QBARS - v21n3 Sixty Years of Growing Rhododendrons and Azaleas in the Arnold Arboretum

Sixty Years Of Growing Rhododendrons
And Azaleas In The Arnold Arboretum
Donald Wyman, Arnold Arboretum

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 543 species, varieties and clones of the genus Rhododendron are being grown in the Arnold Arboretum. There have been many more in the past, species and varieties that have been tried but have not succeeded. Over 130 species and varieties of this genus have been introduced into America by the Arnold Arboretum, including:

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 continuously.

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 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 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:

Ten year, 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. Others have performed well for half a century, some are new and little is known about the limits of their hardiness. Some of the evergreen types can be grown only in the South, others on the Pacific Northwest coast but it is interesting to note that at least a dozen have been growing in New England at the Arnold Arboretum since 1891 and the original plants are still alive!

This might be the place to list those evergreen species and varieties that have proved hardy for 20 years or more in the Arnold Arboretum, and are now alive. A large number have been planted within the last few years, but the hardiness of many of these is still in doubt because the last few winters have not been severe.

There are a few things which should be kept in mind when scanning the list. The collection is in a sheltered valley protected somewhat from searing winter winds, and shaded at least half the time especially from the western winter sun. Some of the plants 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. A variety may have died and been replanted at some time in the past, so the figure for the number of years the individual plant has been alive does not, in every case, represent the time at which the first introduction was made. Merely because a name is not listed here does not mean that the plant is not hardy-it simply means that the variety is not now alive even though it may have been tried years ago and had a long record of many years' growth here under these conditions.

Some of these rhododendrons are of course less hardy than others and these have been marked with an asterisk. 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 (luring 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.

The Hardiest Azaleas

It might also be of interest to note the azaleas that have proved the hardiest over the years, the figure after each plant in the following list being the number of years a single plant of the species has been grown. Since this is a listing of the hardiest azaleas, mention should be made of the native Rhodora (R. canadense ) which used to be in a separate genus by itself (Rhodora) , but is now grouped with Rhododendron. This is truly the hardiest of the azaleas, being native far up north in Labrador, in Zone 2. The next hardiest species (Zone 3) are R. nudiflorum , roseum and viscosum ; with arborescens , mucronulatum , schlippenbachii and vaseyi being hardy in Zone 4. Over 4,000 species and varieties have been catalogued as being grown in the United States. The 66 in the following list are the oldest in the collections of the Arnold Arboretum.

It should be understood that not all failures have been due to lack of hardiness. Mr. T. D. Hatfield, the superintendent of the Hunnewell estate for so many years, noted in 1929 that most of the Mollis hybrids had disappeared because of borers in their roots.

New clones or hybrids are bound to originate in such large collections as this. Rhododendron obtusum arnoldianum ( R. obtusum amoenum x R. obtusum kaempferi ) is just one example of a hybrid from which several clones were selected and named. Propagation of rhododendrons has changed since the early days when most of the evergreen hybrids were grafted on R. catawbiense or R. ponticum under stock. Now many are grown from cuttings, resulting in better plants. Azaleas, especially the hybrids grown in Europe, used to be grafted on the more tender Rhododendron luteum , now even the Ghents can be grown rapidly from cuttings if they are taken when soft and grown under one of the mist systems.

The evergreen rhododendron collection in the Arboretum suffered considerably from lack of attention during the war years when labor was scarce. Afterwards, the collection was badly in need of rejuvenation and most of the plants were severely and heavily, but carefully, pruned in such a way that there were always a few buds or small shoots below the cuts. This pruning was done in early spring and resulted in splendid specimens after a few years time. Now, as a matter of course, the plants are heavily pruned if they get leggy at the base or bear over-long branches with few leaves or side branches at the bottom. We have come to the conclusion that, in our climate, where heavy winter snows can be expected, it pays to keep the plants dense, well rounded and 0-7 feet tall. If branches grow too tall or too long, they are frequently laid low by heavy snow and ice and will not recover. Hence, pruning them every few years to keep them in good shape, results in fine exhibition plants.

The soil acidity in this area is about pH 5.5 but sometimes has gone down to pH 4.0 when the wrong kind of fertilizer was added. This has caused a yellowing of the leaves in Kalmia latifolium , corrected by the application of sufficient limestone to bring it back up to pH 5.5. Various types of manure have been used when available, as well as complete fertilizer (10-10-10) and, if applied early in the spring, all have resulted in good growth.

The collection has been mulched with many things over the years. Cocoa shells and spent hops have not proved satisfactory, causing some minor injury, but oak leaves, pine needles, peat moss, wood chips, etc. have all aided materially in conservation of soil water, something always needed by these plants. In fact we have gone to some trouble to provide extra water during summer droughts.

Winters during which the temperature drops below -10° F are those in which flower buds are killed; the lower it drops, the more buds and varieties are injured.

Although regular spraying has been done for rhododendron lace bug and borers, the azalea bark scale has badly injured some of the broad leaved rhododendrons because it was able to get a good hold on some of the plants in the center of the collection without our knowledge. The timing of the spray (DDT or Methoxychlor 50WP, 4 lbs. plus Malathion 25 WP, 3 lbs. in 100 gal. of spray) is important for it must be applied when the young are in the crawling stage and repeated in 10 clays. The time for this in Boston is July 10-15.

Rhododendron fortunei Hybrids

One interesting project has been the growing of some 80 clones of R. fortunei hybrids or close relatives, mostly seedlings either grown by Mr. C. O. Dexter of Sandwich, Massachusetts or grown from seed or seedlings which he either gave away or sold. Many of these have been discarded because in our climate they just did not perform properly, either being injured by winter cold or having poor flowers, inferior branching or foliage. Those that have proved best are: