QBARS - v14n4 Hardiness of the Ironclads in Central Connecticut

Hardiness of the Ironclads in Central Connecticut
A. F. Serbin. M.D., Hartford, Conn.

How relative is the word "hardy" in describing the ability of a rhododendron to survive in a given climate! If you pause but a moment to realize that the classifications for hardiness comes either from England or from the core of our rhododendron group, the northwest U.S., you must at once conclude that the New England territory has no real yardstick to follow as a guide to hardiness.
The rating of A-hardiness given to a rhododendron in England means that it is hardy anywhere in the British Isles. Excluding the old Waterer "Ironclads," there are very few of these that will withstand our dry hot summers and extremely varied winters many of which are snow-less in subzero periods. It is amusing to the New Englander warming himself before his fireplace on a blowing wintry night as he thumbs through the Royal Horticultural monthly bulletin and reads of "Goldsworth Orange, a hardy hybrid rhododendron" to mention but one of a multitude of "hardies." This variety cannot even take the relatively mild climate of southern New Jersey!
Equally short of the truth as far as we here are concerned is the rating of H-2 (able to withstand 15° F.). If this were so, then such plants as R. 'Arthur Ivens', R. 'Blue Peter', R. 'Cynthia', R. 'John Walter', R. 'Lady C. Mitford', R. 'Lady Longman', R. 'Mars', R. 'Mrs. J. G. Millais', R. 'Mrs. P. D. Williams', R. 'Naomi', R. 'Purple Splendor', R. 'Rubens', R. 'Sappho', R. 'Sweet Simplicity' and many more would be thriving in the Connecticut valley. Alas, such is not the case.
Much more reliability can be placed on hardiness ratings according to zone numbers. In the ensuing paragraphs some of the hardiness features will be described along with the highly important "protection factors" which completely alter and make possible the cultivation of H-2 and even H-3 varieties in Connecticut. Since it is hardly possible to provide an abbreviated hardiness code which will inclusively indicate bud, flower, foliage and wood hardiness, it would appear that the zone classification would indicate in a more satisfactory manner the level of viability. Thus, Zone-4 would include plants able to withstand -10° F. to -20° F. without loss of buds, flowers, foliage, or wood. Of necessity, one should consult the U. S. Department of Agriculture zone map of the U.S.
A fellow rhododendron enthusiast once wryly commented "what do the English or Northwest chaps know about hardiness of rhododendrons in Connecticut?" A trip to both England and the Oregon-Washington areas clearly showed me how much truth there was in this casual utterance.
In dissecting the meaning of hardiness what should we expect of a rhododendron that is truly hardy? A plant that not only lives, but continues to put forth moderate growth each season, leaves that will not exfoliate or burn severely in the winter sun and wind, buds that will not blast, flowers that will last at least 14 to 18 days, and wood that will not split before spring is what we should expect in a rhododendron that is hardy. It should prove equally tough for drought and the vicissitudes of summer. Nor can one pass judgment on the behavior of a single plant. Rhododendrons can default due to many causes such as individual disease caused by borers, fungi etc., chemical irritation, mechanical faults in planting to mention but a few. Therefore the analysis and conclusion of hardiness must come from test plantings of large numbers of a given variety.
When the colorful classification of "Ironclads" came into liberal usage, I envisioned something tough, almost indestructible-a group we could depend on for successful growth and flowering in central Connecticut. I started to collect and propagate as many as were available from Holland, England and the U.S. Observations of these varieties these past 10 years has indicated that for us, at least, the written word of hardiness and "Ironclad" must be taken with more than a grain of salt.
The observations made in this report pertain only to central Connecticut. The Boston and Cape Cod areas as well as Long Island, New Jersey and coastal Connecticut can grow semi-hardy plants in the open that cannot survive under similar conditions in the Connecticut Valley. However, with the addition of protective factors, certain so-called H-3 varieties can successfully be grown even in this harsh climate. The beginner in gardening soon learns that a tender rhododendron can be grown in the safe harbor of an enclosure of his home. Protection from wind and winter sun utilizing a wall or hedge of evergreens, a stout fence or a wind-free north exposure close to a wall are some of the important factors. We have experimentally utilized test gardens here for some years, several of which have been placed about my home. Plants of dubious hardiness for our climate such as R. 'Mars', R. 'Blue Peter', R. 'Purple Splendor' and R. 'Goldsworth Yellow' are decimated on the southern and western exposures of the house. They just about get through the winter with loss of buds and much burning of foliage on the eastern side. They come through the winter in excellent shape on the northern face of the house.
