More on Dexter Hybrids
By Paul D. Vossberg
From a talk given before the New York Chapter
On one of our periodic trips to the Arnold Arboretum with the late Henry Hicks, we decided to see what was going on at Sandwich, Massachusetts. We received reports that a Charles O. Dexter was going into the breeding of Rhododendrons on a large scale. To the best of my recollection this was in the mid twenties. Mr. Dexter received us very graciously and seemed bubbling over with enthusiasm on this project.
The outlook on a good supply of named hybrid Rhododendrons was very bleak during this period. Nurserymen were willing to grasp at anything opening up this field of beautiful flowering hybrids. The hard winter of 1916-17 killed thousands of plants and in looking over some old notes, I found that many of the cast iron varieties perished which were grafted on ponticum, whereas the layered plants came through this bitter winter with flying colors. Apparently there seems to be a range in hardiness in the seedlings of R. ponticum used for understock. Then followed the war period which curtailed production and to top it all, embargo 37 had to come which stopped the importation of plants from Europe. Our nurseries were not equipped to supply the demand for this material at this time. To fill the gap, collectors got busy with the result that carloads and truck loads of collected R. maximum, R. catawbiense and R. carolinianum were planted instead of our much sought for named hybrids. Reviewing all these facts, we considered Mr. Dexter's work very important and were eagerly looking forward to acquiring a new set of rhododendrons.
We were informed that Mr. Paul Frost encouraged him in this work by arranging the purchase of a number of R. fortunei hybrid seedlings from the old Farquhar Nurseries nearby. Any of the Dexter Rhododendrons having an original number of one (1) to twenty (20) happen to be one of these Farquhar plants and not a Dexter hybrid. He spoke of Ernest Wilson giving him advice on hybridizing and aiding him in getting pollen from England. Pollen from R. smirnowii, discolor, decorum, haematodes, griffithianum and a few brilliant, complex named hybrids, including one called 'Pygmalion', were applied to the collection of these fortunei hybrid seedlings. At that time Mr. Dexter stressed-the use of griffithianum so much that we lost interest in his plants for our area. Since griffithianum was considered more tender than fortunei, we thought that his seedlings would not stand up for us.
Some years later a collection was planted on a nearby estate which gave me a good chance to study them for bud and plant hardiness. The winter of 1933-34 was very hard on broad leaved evergreens; great fluctuation of temperatures and 14°- 20° F below zero in the interior of Long Island (N.Y.). The writer was very much surprised to see most of this collection nearby come through with very few casualties. Three of them showed no killing of flower buds. Many of the acknowledged ironclads had sparse flower trusses that year.
After this experience with the Dexter hybrids in our locality, we changed our opinion on their value and at the same time could only draw one conclusion: that his pollen was incorrectly named or insects and wind possibly carried pollen from the hardy Anthony Waterer and Parson hybrids which I saw scattered throughout his plantings. He may have used this pollen from hardy plants and neglected to tell us. Some of his seedlings show traces and ear-marks of 'Everestianum', 'Henrietta Sargent', 'Lady Armstrong' and 'Mrs. C. S. Sargent'. To my knowledge there were only five collections of original seedlings on the eastern seaboard. The other collections were plants propagated from the best on the Dexter place at Sandwich, Mass. This may create some confusion at a later date when selections are made for naming.
About five years ago a committee was formed to examine these collections with the idea of selecting the most outstanding plants for propagation. To have passed final judgment on a seedling in any of these collections would have been a grave mistake. In many cases these hybrids were growing under adverse conditions, showing a spindly plant with a lovely truss of flowers. Then, again, a seedling may mature into a lovely plant, whereas the plant grown from this clone by vegetative propagation may turn out to be a big disappointment. For this and many other reasons we decided to grow these selections under average nursery conditions and if they produced blooming plants with good habits, acceptable to the public in three to four years, they may be considered worth naming and dissemination.
The term "Dexter hybrids" has been questioned by many. Personally, I consider most of these fortunei hybrids. Not to discredit Mr. Dexter's intensive work, most of the Rhododendron growers are going along with the Dexter classification at present. In time this may crystallize into a dozen or so named varieties credited to Charles O. Dexter. Parentage: unknown because records were rather vague on most of his seedlings. For show purposes and classification the best varieties may be grouped into fortunei and smirnowii hybrids. Here at Westbury (N.Y.) we have about one hundred of the best selected from twelve collections along the eastern seaboard.
