. A. 1990. The Larger Rhododendron Species . B.T. Batsford: London.
Cox, P. A. & K. N. E. Cox. 1997. The Encyclopedia of Rhododendron Species . Glendoick Publishing.
Cullen, J. 1980. A Revision of Rhododendron 1. Subgenus Rhododendron sections Rhododendron & Pogonanthum . Notes from the Royal Botanic Garden Edinburgh. Vol. 39, No.1
Philipson, W. R. & M. N. Philipson. 1986. A Revision of Rhododendron III. Subgenera Azaleastrum , Mumeazalea , Candidastrum and Therorhodion . Ibid. Vol.44, No. 1.
Judd, W. S. & K. A. Kron. 1995. A Revision of Rhododendron VI. Subgenus Pentanthera (Sections Sciarhodion, Rhodora , and Viscidula ). Ibid. Vol. 52, No. 1.
Kron, K. A. 1993. A Revision of Rhododendron Section Pentanthera . Ibid.Vo. 50, No.3.
Rosenørn, S. 1986. De klimato-logiske forhold under vintrene 1981-82 og 1984-85. Climato logical conditions during the winters 1981-82 and 1984-85. Ugeskrift for Jordbrug, 131.219-221.
Royal Horticultural Society. The Rhododendron Handbook ; Part I, Rhododendron Species, 1967; Part II, Rhododendron Hybrids, 1969.
Royal Horticultural Society. 1998. The Rhododendron Handbook
Stevenson, J. B., ed.1930. The Species of Rhododendron . London.
Søndergaard, P. 1989. Experiences with cultivation of plants from the southern hemisphere in West Norway compared with observations from Denmark and the Faeroe Islands. In "A century of tree-planting in the Faeroe Islands." Annales Scient. Faeroensis, Suppl. XIV. 165-180.

* Name is not registered.