JARS v39n2 - Rhododendrons of North Germany: Hybridizer - Hans Hacahmann

Rhododendrons of North Germany: Hybridizer - Hans Hachmann
Jon M. Valigorsky, M.D.
Pittsfield, Massachusetts

On the moors and marshes of the North German State of lower Saxony by the North Sea is the heart of the West German nursery industry. The area encompasses the Ammer River and is known as the Ammerland. Dietrich Hobbie created his hybrids here and his nursery remains fully operational today at Linswege/Westerstede. A satellite of the nursery industry is in the state of Schleswig-Holstein just north and east of the great port of Hamburg. The town of Barmstedt lies at the most northern border of this region surrounded by a ring of forests. It was here we came to meet Hans Hachmann, at his nursery.
The steps leading to that meeting begin with my wife, Helga, a native of Hamburg who started the madness many years ago when she suggested we plant some rhododendrons. At the 1981 San Francisco ARS meeting we met nurseryman Alfred Raustein of Long Island who has introduced many of Hobbie's hybrids in this country. He suggested that we join the German Rhododendron Society. He himself was a member, interesting things were happening in Germany, and he graciously provided us with the address of Dr. Lothar Heft, Curator and Director of the Bremen Rhododendron Park. A lively correspondence culminated in our visit to Bremen in late August of 1983. When asked for additional sites to visit, Dr. Heft suggested the Hobbie Nursery and "Why not visit Hachmann in Barmstedt." "It is an easy day trip from Hamburg and he has developed a number of red hybrids that are hardy." His introduction was delivered with the traditional North German reserve and understatement which caught us unaware. We had expected to meet a nurseryman who did a little hybridizing on the side. What we did discover was absolutely staggering. Some statistics are illustrative. Since 1952 Hans Hachmann has made more than 2,400 crosses of rhododendrons. Lionel de Rothschild, it may be recalled, made 1,210. Hachmann has grown almost one-half million seedlings to flowering size using only the resources afforded to him as a wholesale nurseryman. It has been accomplished with a staff of, at most, seventeen gardeners in contrast to the 250 Rothschild employed. A single-minded determination characterizes all great achievements and, once again, that single-mindedness has been devoted to improving the Genus Rhododendron.

The Hachmann Nursery
The Holstein nurseries specialize in the growing of the largest assortment of young plants for subsequent container culture. The Barmstedt Nurseries specialize in the development of new plant introductions to the nursery trade They also have the distinction of being innovators, intensely involved in improving the science and art of ornamental plant culture.
The nursery was founded in 1929 by his father Johannes Hachmann, Sr. growing, at first, wild roses, ground covers and forest evergreens and, after the war, fruit trees and hedge plants. Hans Hachmann joined the firm in 1951 at the age of 21 after his apprenticeship in the Ammerland. He brought a love of rhododendrons which had been nurtured by his contact with Dietrich Hobbie. He had also been accorded the great honor to be chosen to work in the greenhouses of the rose grower, Wilhelm Kordes. The nursery today is very much a family operation and Hachmann now has the assistance of his son Holger, age 27, a master gardener who is gradually taking over responsibility for plant production. His wife Edith is his private secretary and directs much of the business end of the operation. She is, in addition, his most severe but well-meaning critic in evaluating a new rhododendron hybrid.
The Hachmann Nursery is a model of space utilization comprising only 27 acres in total size with 16 acres devoted to outdoor culture. The Danish Egedal elevated bed system is employed. Cultural beds are laid out with laser-like geometric precision. So great is the uniformity of culture that the plants of a given variety are identical in size and height. This yields a picture of incredible order as the planting beds, containing thousands of rhododendrons, extend upwards along a gentle slope. At any one time, 50% of the total plantings are young plants and 50% are ready for shipment. Rhododendrons and azaleas constitute 65% of the production of the nursery, well over 200,000 plants per year. A remaining 150,000 consist of a broad assortment of needled evergreens and ornamental shrubs. The entire area is irrigated using ground water feeding overhead sprinklers. A weather station on site provides continuous chart recording of temperature and humidity.

