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Journal American Rhododendron Society

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Volume 20, Number 2
April 1966

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Azaleas Suitable for Different Areas in Connecticut
By J. W. Oliver
Talk given at University of Connecticut, Storrs, Jan. 26, 1966

        For a plant to be suitable for an area it must be reasonably hardy in that area. Though there are other factors that enter into the overall definition of suitability, hardiness, perforce, is the number one consideration. Since hardiness involves health and vigor, proper care, and a number of factors other than climate, I think it necessary at the outset to review some of the basic cultural considerations necessary to bring about optimum conditions (of plants and their surroundings) under which plants are most likely to withstand the rigors of winter.

        Frost pockets at the nursery or at the home are not ideal places for growing azaleas. Unfortunately, most nurserymen, when choosing a site, look for smooth ground with deep top soil which is almost invariably found at the bottom of a valley. Cold air being heavier than warm air naturally settles to the lowest point and is trapped there for lack of further air drainage. Result: a low of 25 degrees may exist in the frost pocket when, at exactly the same moment, only a low of 32 would prevail on ground only a few feet higher on a slope or hillside. Thus, an azalea is usually put to its most severe test in late October or early November when we experience what gardeners call the first "black frost." In the case of azaleas normally considered hardy, in most instances, more bud damage is caused by the first one or two severe freezes than by all remaining winter weather.
        Aside from the question of frost pockets, exposure is next on location preference-north, northwest or northeast exposures are best. A tender azalea may survive on the north side of a house, or on the extreme northern slope of a hill, when it would have no chance of survival if planted on a southern exposure.

        A soil that is porous and subject to good drainage - but containing a goodly portion of moisture-retaining organic matter - is a must. While such soil is essential to healthy growth, a soggy soil causes heaving during freezing and thawing periods. Such heaving destroys root hairs and small rootlets and exposes the remainder of the root system to the drying sun and wind. It is hardly necessary to mention the pH range required by azaleas, but we do need to be reminded of the preferable chemicals to be used when it is necessary to change the soil pH. As an acidifier, plain agricultural sulphur can be used without toxic effect. Avoid repeated use of aluminum sulphate or magnesium sulphate.

        The use of proper mulch material in the right way is one of the most important considerations in maintaining healthy azaleas and protecting against winter damage. Here I must emphasize "the right way." Most gardeners and too many nurserymen - do it the wrong way. They rake leaves, or other mulch materials, around the plants in early fall when mulches should be removed, and, then in spring cleanup they remove the mulches when most needed. The right time to apply mulches is after cold weather (several freezes) has set in and the ground is frozen to a depth of about one inch. By this time plants are hardened and well conditioned for winter. The mulch then applied should prevent further deep freezing as well as thawing and thus stabilize the root temperature, hence the lower roots can replenish moisture lost by transpiration on dry windy days. This is most essential during open years such as we have had recently.
        Speaking of recent years, we must not overlook droughts. If the soil is of the right texture, a proper mulch will conserve moisture to the extent that plants may fare reasonably well with less than half normal rainfall. And, I can not emphasize this too strongly, a plant that has suffered badly for want of moisture during the summer-no matter how hardy the variety-stands a slim chance of escaping winter damage. Another advantage of the summer mulch (as in winter but even more important) lies in preventing great root temperature fluctuations. It has been proven that temperature fluctuation-between the heat of the day and cool of the night may have a range of 40 to 50 degrees, whereas the difference between high and low temperatures around the plant with a two inch mulch (say pine bark or pine needles), will be less than 20 degrees.
        Now why is the mulch harmful in late summer and early fall? Perhaps the is more widely misunderstood than any other phase of mulching practice. Under normal conditions plants will have completed their growth by late summer. Then the days become shorter and nights cooler, which should bring about the normal process of hardening off. It is at this time that the summer stored-up heat begins to leave the surface of the ground, but, if the surface is insulated by a mulch, the root system is kept too warm and the plant remains too tender. It is the mulch at the time of the first two or three frosts-in most cases at least that determine whether the plant is to have flowers next spring or even escape bark splitting.
        By actual tests it has been proven that, when we have our first few frosts in the fall, the lowest temperature recorded at night over a mulched area (at one foot above the surface) is from five to eight points lower than an adjacent soil with no mulch.
        In other words, the temperature around a small mulched plant may be as low as 26 degrees while a nearby un-mulched plant will escape without freezing. Many nurseries have learned the hard way - if you mulch small rooted cuttings or liners in autumn and have a very early freeze (such as occurred during the first weeks of October 1964 and 1965) you will have mostly dead (bark split) liners in the spring. The answer is that mulches insulate against the earth's heat which otherwise would counteract cold immediately above the ground. Some years when there is only a gradual cooling off-with no early or sudden freezes - plants, even though mulched, will escape fall damage.
        Certainly new mulches should not be applied during the period from September 1 to December 1 and certainly all mulch should be removed from around small plants in September only to be replaced after the ground is frozen. Strict observance of this rule will save both dollars and headaches.

        Given proper drainage and soil texture, it is scarcely possible to water too much during the growing season. However, except for an extended severe drought, little or no watering should be done after late August until the plants have hardened off for winter. Late watering may cause renewed late growth. However, after all danger of renewed growth is past - and assuming that there has been insufficient rainfall - the ground should be saturated to a depth of several inches. The warmth of the soil escapes-to counteract early settling frost-mostly through vapor. Without moisture there can be no vapor such as we think of in this instance. Furthermore, moisture around the lower root level is an absolute must if the root system is to function in overcoming desiccation through excessive transpiration during periods of extreme cold. Frequently, in fact most of the time, there is ample rainfall, or snowfall, to supply deep moisture, but if the ground is already frozen, that moisture can not settle to the unfrozen lower roots. Thus, we have two reasons for watering after the growing season is past, i.e., one, to provide moisture to evaporate and give off warmth that will combat early frosts; two, to supply moisture to lower roots so as to overcome effects of desiccation after soil around the upper roots has frozen solid.

        I think the requirements of acid-loving plants are so well understood by the average grower that it would be redundant here to dwell at length on this subject. Suffice it to say that more damage is done by overfeeding than by underfeeding. And, let me warn against late feeding, which like late watering, tends to extend growing into the period when plants should be hardening.

        Nurserymen generally prune azaleas too much. It is necessary sometimes to cut small plants back so as to have them "shape up" but repeated pruning in an effort to make compact plants out of those intended by nature to be open and leggy can only result in disappointment. The abnormal new growth will not be hardy, and, in general the shrub will not produce flowers equal to the plant that is allowed to find its own form. Be patient. In time the leggy plant will send out lateral growth and take on a pleasing form. There are thousands of varieties, i.e. species and clones, on the market. Of these, I dare say two-thirds are by nature tall and loose growers. For background plantings or in woodland settings, these plants make a most pleasing display. But, if you have a 'Palestrina' (for example) growing where you should have a 'Delaware Valley White,' don't prune it but substitute the proper plant.

        You can not change the climate of your vicinity, but proper care along the lines indicated so far in this paper should enhance the chances of plant survival. You must not expect perfection in your choice of winter-hardy plants. The winter of 1935-36 for example, was the worst in a period of over fifty years. Surely, we should not limit ourselves only to plants hardy enough to have escaped injury during that year. I would be willing to grow in my own garden plants that bloom well in an average of three out of four years.
        In assessing the effect of winter in Connecticut, I think too much emphasis is placed upon extreme cold. We talk about sub-zero - how cold did it get - forgetting or overlooking the fact that we may experience real killing frosts long before the plants have hardened. When gradual cooling off periods extend into November without severe freezes I think azaleas generally will survive our cold winters - even temporary periods of 15 or 20 below zero. But, if we have a sudden drop to around 25 above early in October, especially after a severe drought, we are certain to have few, if any, azalea blossoms the following spring. And this is so without zero weather. I dare say that whether you live in mid-Connecticut, the coastal area, or Long Island, if you had taken the trouble to examine the flower buds on your azaleas about the 15th of October you would have found most of them dead. Many nurserymen, even to this day, do not know that their azaleas are bud blasted and many of them bark split. Next year they will blame it on the bad winter. For two years now we have had devastating freezes in early October. I think you will have to go back to about 1928 to find a record of such early killing frosts. I mention this to lend courage to those of you who are still surveying the losses of the last two years. Certainly, average conditions must come sooner or later.
        I have found that the book The Climate of Connecticut by Joseph J. Brumbach, (State Bulletin No. 99) published in 1965 presents much useful climatological information. Most of the information given therein applies only to what might be called the general climate as distinguished from the micro-climatical conditions such as frost pockets, etc. The statistical data and calculated probabilities are based upon detailed weather reports covering the thirty-three-year period 193063. As representative of different parts of the state I suggest we review some data based upon weather reports of New Haven, Norfolk, Hartford and Storrs. For example, the average seasonal mean temperatures are:

  Winter Spring Summer Fall
New Haven 30.5 47.0 69.6 53.8
Norfolk 22.7 42.6 65.8 47.7
Hartford 28.7 48.0 70.5 52.6
Storrs 27.4 45.3 67.8 51.2

        Now if you are located in an area that approximates the figures given for New Haven, you can grow successfully many of the evergreen azaleas - species and varieties. But if you are in an area where the average weather conditions approximate those given for Norfolk you might well forget about evergreen azaleas and confine yourself to deciduous types.
        Since the chance of an early freeze is so important - i.e. before plants have hardened - I think we might well consider the following calculated probabilities:

Probability of First Fall Occurrence
  25% Chance
(1 out of 4)
90% Chance
(9 out of 10)
New Haven    
32 degrees      Oct. 17      Nov. 13
24 degrees      Nov. 12      Dec. 7
16 degrees      Dec. 2      Dec. 20
Cream Hill    
32 degrees      Sep. 30      Oct. 22
24 degrees      Oct. 28      Nov. 24
16 degrees      Nov. 18      Dec. 16
32 degrees      Oct. 10      Nov. 2
24 degrees      Nov. 10      Dec. 3
16 degrees      Dec. 1      Dec. 18
32 degrees      Oct. 2      Oct. 18
24 degrees      Oct. 26      Nov. 21
16 degrees      Nov. 24      Dec. 11

        In Fairfield we had a low near 24 on October 8, 1964 and October 4, 1965. Thus, you can see that we have had two successive years in which the devastating cold was about one month ahead of schedule. While considering the effects of early freezing temperatures I think it again necessary to review the effects of mulching. It has been demonstrated that on October cold nights (when the ground is still giving off heat that counteracts settling cold air) a thermometer placed 12 inches above a mulched area will show a low for the night of some 6 to 8 degrees colder than one only a few feet away but placed 1.2 inches above an un-mulched area. So, by keeping the roots of small plants warm with a mulch you are inviting freezing temperatures at the level of the flower buds several days, if not several weeks, ahead of nature's schedule.

Plants For The Different Areas
        For the coldest areas similar to the northwest part of the state, say near Norfolk, the only suitable azaleas are to be found in the deciduous group. Most of the native species - especially, R. vaseyi (pink as well as the white form, 'White Find') and R. viscosum should do well. Reports from the University of Minnesota show that R. mollis and some of the native species (such as roseum, nudiflorum, and viscosum), and crosses between them, did well near Minneapolis following the winter of 1961-62 when temperatures for a prolonged period reached about minus 30 or lower each night.
        Most of the Ghent and Exbury hybrids should prove suitable throughout the entire state of Connecticut. Throughout the Connecticut River Valley area - say Hartford and Storrs - only a few of the hardiest evergreen types will prove suitable. And here I must offer my own definition of "suitable." To me a plant of good flower and good form if it receives no appreciable plant damage or bud-blasting more frequently than once out of every four years would be suitable or worth growing. If it fails to perform well on an average of 2 out of 4 years then it is unsuitable. For this central area I believe the following azaleas would be satisfactory from the standpoint of climate.

'Amoena', magenta 'Sherwood Red', (Kurume) red 'Mary Dalton', (Gable) salmon red
R. poukhanense, rose purple 'Daphne', (Kurume) lavender, light center 'Springtime', (Gable) clear pink
'Delaware Valley White' 'Boudoir', (Gable) watermelon pink 'Atlanta', (Kaempferi) light purple
'Yodogawa', double purple pink 'Caroline Gable', (Gable) pink 'Fedora', (Kaempferi) salmon rose
'Hinomayo', (Kurume) pink 'Corsage', (Gable) orchid 'Mayflower', (Glenn Dale) salmon pink
'Sherwoodi', (Kurume) orchid 'Herbert', (Gable) crimson purple 'Palestrina', (Vuyk) white

For the coastal area - a strip a few miles wide along the Long Island Sound - most of the Kurumes, possibly 75% of the Gables and 30% of the Glenn Dales are suitable. Here the number is so large that it becomes a matter of personal choice. So, I set forth here a list, as my first choice, were I starting a new nursery to serve customers in this particular zone:

'Addy Wery', tall growing, blood red
'Daphne', large lavender with white center
'Hino-crimson', good form, brilliant red
'Peach Blow', salmon red
'Sherwood Red', similar to 'Addy Wery' but low compact
'Louise Gable', double salmon pink
'Rosebud', like double pink pearl rose buds
'Springtime', clear pink
'Stewartstonian', a true red
'H-12-G', bright scarlet double, very late
'Guy Yerkes', salmon pink
'Polar Bear', white, hose-in-hose, superior to 'Snow'
'White Cloud', light pink, hose-in-hose
'Cavalier', orange red
'Copperman', brilliant red
'Crusader', geranium red, very late
'Dayspring', center white, shading pink, very early
'Dream', deep rose pink, frilled
'Emblem', rose red
'Epilogue', light rose pink, latest of all to bloom
'Gaiety', light rose pink
'Geisha', white, peppermint striped, sometimes red sports
'Glacier', white, excellent foliage but can not take much sun
'Glamour', dark rose pink
'Greeting', coral rose, excellent form and foliage
'Louise Dowdle', brilliant pink
'Martha Hitchcock', white center, margined magenta  (In young plants the flowers are usually all magenta but mature plants  invariably have blossoms predominantly white.)
'Mayflower', pink, hose-in-hose, about the hardiest of all
'Phoebe', jasper pink, very popular
'Reward', lavender pink
'Shimmer', white, sanded and striped, similar to 'Geisha' but two weeks later
'Vestal', the best of all late whites
'Witchery' clear pink, large loose grower
'J. T. Lovett', dwarf Macrantha
'Balsaminaeflora', orange red, sometimes sold as 'Rosaflora,' very late
'Flame Creeper', flame color, very low creeper
'Goodtimes', compact Kurume, pink
'White Gumpo'
'Pink Gumpo'

        There are many Satsuki hybrids (sometimes called Chugai hybrids) that have recently been brought into the country. Most of them have been supplied by the Chugai Nursery Company of Osaka, Japan. I believe many of them are suitable for rock gardens in the coastal area if given some protection, but I am unwilling to offer a specific list until we have had more experience with them. Certainly they have the growth habit and kind of blossom that appeal to rock garden enthusiasts.
        Most dwarf azaleas are derivatives of the R. indicum or a sub-division known as R. eriocarpum. All indicums, or Macranthas, are so tender as to require some winter protection in the vicinity of New York City or the Long Island Sound area in most years, but it is not very difficult to give adequate protection to small plants in a rock garden. The real difficulty arises when the nurseryman tries to grow them in quantity in the open fields.

Deciduous Azaleas For The Entire State
        Recent introductions of Knap Hill and Exbury deciduous azaleas offer a wide range-flower as well as growth habit-of azaleas, that in my opinion should replace most of our native, mollis and Ghent varieties hitherto grown by American nurserymen. Exburys are not difficult to propagate, so within a few years named varieties should be plentiful. A word about seedlings in this group-I think 9 out of 10 seedlings are good. But I think it wrong to sell them without emphasis upon the fact that they are seedlings. Where specific colors are demanded, it is best to stay with named varieties. So, I offer the following list of Exburys and other deciduous azaleas as my choice to supersede the great range hitherto grown.

vaseyi, pink
vaseyi 'White Find', white
viscosum, white, fragrant, very late
schlippenbachii, pink, very early
'Daviesi', creamy white, very fragrant
'Nancy Waterer', golden yellow
'Aurora' (Exb.), salmon pink
'Berry Rose' (Exb.), orange red
'Cecile' (Exb.), red with orange shading to yellow
'Eisenhower' (Exb.), reddish orange
'Flamingo' (K.H.), rose with orange
'Gibraltar' (Exb.), burnt orange
'Ginger' (Exb., orange
'Hugh Wormald' (Exb.), lemon yellow, blotch slightly orange
'Persil' (Slocock), white, pale yellow blotch
'Satan' (Slocock), scarlet
'Strawberry Ice' (Exb.), coral, orange and yellow
'White Swan' (Exb.), white, yellow blotch

        Perhaps there are others as good or better than these I have listed, but I have not seen them in bloom. If we azalea growers introduce new and better varieties, educate our customers, and properly display our products, there is no reason why we should not double or triple our sales in a short period of time.
        These comments and suggestions are based upon my study and experience. As such, they are bound to be affected to some degree by my own prejudice. What we need is a test garden independently and scientifically run, from which every nurseryman could get periodic reports that would meet the test of true objectivity.

Volume 20, Number 2
April 1966

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