So much for what you can do with adequate protection for semi-hardies. But what can we say for the granite tough H-1 and H-2 varieties? Can they live out in the open? Many of our nurserymen boast in their advertisements in commercial journals that their rhododendrons are grown out in the "open field without protection." The majority of these growers are to be found either in Long Island or New Jersey, but rarely above the Philadelphia zone line. Our experience here is that most of the hardies can indeed be grown out in the open if provided with a mulch of 4 inches of dead leaves or salt hay and if they are 3 or more years of age. However, R. 'Amphion', R. 'Atrosanguineum', R. 'E. S. Rand', R. 'F. D. Godman', R. 'Ignatius Sargent', R. 'Kettledrum', R. 'Lee's Dark Purple' and R. 'Mrs. P. den Ouden' (really not hardy at all) will suffer varying degrees of foliage burning and appear quite miserable by April. In an average year such toughies as R. 'Atrosanguineum', R. 'Dr. Dresselhuys', R. 'Dr. Rutgers', R. 'Kettledrum', R. 'Lady Armstrong', R. 'Lee's Dark Purple' and even the highly rated R. 'Mrs. C. S. Sargent' will lose half or more of their flower buds. If these same plants are grown under our oak trees, or given a wall of evergreen trees as a wind and sun break, we have quite a change in the outcome. They come through with flying colors. A mere enclosure using snow fencing will bring the plants through the winter in better condition.
At this point some mention should be made regarding some varieties included in the "ironclads" which to my mind should be called "tin-clads" for their armor is thin indeed. R. 'Dr. Lovink', R. 'F. D. Godman', R. 'Lee's Dark Purple' and R. 'Mrs. P. den Ouden' are certainly not hardy here. The first and last hybrids of this group do poorly even with protection.
Which, then, are the hardiest of the hardies? Among the reds we rate R. 'America' and R. 'Nova Zembla' as the best. As an all around plant I personally consider R. 'Nova Zembla' the finest of the reds for its form, flower and its unexcelled toughness. Yet this variety which has been available for some 8 to 10 years on the market is little known among nurserymen and difficult to obtain.
In the field of the hardy whites we encounter plenty of hardies as defined above. Alas, the color of white lacks quality in purity many of which are dirty or muddy-white. R. 'Cunningham's White' and R. 'Gomer Waterer' are not bud-hardy here. I consider R. 'Boule de Neige', a delightful dwarf, the best of the whites, again referring to its form, flower and extreme weather-proof virtues. In some ways it must be considered among the few rated as "hardiest of all" due to its heritage ( caucasicum x catawbiense hybrid). I find it hard to understand why this hybrid is looked upon with disdain. Indeed, the Dutch exporters no longer list it. In this day when hardy dwarfs are hard to find, is it not curious that a fine-leafed, low growing, dense shrub which is almost indestructible and never fails to faithfully produce a mass of flowers should be on its way out? However limited the supply, the demand for this plant continues to exceed the supply, thanks to the astute observations of the average rhododendron enthusiast.
In the pinks nothing supersedes the true clones of R. 'Roseum Elegans', and to a lesser degree, R. 'English Roseum', and R. 'Roseum Superbum'. All three are variations of the same Waterer cross (1851). This group is truly weather-repellant and singularly free from disease. Its buds have taken -25° to 30° in the open field! The glossy, dark convex leaves resist the sun and wind beyond expectation.
Among the purples (and there are no true purples) R. 'Caractacus', R. 'Catawbiense Grandiflorum', R. 'Everestianum', R. 'Purpureum Elegans', and R. 'Purpureum Grandiflorum' are almost equal in hardiness rating behaving quite satisfactorily from a survival point of view. Of this group I prefer R. 'Caractacus' to the rest, but without particular enthusiasm.
As a safe rule of thumb, I would therefore urge rhododendronists above the Philadelphia zone line not to plant any but H-1 varieties in fully exposed areas (plants 3 or more years of age). With the protection of a house wall, hedge or fence, live dangerously and try your H-2 and even H-3 varieties. You may lose one or two in your venture, and where timidity may save you a few dollars and momentary anguish, if your H-3 lives and blooms, your joy and sense of accomplishment will more than repay your efforts.