QBARS - v24n1 Seventy Five Years of Growing Rhododendrons in the Arnold Arboretum

Seventy Five Years of Growing Rhododendrons
in the Arnold Arboretum
Dr. Donald Wyman, Horticulturist, Arnold Arboretum
Reprinted from Vol. 29, Number 6, Arnoldia

Mr. H. H. Hunnewell of Wellesley started buying and planting large numbers of azaleas and rhododendrons on his estate in Wellesley, in 1856. He became so enthusiastic about these plants that he staged the first Rhododendron Show on the Boston Common in 1873. Later, it was he who urged Professor Sargent to plant these beautiful shrubs in the Arnold Arboretum.

At the present time over 550 species, varieties and clones of the genus Rhododendron are being grown in the Arnold Arboretum. There have been more, in the past, species and varieties which have been tried but have not succeeded over the years. Over 130 species and varieties of this genus have been introduced into America by the Arnold Arboretum. Even during the ten-year period from 1952-1962 at least 30 species and varieties were introduced, so it is still a fascinating group with which to work, with new varieties appearing constantly.

In the many years we have grown these colorful plants, hardiness has been a very important factor. Hundreds have been grown and failed to survive our rugged winters, where average annual minimum tempera­tures go to -5 or -10° F each winter. This is low enough to kill the flower buds on many of the more tender species and varieties and every few winters, temperatures will fall as low as -10 to -20° F, low enough to kill the flower buds of many more.

At first (in the late 1890's and early 1900's) the plants grown in the Arnold Arboretum were a few of the native American species especially those of the eastern seaboard, plus the varieties and hybrids popular in Europe at the time. Then, as the results of the expeditions of E. H. Wilson (and a few earlier trips to Japan by Professor Sargent) Asiatic species and hybrids were introduced, many of which proved hardy at the Arnold Arboretum.

In 1917, Ernest H. Wilson published a list of what he termed were the "Iron Clad" rhododendrons, meaning those which had been planted in the Arnold Arboretum and were able to survive many winters successfully. It consisted of: 'Atrosanguineum', 'Charles Dickens', 'H. W. Sargent', 'Caractacus', 'Roseum Elegans', 'Lady Armstrong', 'Mrs. Charles Sargent', 'Henrietta Sargent', 'Purpureum Elegans', 'Everestianum', 'Album Elegans', 'Album Grandiflorum', 'Purpureum Grandifforum' and catawbiense album .

Ten years later he dropped 'H. W. Sargent' and 'Caractacus' from this list and merely talked about the "Dozen Iron Clads". Today, this group of a dozen R. catawbiense hybrids is as good as it ever was, even though there must be several thousand evergreen rhododendrons being grown, many of them in the United States. Some have performed well for half a century, others are new and little is known about the limits of their hardiness. Some can be grown only in the South, others in the Pacific Northwest but it is interesting to note that some of the species and varieties have been growing in New England at the Arnold Arboretum since 1891 and the original plants are still alive!

The evergreen rhododendron collection of the Arnold Arboretum is in a protected valley where continuous wind does not reach them and where they are shaded at least half of the day from the western sun. Some of the plants now growing in the Arnold Arboretum are still the originals first brought over to America from Europe. One of the first importations was from the Waterer Nursery in Woking, Surrey, England, in 1886, and another large shipment of introductions was received from T. J. Seidel, Saxony, Germany, in 1908.

The following list of evergreen rhododendrons is of those which have had individual specimens growing 10 years or more in the Arnold Arboretum. The figure to the right of the name is the age of the oldest plant in each species or variety, but does not necessarily indicate when the plant was first grown in the Arboretum. For instance, Rhododendron 'H. W. Sargent' in the following list has the number 16 to the right of the name indicating our oldest living plant is that age. However, this variety was first grown in the Arboretum in 1886 and these plants lived until 1939 when they had to be replaced.