The Comparison Garden
The property, encompassing 3.7 acres including the area surrounding Hachmann's house and business offices, is extensively planted with rhododendrons, heather, needled and broad-leafed evergreens. This is a Hachmann innovation. It is a show or display garden but, more precisely, he characterizes it as a comparison garden. Within the garden are over 800 of the world's best rhododendron hybrids developed over the years. This includes a small number of American varieties such as 'Scintillation', 'Janet Blair', 'King Tut', 'Taurus' and 'Trude Webster'. Along side of these and with impeccable taste, Hachmann has planted 260 of his own selected clones selected from over 30 years of hybridization. It is in this way that the performance of his hybrids can be compared with the standards, past and present, of the nursery trade. Cultural conditions are identical. The display garden is in virtual full sun and unprotected from the fierce North Sea wind. This is also the final test site and a hybrid may remain in this garden for as long as 15 or 20 years before being chosen for introduction. The comparison garden is also an extremely effective sales tool and essentially represents the Hachmann Nursery portfolio. The central portion of the garden is devoted to a heather comparison display and these were in gorgeous bloom when we visited. Unique selections of dwarf and medium-sized conifers offered by the nursery are inter-planted among the rhododendrons and heather.
Mr. Hachmann was busily arranging plant shipments when we arrived on a mid-September day. Sunny skies were replaced by a succession of typical north German showers of varying severity. I believe he sensed the seriousness of our intentions as we emerged dripping wet but awe-struck from the comparison garden/having spent at least one and one-half hours attempting to comprehend the enormity of his contribution to rhododendron hybridization.
We were graciously welcomed for afternoon coffee and cake with his wife Edith and son Holger. He was intensely interested in the varieties we grow in the cold Berkshires of western Massachusetts. Since his plants were not in bloom we spent the balance of the afternoon reviewing several hundred slides of his hybrids. The photographs for this article attest to his consummate skills as a photographer.

Hachmann As Hybridizer
He began hybridizing in 1952, at first, without specific goals other than a desire to create something different. In addition to rhododendrons he has hybridized Calluna, Spirea and with Potentilla achieved a breakthrough in 1969 with 'Hachmann's Giant'. In 1974, Hachmann's Forsythia 'Goldzauber' (Golden Magic) was commercially introduced. Hybridization of rhododendrons continued in the 1950's and 1960's. Since the German market was saturated by Hobbie's repens hybrids, he concentrated principally on larger elepidotes. His initial goals were then formulated. They included large flowers, and improved hardiness. Since Barmstedt is in the coldest area of North Germany a combination of bud hardiness, wind and sun hardiness (foliage) were important and, finally the creation of new colors. In the early 1960's he obtained the Rhododendron yakushimanum and has exclusively used the 'Koichiro Wada' form to produce, as additional goals, more compact plants with dense foliage which exhibit a tolerance for alkaline soils. In 1974 he introduced his first elepidote hybrid 'Bellini'. This was a cross of the old Seidel hybrid 'Omega' with wardii and has a cream-yellow flower. In 1976 he created a sensation in European nursery circles with the introduction of 'Gletschernacht', translated as 'Glacier Night' and introduced in 1983 in the United States by Wayside Gardens as 'Starry Night'. This is a cross of russatum x 'Blue Diamond,' and not impeditum x russatum as originally stated in their catalogue. His Potentilla 'Gold Star' is also carried by Wayside. The following years, in 1977 and 1978, Hachmann launched his assault on the European nursery market with the hybrids 'Bernstein' (Amber), 'Hachmann's Feuerschein' (Hachmann's Fireshine); 'Holstein'; 'Nicoline'; 'Sammetglut' (Velvet Glow); 'Simona', and the yakushimanum hybrids 'Morgenrot' (Morning Red or Aurora), 'Polaris', and 'Silberwolke' (Silver Cloud). Each year has seen additional introductions, time-tested varieties selected from the tremendous reservoir of the comparison garden.

An Analysis Of Hachmann's Hybrids
The varieties are listed using the German names. The English name is in parenthesis followed by HH for Hans Hachmann and a five or six digit number. The first two numbers are the year of the cross with the next numbers being the consecutive cross number followed by the year of introduction. For example: 'Hachmann's Feuerschein' (Hachmann's Fireshine, cross made in 1960, consecutive cross number - 347, year of introduction 1977). Hachmann has generously provided the parentage of all introduced hybrids.

Red Hybrids
'Hachmann's Feuerschein' (Hachmann's Fireshine, HH60347, 1977). This is Hachmann's most spectacular hybrid. A great red elepidote, perhaps satisfying the quest of Northeast American hybridizers for a hardy scarlet red. The cross is Nova Zembla' x Mars'. From 'Mars' it has inherited the large flowered scarlet trusses with white tipped anthers and from 'Nova Zembla' superior foliage. Actually it is better than either parent. To quote Hachmann: "It is our reddest hybrid which will not turn bluish at the end of the bloom." It is a medium grower of extraordinary vigor maturing at 4 to 5 feet. It appears to branch readily and develops exceedingly dense foliage, all the way to the ground. It is extremely floriferous and bud hardy to at least -27°C (-17°F), just somewhat less than 'Nova Zembla'. It is late blooming in Barmstedt, in the first week of June.
'Blinklicht' (Blinklight). This is a sister seedling to 'Hachmann's Feuerschein' exhibiting significantly less bud hardiness to -20°C (-5°F). It is lighter red in color.
'Sammetglut' (Velvet Glow, HH59253, 1977), 'Mars' x 'Nova Zembla'. This is the reverse cross again with a truss closely resembling 'Mars' including deep scarlet red flowers with white anthers. The plant is upright, somewhat loose growing and extremely floriferous. It blooms earlier, 24-30 of May. It too is hardy to at least -27°C (-17°F). This has been introduced in the eastern United States as 'Crimson Classic.'

R. yakushimanum Hybrids
Hachmann began working with the Koichiro Wada form of Rhododendron yakushimanum in 1963. He has developed a remarkable group of first generation hybrids in the red color range which fade minimally as the truss ages. These are compact low growing mounds with dense foliage and excellent wind and sun hardiness. They include:
'Morgenrot' (Morning Red or Aurora, HH63659, 1978). The cross is R. yakushimanum x 'Spitfire'. The flowers are dark red in bud fading to a lighter red. The plant exhibits a superior compact rounded habit. It is hardy to -8°F.

'Hachmann's Polaris' (HH63657, 1978). This is R. yakushimanum x 'Omega'. 'Omega' is a R. catawbiense x unknown hybrid of Seidel of Dresden and is pure carmine rose. 'Polaris' has carmine rose buds with a ruffled flower which fades to a carmine pink with a lighter throat. It is extraordinarily floriferous even as a young plant and as it matures each branch forms 3-5 flower buds. It is the most consistently winter hardy of the Hachmann hybrids to -17°F. It won a gold medal in 1980 in Basel.
T.J.R. Seidel of Dresden, now East Germany, hybridized rhododendrons from 1880-1930. He used R. catawbiense and R. smirnowii extensively. Today his hybrids are highly prized in Germany for their great vigor. One reason undoubtedly was his practice of sowing the seeds outdoors directly onto the surface of the ground! Dr. Heft of Bremen, who related this anecdote, cites this practice as his definition of an optimist!

'Fantastica' (Fantastica, HH681093, 1983). This is 'Mars' x R. yakushimanum . This is superior to most American registrants of this cross with exceedingly large trusses which dwarf the foliage. It is hardy to -15°F. According to Hachmann: "At the time, my most beautiful yak hybrid. Buds are bright red; on blooming the outer flower retains the bright red color, inside the blossom is pink-white. The trusses are long-lasting. The foliage is an excellent dark green."
From the cross 'Sammetglut' x R. yakushimanum , made in 1968, #1097, has come a group of outstanding hybrids in various shades of red fading to rose and pink with a characteristic compact yak growth habit. Those introduced include: 'Anilin', 'Anuschka', and 'Barmstedt'. Hardiness is in the range of -13 to -15°F. Additional first generation yak hybrids that have been introduced include: 'Blurettia' (HH681101, 1982) with Blue Peter' as the female parent, a violet pink in color, hardy to -15°F., 'Julischka' (HH691109, 1980) with 'Thunderstorm', the female parent, a pink-red color. It is bud hardy only to -5°F., 'Schneewolke' (Snow Cloud, HH63664, 1982) a compact white with a striking red blotch. 'Mrs. J.G. Millais is the female parent. 'Mrs. J.G. Millais' is an A. Waterer hybrid, white with large yellow blotches and of H-2 hardiness. 'Schneewolke' is hardy to -13°F.

Second generation yak crosses include two introductions in 1983. These are:
'Kalinka' (HH691131, 1983) rose-pink in color, a cross between 'Morgenrot' x a seedling of cross #675 ('Mars x yakushimanum ). Cross #675 of 1963 was an earlier cross of 'Mars' x yakushimanum .
'Tatjana' (HH691134, 1983) a cross between a sister seedling of 'Hachmann's Feuerschein', called 'Nachtglut' (Nightglow), not to be confused with a later hybrid called 'Nachtglut' from 1976, cross #1564. The cross is #347 'Nachtglut' x #675 ('Mars' x yakushimanum ). Large well formed trusses are ruby pink.

Yellow R. wardii Hybrids
The key to his successful yellow hybrids lies in two key crosses: The first again uses Seidel's carmine rose 'Omega' and is '0mega' x wardii . This cross, #654 in 1963, produced the named clone 'Hachmann's Marina' with pure light yellow flowers and this hybrid was introduced in 1978. The clone of wardii Hachmann has used was obtained many years ago from Mr. Hobbie. It closely resembles the 'astrocalyx-clone' with dark lemon/yellow blooms and without crimson coloring internally. New leaf growth is light blue-green.
The second cross was #298 ( wardii x 'Alice Street'). The parentage of 'Alice Street' is extensive, reaches well back over 100 years and illustrates the enormous gene pool available in a documented hybrid. This can easily be traced in the appendix to David Leach's Rhododendrons of the World as follows: 'Alice Street' is 'Diane' x wardii , an H-4 light yellow flowered hybrid introduced by M. Koster and Sons of Boskoop, Holland in 1953. 'Diane' is 'Mrs. Lindsay Smith' x a campylocarpum hybrid from the same nursery crossed in 1920, creamy white shaded primrose yellow in the center, H-3 in hardiness. 'Mrs. Lindsay Smith' is a cross of 'George Hardy' x 'Duchess of Edinburgh' (M. Koster and Son), crossed in 1910, a white slightly spotted red on the upper lobe and H-3 in hardiness. 'George Hardy' is R. griffithianum x R. catawbiense made by Mangles well before 1922, a blush color fading white. 'Duchess of Edinburgh' is R. brookeanum var gracile x R. lobbii crossed by J. Waterer, a light crimson with lighter center. The year of the cross is unknown but it won a first class certificate in 1874. Both parents are Vireyas.

Hachmann hybrids in yellow are:
'Graf Lennart' (Count Lennart, HH691146, 1983), the cross is ( R. wardii x 'Alice Street') x 'Hachmann's Marina'. A beautiful orange-yellow in bud, the blossoms open a pure light yellow. A floriferous hybrid, it is marred slightly by a lax truss. It has typical wardii blue-green foliage. It blooms early in Barmstedt, around May 10-20 and is hardy to -22°C (-8F).
'Goldkrone' (Gold Crown, HH691146, 1981), a sister seedling to 'Graf Lennart', a beautiful gold-yellow flower with red spotting on the upper lobe and a tighter truss. Bud hardiness is -20°C (-5°F). The plant habit is excellent forming a round compact form.
'Goldrausch' (Golden Ecstasy, HH691126, 1983) is the reverse cross 'Hachmann's 'Marina' x ( wardii x 'Alice Street'). It is orange in bud. The flowers are golden yellow and bud hardy to -22°C (-8°F).
'Hachmann's Brasilia' (HH6911 27, 1982), the parentage is an orange sister seedling of 'Hachmann's Marina' ('Omega' x wardii ) x ( wardii x 'Alice Street'), this is a unique tri-colored orange flower with yellow and red shading, a striking new color in rhododendrons.
'Sandra' (1982), a sister seedling of 'Hachmann's Brasilia', a cream color with pink and orange shading. Bud hardiness of 'Sandra' and 'Hachmann's Brasilia' is -22°C (-8°F).
'Goldbukett' (Golden Bouquet, HH66935, 1980), the cross is 'Scintillation' x wardii with creamy yellow blossoms accented with a beautiful red-brown blotch. In foliage, it closely resembles 'Scintillation.' This hybrid is an elegant demonstration of the potential of 'Scintillation' in hybridizing for yellow. Moreover, it is the most hardy of the Hachmann yellows to -26°C (-15°F). It won a gold medal at the IV International Garden Exhibition in Munich, 1983.
'Festival' (HH711 20), the cross is a second generation yakushimanum hybrid. It is 'Hachmann's Polaris', ( yakushimanum x 'Omega') x wardii . This striking hybrid is to be introduced in 1986. The flower buds are orange. The flowers are golden yellow with an orange blotch. The plant is low-growing of compact habit and is quite floriferous. Hardiness is not given but should be -5 to -8°F.
An orange hybrid is 'Marimba' (HH671049, 1980), the cross is R. fauriei var. rufescens x 'Goldsworth Orange'. The flower is a medium salmon orange and closely resembles Gable's 'Marybelle'. The hardiness is not given.