This last winter was really a good test for bud hardiness. I wish to digress a little on this subject for the simple reason that after forty years of association with named hybrids, I still cannot pin down the answer to this. We know and have proof that a young vigorous plant will form its flower buds later than a slow growing, mature plant and is more apt to be injured by a sharp drop in temperature during the months of October and November. For a number of years we have examined flower buds during the Christmas holidays and found black pips in evidence. In old established varieties we find some bud killing during winters considered mild and the reverse in winters like the one we just passed through. We have tried various foods, some higher in potash and others in phosphorous with no answer forthcoming. Moisture content of soil during the months preceding winter proved that excessive moisture caused splitting of cambium at the base only on varieties listed as B and C hardiness. This spring the writer enjoyed a trip to Ohio where winter conditions are said to be worse than ours on Long Island (N.Y.). You can imagine my surprise to see plants of 'Pink Pearl', 'Purple Splendour' and 'Gomer Waterer' with complete flower trusses. Our 'Pink Pearl' showed complete bud blasting; 'Purple Splendour' had a few trusses with 70% bud kill; 'Gomer Waterer' almost complete bud kill. I was shown a row of 'F. D. Godman' at this same nursery in Ohio with many complete dead flower buds. At Westbury (N.Y.), 'F. D. Godman' came through with complete trusses. What is the answer?
The Dexter hybrids also show strange traits on bud hardiness. We assumed that plants doing well at Sandwich, Mass., would also bloom profusely on Long Island, N. Y., but this was not the case. Cape Cod Bay and the Atlantic Ocean, with the numerous small lakes nearby, must create a moderating influence on temperatures and minimize daily temperature fluctuation. In the final analysis there may be only about a dozen outstanding varieties with an "A" rating for hardiness. The remaining may come under "B" and "C" rating. Most of the best selections run in the pink shades with flower trusses six to seven inches high and the same distance across; 12-16 flowers to a truss and individual flowers measuring from 3 1/2-4" across. Some of the creamy pink varieties have a delightful fragrance. The true red shades are rather scarce. The best collection of reds can be found in the Swarthmore collection, with a few at the Morris Arboretum in Philadelphia, Pa. The cream colored varieties seem to have a brownish yellow cast to the foliage. The best one in this class as far as foliage is concerned is one tabulated N. Y. B. G.-A, located at the New York Botanic Garden. As with most fortunei and fortunei hybrids, the new growth obscuring the blooms may be objectionable to some. This seems to be more noticeable on young vigorous plants. On the Ben Mosely estate at Ipswich, Mass., where one may see mature plants some twelve feet high, the flower trusses are so plentiful that the slight new growth protruding does not hurt the general effect. At Halesite, Long Island, N.Y. there was a crimson red with a label 'Mrs. Henry B. Gardner' attached to it. The foliage is dissimilar from all other plants from Sandwich. This plant has been a consistent bloomer with us at Westbury. A large flowering brilliant pink, purchased by the Coe Estate and named 'Mrs. Wm. R. Coe', compares favorably with some of the glorified English hybrids found at Cornwall. A good white in this strain seems to be hopeless. Out of three on trial the best is a far cry from being favored by gardeners. One of the Dexter hybrids proved such a grateful plant in habit, foliage, flower and hardiness that the writer named this one 'Scintillation'. The satiny pink flowers remain longer in bloom than most varieties and the foliage and plant habit make this a very desirable plant even when not in flower. We found very few purples; one (N.Y.B. G-2-3), a purple at the New York Botanic Garden, should compete with our standard hardy purples for an early crop. A lavender at Ipswich could be termed a glorified 'Everestianum'.
Since our winters vary a great deal, it may take another two years to gather comprehensive reports from all who have had experience with these Dexter hybrids. At present the propagated clones of the best selected are being tested in various parts of our northeastern states.
The above is not the complete story on the Dexter hybrids. In the past years collections were shipped to the Pacific coast and Ohio. Some collections were also shipped within our area. We were unaware of their locations when making our tours of inspection from Maine to Delaware. Reports from these collections would be appreciated by all lovers of rhododendrons.