Some of these rhododendrons are of course less hardy than others. Some winters, when the temperature may reach -15°F. or lower, the flower buds of many will be injured, and the winter burning of foliage can occur on almost any rhododendron when conditions are right. However, it is of interest to know that during the last fifty years the temperature has reached -26°F. at least once (1934) and -18°F. several times, so that some of these rhododendrons have been able to survive rather low temperatures.

Flower colors are given by general name and figure designations in paren­theses according to the Nickerson Color Fan of the American Horticultural Society. In the description of the flower truss size, the first figure is height of the truss, the second figure is the width at the base. Corolla color markings on the inside of the corolla have been given in many cases because sometimes this is the best means of distinguishing one variety from another. In fact, detailed color information and truss measurements of some of these varieties is not available in books on rhododendrons and has been made directly from the plants growing in the collections of the Arnold Arboretum, so it should have some value. The truss measurements will vary with soil and weather conditions, and those recorded here are on the small side because measurements were taken during a dry year. Also, it is a well known fact that flower truss size, like the size of carnation or rose flowers, will be greater when proportionately fewer trusses are allowed to develop. No attempt whatsoever was made to reduce the number of flower clusters produced on the plants in this collection.

Evergreen Rhododendrons living more than 10 years in the Arnold Arboretum (Figure to the right of the name is the age of the oldest plant.)

x arbutifolium 60; flowers magenta (3 RP 6.3/10); flowers only an inch in diameter with brown spots on the inside of the corolla; truss small.
brachycarpum 32; flowers cream to pink; truss 4"x4"; corolla markings spotted green or brownish yellow.
carolinianum 58; flowers mauve but vary from white to pink; 3"x3", compact, rounded.
catawbiense 90; flowers cyclamen purple (2 RP 7/6); truss 4½" x 6½"; corolla markings yellowish, few.>
catawbiense album 59; flowers white.
catawbiense compactum 44; flowers cyclamen purple (2 RP 7/6).
catawbiense grandiflorum 27; flowers mauve.
caucasicum 60; flowers creamy white to pale rose; truss 3½"x5" loose and rounded; corolla markings chartreuse green (2.5GY 8.5/6); style and stigma reddish; sometimes subject to some winter damage.
caucasicum ' Jacksonii' 31; flowers rosy pink with deeper pink stripe on outside of petal.
discolor 22, flowers white to pale pink; truss loose; corolla markings yellow.
fargesii 16; flowers white, pink, rose; truss loose, flat-topped; corolla markings red.
x fortunei hybrids 39; flowers white to deep pink; (A few years ago the Arnold Arboretum was testing 80 clones in this hybrid group, many of them thoroughly hardy. More recently the less ornamental clones have been discarded together with the ones not thoroughly hardy. Some have been named like 'Ben Mosely', 'Elizabeth Poore', 'Scintillation', 'Tom Everett', 'Roslyn' and 'Westbury'. All these seem to be hardy in normal winters in protected places in the Boston area and would be worthy of further trial.)
keiski 29; flowers lemon yellow; occasionally subject to some winter damage.
x laetvirens 83; flowers moderate purplish pink (2.5 RP 7/8).
maximum 42; flowers pink to white; truss 4½"x5", compact; corolla mark­ings spotted with greenish yellow.
maximum album 42; flowers colored white.
maximum superbum 17; flowers deep pink.
metternichii 46; flowers white to rose colored.
micranthum 60; flowers white; truss 2"; terminal containing as many as 50 small flowers.
minus 89; flowers rose colored to white, somewhat similar to those of Rhododendron carolinianum.
x morelianum 50; flowers light reddish purple (10 P 6/11); truss 4½ x 6" dense and pyramidal; corolla markings brownish; stamens and pistil pinkish.
x myrtifolium 17; flowers moderate purplish pink (2.5 RP 7//8); truss 1½"x1½" open; no corolla markings.
oreodoxa 12; flowers white to lavender pink to rose colored; few flowers to a truss.
x praecox 13; flowers bright rose purple; there may be winter injury from low temperatures on this hybrid.
praevernum 32; flowers deep purplish red (10 RP 2/6); possible injury in very severe winters.
smirnowii 51; flowers rosy red with a frilled margin; truss 4" x5", loose and open; styles red; filaments pink; corolla markings brownish.

ungernii 34; flowers white to pale pink.
X watereri 60; flowers pale to deep rose pink; truss5"x6" dense and pyramidal; corolla markings brownish.
X wellesleyanum 46; flowers white, slightly tinged pink.

Varieties:

'Adalbert' 60; flowers strong reddish purple (2.5 RP 5/14); truss 4" x 5½", dense and rounded; corolla markings a deep purplish red (10 RP 2/6); pistil and stamens pinkish; margin of petals slightly frilled and almost identical with those of 'Echse' except for the color of stamens and pistil.
'Alaric' 60; flowers a strong reddish purple (3. 5 RP 5/13); truss 3½" x 5" dense and rounded; corolla markings few but pronounced deep purplish red (10 RP 2/6); stigma nearly black, styles and stamens purplish.
'Albert' 60; flowers strong purplish pink (7.5 RP 7.8/8); truss 3½" x 6" dense and round; corolla markings brownish.
'Album Elegans' 78; flowers pale purplish pink (7.5 RP 9/4); truss 5" x 6", dense and pyramidal; corolla markings primrose yellow (1GY 9/8); style pink.
'Album Grandiflorum' 83; flowers white; truss4"x5" dense and pyramidal; corolla markings greenish yellow and mostly inconspicuous; stigma red; margins of petals slightly frilled.
'Album Splendens' 54; flowers white.
'Alexander Dancer' 16; flowers magenta; truss 4½ "x5" dense and pyramidal; style and filaments pink; corolla markings are dark red and few.
'America' 18; flowers deep purplish red (10 RP 3.5/12); truss 3½"x6"; corolla markings dark red spots; red filaments.
'Amphion' 18; flowers strong reddish purple (3.5 RP 5/13) to a white cen­ter; truss 4½"x6½, rather loose.
'Anton' 60; flowers mauve (7.5 P 6.5/8) ; truss 4½ "x6" dense and pyramidal; corolla markings greenish to brownish; stamens and pistil pinkish; petals slightly frilled.
'Arno' 60; flowers rose purple (7.5 P 7.5/5) to white; truss 4"x6" dense and pyramidal; corolla markings dark purple and prominent.
'Atrosanguineum' 73; flowers red; truss 2½"x4½", loose, rounded; corolla markings dark purple; pistil and stamens dark purple.
'Bella' 60; flowers light purplish pink (2 RP 8/6) fading considerably; truss 3"x5" dense and rounded; corolla markings strong orange yellow; stamens and pistil pinkish; petals frilled.
'Ben Moseley' 13; flowers strong reddish purple (2.5 RP 5/10); truss 4½"x5½" dense and rounded; corolla marking reddish; usually 6 petals in the corolla and slightly frilled.
'Bicolor' 78; flowers deep purplish pink; truss 4"x5½" dense and rounded; corolla markings deep purplish red; stamens and pistil pink.
'Boudoir' 15; flowers moderate purplish red (4.5 RP 5/10); corolla markings darker.
'Boule de Neige' 60; flowers pure white; truss 4"x6" compact and rounded; corolla markings chartreuse green (2.5 GY 8.5/6); petals slightly frilled.