Large Flowered Cream and Multicolored Hybrids
Several cream and unique new multicolored large flowered hybrids also have been introduced. In their pastel blossoms these mirror the nuances of the finest Exbury azaleas.
'Bernstein' (Amber, HH65909, 1978). This is a cross of 'Goldsworth Orange' x 'Mrs. J.G. Millais' with light amber colored yellow flowers with a brown-red blotch. This hybrid, in almost 20 years of trial, has exhibited outstanding winter hardiness sustaining some damage after -15°F. in the winter of 1981/82. It is reliably bud hardy to -5°F.
'Maharani' (HH64733, 1978), 'Harvest Moon' x 'Letty Edwards', a large flowered hybrid of cream yellow flowers with a brown-red blotch of hardiness comparable to 'Bernstein' with foliage damage after severe winters under -5°F.
'Simona', a sister seedling to 'Maharani', multicolored, rose pink buds open to a light pink rimmed flower the inner surface of which is yellow over laid with a red blotch. Flower buds are extraordinarily large in the fall. Blooming in early May, it ultimately forms a large bush 6 feet in height and is hardy to -8°F.

Large Flowered With Blotches
Hachmann's hybrids in varied colors with blotches are all derived from the Seidel hybrid 'Humboldt', catawbiense x ?, rose with dark markings. They include:
'Holstein'(HH59268, 1978), 'Humboldt' x catawbiense ' Grandiflora' resulting in a lilac colored flower with a red-brown blotch of considerable hardiness at least to -27°C (-17°F).

'Kokardia' (HH57185, 1978) is 'Humboldt' x 'Director Hjelm'. The male parent is a second generation fortunei hybrid, deep carmine rose in color with a bronze blotch. 'Kokardia' is pink with a striking red-brown blotch. A great favorite of Hachmann, it is floriferous and hardy to -15°F. It blooms in late May to early June.
'Hachmann's Ornament' (HH511 85, 1978) a sister seedling of 'Kokardia' dark pink with a dark purple blotch. It blooms earlier, around May 20 and is hardy to -8°F.
'Diadem' (HH691139, 1983) is an improved 'Furnivall's Daughter' employing 'Hachmann's Ornament', as the female parent, x 'Furnivall's Daughter'. It has a beautiful pure light pink blossom with a red-brown blotch. Bloom is particularly long lasting. It is very floriferous and exhibits a dense compact growth habit. It is hardy to -8°F.

R. insigne Hybrids
Two insigne hybrids have been introduced: 'Hachmann's Rosarka' (HH63694, 1983) is insigne x 'Spitfire', a light carmine red which blooms for an unusually long time in a compact bush and is bud hardy to -11°F.
'Brigitte' (HH65888, 1980) insigne x 'Mrs. J.G. Millais'. Blossoms are a very delicate pinkish-white with yellow-green blotch. These contrast with the shiny dark green foliage of great density and a compact rounded growth habit. Bud hardiness is -11 °F.

The European nursery market has been dominated by Hobbie's repens and williamsianum hybrids yet Hachmann has made a number of crosses and has six of the former and twelve of the latter under evaluation. Actually three williamsianum hybrids have been introduced including a pure pink clone, 'Hachmann's Kristina', in 1983. A repens hybrid 'Buketta' (HH63684, 1979) 'Spitfire' x 'Fruhlingszauber' is noteworthy for the large number, 10-12, dark red flowers/truss. It is hardy to -5°F.

Lepidote Hybrids
'Azurika' (HH63672, 1979) is russatum x impeditum with a dark blue-violet flower, extremely floriferous with multiple trusses on many branches. It has an excellent compact growth habit and H-2 hardiness to-8°F.
'Hachmann's Violetta,' a sister seedling of Azurika, more blue-violet in color with multiple trusses and comparable hardiness.
'Azurwolke' (Blue Cloud, HH61383, 1977). russatum x 'Blue Diamond', a sister seedling of 'Gletchernacht' (Starry Night) with pure blue flowers on a round tidy bush is bud hardy to -11°C.
'Gletchernacht' translated literally as 'Glacier Night', an incomparably dark violet-blue. Hardy to -20°C (-5°F).

Deciduous Azaleas
Hachmann has worked with both evergreen and deciduous azaleas utilizing Knaphill and Exbury varieties and has introduced a number of new hybrids principally in orange and red colors which exhibit improvements in flower size, truss configuration and flower color. His two favorites, as illustrated, are:
'Feuerwerk' (Fireworks HH63703, 1977) 'Cecile' x 'Fireball'. To quote Hachmann: "The largest pure red azalea of all azaleas available in Europe. It is very floriferous, exhibits loose upright growth and is hardy to -17°F."

'Goldpracht' (Golden Splendor HH711 204) The cross is his named hybrid 'Limona' (HH63704, 'Cecile' x 'Klondike') x 'Marion Merriman'. The blossoms are deep yellow with an orange blotch.
As his accompanying photographs attest, Hachmann has extraordinarily achieved his hybridizing goals. The work continues. Thus far, he has introduced 70 of his hybrids; 260 selections are named and under trial in his comparison garden. He continues to make 70 to 150 crosses per year growing, on the average, 15,000 to 20,000 seedlings. He reports he made 240 crosses in 1984! Final selection is made in eight to twelve years. Selections are named and removed from the field to the comparison garden, for further evaluation. Although many culls are discarded, most crosses with careful culture, yield plants that are sold as unnamed seedlings.
As the hybridizer serves the nursery, so his nursery supports the hybridizer. Continued success and recognition has come from competition in the International Garden Expositions in Europe where, in recent years, Hachmann hybrids have consistently been medal winners. As demand has increased, more and more of his nursery resources have been diverted to their production. Although the early introductions were patented, Hachmann now has discontinued the practice. He finds that patented plants inhibit the introduction of newer varieties into the trade. His preferred market is the retail garden center although he continues to sell 200,000 rhododendrons per year wholesale for container culture. He understands that success will be determined by the marketplace. However, he has single mindedly pursued his hybridizing goals independent of marketing consideration but rather in the service of the genus. Indeed, all of his introductions have excellent commercial attributes. However, these may be expressed under the conditions in which they are propagated. Propagation, thus far, has been exclusively by grafting onto 'Cunningham's White' root stock. 'Cunningham's White' is used for its tolerance to alkaline soil conditions and also its extreme resistance to drying out. Grafting success is extraordinarily high, above 90% and the German gardening apprenticeship system insures that grafting skills are carefully developed.
Thirty-five thousand rhododendrons are grafted each year. Moreover, they believe that a grafted plant is more vigorous, makes up faster into a saleable plant and is superior in every way to a rooted cutting. This is directly contrary to our belief that a plant on its own roots ultimately is superior. These German nurserymen, however, are open-minded. A very careful study is in progress comparing propagation of standard hybrid by grafting and from rooted cuttings.
Thirty-five of the hybrids detailed in this report were registered in the International Rhododendron Register of the Royal Horticultural Society in 1983 and 1984. Since this article originally appeared in the Rosebay in April of 1984, thirty-five additional hybrids have been registered for a total of 70! These represent crosses made only up to 1971. The question remains! What does Hachmann have waiting in the wings?
A long cherished dream to see the great rhododendron gardens of England will be realized this spring when he will visit Exbury at the invitation of Edmund de Rothschild.
There is no reason to doubt that certain of Hachmann's clones would do well in American gardens, east and west coasts, even in the colder areas of the inland northeast. Peak bloom in Barmstedt is May 25 to June 3, identical to ours.
Years of struggle and hard work have been rewarded, the achievements are truly colossal. Hobbies mantle has passed to this premier German hybridizer.

References :

1. Maethe, H., An Barmstedt fuhrt kein Weg vorbei. Deutsche Baumschule; Aachen. 33:378-401, September 1981.
2. Hachmann, H. Hachmann Barmstedt in Holstein, Nursery Catalog; Fall 1983, Spring 1984.
3. Hachmann, H. Sortimentslistei; 1983.
4. Hachmann, H. Personal communication, 26 June 1984, 3January, 1985.
5. Stuck, G. Rhododendron Hybriden aus Holstein, Rhododendron und immergrune Laubgeholze Jahrbach, 1983; Deutsche Rhododendron Gesellschaft, April 1984.
6. Leach, D.G. Rhododendrons of the World. New York; Charles Scribners Sons; 1961.