'Boursault' 12; flowers strong reddish purple (9 P 4.5/12); truss 3½"x5" dense and rounded; style and filaments pink; corolla markings yellow­brown.
'Candidissimum' 54; flowers strong purplish pink to white (5 RP 7.8/8): truss 4"x6" dense and rounded; corolla markings citron green, few and inconspicuous.
'Caractacus' 51; flowers magenta (3.5 RP 5/13); truss 4°x5" dense and rounded; corolla markings citron green and not conspicuous; pistil and stamens red.
'Charles Bagley' 58; flowers deep pur­plish pink (4 RP 6/14); truss 5"x 5½" dense and pyramidal; corolla markings citron green; pistil and stamens pink.
'Charles Dickens' 73; flowers crimson red loose to dense and rounded; corolla markings dark purplish red (10 RP 2/6); stamens dark red.
' Comte de Gomer' 63; flowers white edged crimson.
'Coriaceum' 78; flowers white; corolla markings green.
'Cunningham's White' 42; flowers white.
'Daisy' 61; flowers deep purplish pink (2.5 RP 6/10); truss 3"x4½" dense and rounded; corolla markings brilliant yellow (5 Y 9/10); stigma red; style pinkish; petals slightly frilled.
'Daisy Rand' 14; flowers strong reddish purple (2.5 RP 5/12); truss 3" x 4½" dense and rounded; corolla markings purplish red (10 RP 2/6); stigma black, style whitish and a few stamens purplish.
'Delicatissimum' 52; flowers strong purplish pink (5 RP 7.8/8); 4"x5" loose to dense and rounded; corolla markings a pronounced greenish yellow; stigma red.
'Desiderius' 61; flowers deep purplish pink (2.5 RP 6/11); corolla markings brilliant yellow green (2.5 GY 9/8).
'Dr. H. C. Dresselhuys' 11; flowers strong purplish red (8.5 RP 4.4/12); truss 5"x6" and rather loose.
'Duke of York' 54; flowers light purplish pink (2 RP 8/6) to white; truss 5½" x 7" loose and rounded; corolla markings brilliant yellow green (2.5 GY 9/8); 7 petals usually in each corolla.
'Echse' 60; flowers strong reddish pur­ple (2.5 RP 5/14); truss 5"x5½" dense and pyramidal; corolla markings deep purplish red (10 RP 2/6 and pronounced; stigma deep red; style and stamens white.
'Eva' 60; flowers moderate purplish pink (2.5 RP 7/8); truss dense and rounded; corolla markings citron green; stigma red; style and stamens pink; edge of petals frilled.
'Everestianum' 14; flowers moderate purplish pink (2.5 RP 7/8); truss 4½"x6" dense and pyramidal; corolla markings greenish to brown; pistil and stamens pinkish; petals slightly frilled.
'Fee' 60; flowers deep purplish pink (2 RP 6/10); truss 3½"x4½" dense and rounded; corolla markings are pronounced purple; stamens and pistil pinkish.
'Flushing' 57; flowers magenta (3.5 RP 5.2/13); truss 4"x6" dense and rounded; corolla markings deep purplish red (10 RP 2/6); stigma and style red; stamens purplish.
'General Grant' 13; flowers magenta (3.5 RP 5/13); truss 3" x 4½"; corolla markings deep purplish red (10 RP 2.5/10) and few; stigma nearly black; stamens purple, styles lighter.
'Gomer Waterer' 16; flowers magenta (2.5 RP 7.3/6) to white; truss 4"x6" dense and rounded; corolla markings yellow-brown.
'Hannah Felix' 60; flowers strong reddish purple (2.5 RP 5/14); truss 3"x4½" dense and rounded; corolla markings a pronounced beet-root purple (5 RP 2.5/10); stigma red, style and stamens reddish; corolla color almost uniform color except under the markings where it is whitish.
'Henrietta Sargent' 77; deep purplish pink (4 RP 6.5/12); truss 4"x6" dense and rounded; corolla markings brown to greenish; stigma red, style pink.
'H. W. Sargent' 16; deep purplish pink to strong purplish red (4 RP 4.5/12 to 8 RP 4.2/14); truss 3"x4½" corolla markings deep purplish red (10 RP 2/6); pistil and stamens deep purplish red.
'Ignatius Sargent' 56; flowers moderate purplish red (6 RP 4/14); truss 3½ "x5" dense and rounded; corolla markings brown and pronounced; stigma nearly black; styles red; petals slightly frilled.
'James Bateman' 73; flowers deep purplish pink (4 RP 6.5/12); truss 4"x6" dense and rounded; corolla markings deep purplish red (10 RP 2/6); stigma red, style pink.
' Kettledrum' 13; flowers strong reddish purple (2.5 RP 5/14); truss 3"x4½" dense and rounded: corolla markings citron green (10 Y 7/8); pistil and stamens reddish.
'Lady Armstrong' 53; flowers deep pur­plish pink (5 RP 6/13); truss 5"x 5½" dense and pyramidal; corolla markings ruby red (7.5 RP 2.5/12) and few; stigma red; style pinkish near tip.
'Lady Gray Egerton' 14; flowers pale lilac.
'Lee's Dark Purple' 13; flowers strong purple (7.5 P 5/10); truss 4½"x6" dense and rounded; corolla markings brownish and conspicuous; pistil and stamens red; one of the darkest purple of any of the varieties.
'Luciferum' 12; flowers light reddish purple (9 P 6/8) with a white center; truss 3½"x5" open and rounded; corolla markings greenish yellow: petals frilled.
'Madame de Bruin' 13; flowers bright red.
'Melton' . 73; flowers strong reddish purple (3 RP 5/10); truss 3½" x 5" dense and rounded; corolla markings maroon (5 R 2/6); stigma black; stamens and style whitish.
'Mrs. C. S. Sargent' 52; flowers deep purplish pink (2.5 RP 6/10); truss 4½ "x6" dense and rounded; corolla markings greenish; pistil and sta­mens strong reddish purple; petals frilled.
'Mrs. Harry Ingersoll' 78; flowers strong reddish purple (3 RP 5/18); truss 4"x5" dense and pyramidal; corolla markings citron green (10 Y 7/8); stigma whitish; stamens purplish. A whitish condition of the petals underneath the markings gives a unique color.
'Mrs. P. den Ouden' 19; flowers strong purplish red (8.5 RP 4.4/12); truss 4"x6" and dense.
'Norma' 78; flowers strong reddish purple (3.5 RP 4/15); truss 4"x5" dense and rounded; corolla markings greenish; pistil and stamens purplish.
'Parson's Gloriosum' 15; flowers light purplish pink (2.5 RP 8/6) _; truss 4"x6" rather loose.
'Parson's Grandiflorum' 78; flowers moderate purple (4.5 RP 5/10); truss 5"x4" dense and pyramidal; style and filaments pink, corolla markings yellow-green and few.
'Parson's Rubrum' 27; flowers red.
'P.J.M. Hybrids' 14; flowers deep reddish purple.
'President Lincoln' 16; flowers light purplish pink (5 RP 8/4 to 2 RP 8/16); truss 4½"x5" dense and rounded; corolla markings brownish; stamens and pistil pinkish.
'Professor F. Bettex' 14; flowers strong purplish red (10 RP 3.5/12).
'Promethius' 63; strong purplish pink (5 RP 7.8/8); truss 3"x4" dense and round; corolla markings white to yellow.
'Pulcherrimum' 55; flowers strong reddish purple (3 RP 5/14).
'Purpureum Elegans' 78; light purple (4 P 6/8 to 9 P 4.5/12); truss 4"x6" dense and rounded; corolla markings citron green (10 Y 7/8) and prominent; pistil reddish; stamens pink­ish.
'Purpureum Grandiflorum' 83; flowers light purple (7 P 6/9); truss 4"x7" dense and rounded; corolla mark­ings citron green (10 Y 7/8); pistil and stamens reddish.
'Roseum Elegans' 27; light reddish purple (9 P 6/8); truss 4"x5½" dense and slightly pyramidal; corolla markings brilliant yellow green (2.5 GY 8/8) and few; center of flower white.
'Roseum Superbum' 54; flowers strong pink (10 RP 7.5/10) truss 3½"x6" and loose.
'Sultana' 63; flowers pale purplish pink (7.5 RP 9/14); truss 4"x6" dense and pyramidal; corolla markings (7.5 YR 7.3/8); anthers purplish.
'Viola' 12; flowers porcelain white.