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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Volume 34, Number 1
Winter 1980

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Cold-Hardiness, Nutrition, Mycorrhizae and
Companion Plants as they Relate to the Ericaceae

By Michael A. Dirr, Ph.D., Mercer Fellow
The Arnold Arboretum, Jamaica Plain, Mass.
Reprinted from The Rosebay

Rosebay Note: Dr. Michael A. Dirr is internationally known for his work as an academician and researcher. Prior to holding the coveted Mercer fellowship, he was an associate professor of ornamental horticulture at the University of Illinois. Presently he is the director of the University of Georgia Botanical Garden, Athens, Georgia. His contribution to this issue of the Rosebay is a sequel to a recent talk before the Massachusetts Chapter.
        When one presents a talk to a specific plant society he is never sure of what to offer or the response to expect. The Massachusetts Chapter of the American Rhododendron Society was a great group for, although my talk included things of peripheral interest, enthusiasm and questions emanated from the audience. The following text is an attempt to record some of my rambling remarks.

        In the coastal area of New England, a much greater diversity of rhododendrons can be successfully cultured than in the Midwest. You may consider your climate harsh, but by comparison it resembles a banana belt. Low temperatures (-20 to -25 degrees F.) plagued most of the Midwest during the winter of 1976-77. Desiccating winds are always problematical and severely limit successful broadleaf culture unless plants are properly protected. The soils are heavy (clay based) and often poorly drained while pH may range from 6 to 7 plus. Raised beds with appropriate soil-amendments (peat, other organic materials) and chemical agents (ferrous sulfate) are often necessary. E. H. Wilson spoke of the 'Iron-Clads' but they are not safe under Midwestern conditions. Tremendous "winter burn" was evident with flower bud damage and in some cases plant loss. 'Nova Zembla', 'America', 'Lee's Dark Purple', 'Boursalt' and 'Roseum Elegans' showed the ill-effects.
        A few of the better performers include R. dauricum var. sempervirens, R. mucronulatum, 'PJM', 'Pioneer', 'Herbert', 'Gibraltar', 'Polar Bear', R. poukhanense, R. schlippenbachii, 'Karens', Northern Lights hybrids (produced by University of Minnesota Landscape Arboretum by crossing R. kosterianum x R. prinophyllum) and a few others. The deciduous types are preferable to the semi-evergreen and evergreen types simply because the problem of wind desiccation is avoided. Many of the Exbury group, Mollis hybrids, etc., could be grown but have not been tried to any degree. 'PJM' is by far the best broadleaf performer with flower bud hardiness to at least -35 F and root hardiness to -9 F (hardiest of all plants tested to date). Winter injury (cold hardiness) is something of interest to everyone in the north temperate zones. When we speak of a plant's cold hardiness it refers to that plants' ability to withstand low temperatures - nothing less and nothing more! Forget desiccation, poor drainage, improper nutrition and other variables which contribute to decline. Plant cells are injured by low temperatures in two possible ways. The first is called intracellular freezing and refers to ice crystal formation within the membrane. This type of freezing is always lethal. Supposedly this only occurs at rapid temperature drops of something like -20 F per minute. At least one cold hardiness researcher believes this can occur more often than most people believe. The winter browning of Thuga occidentalis is thought to be caused by intracellular freezing. More about this later. Most damage is caused by intercellular freezing. In this situation water freezes in between the cell wall areas with the resultant injury stemming from dehydration effects. Considerable research is necessary before a clear understanding of freeze injury emerges. Rapid freezing proved more injurious to leaves of Rhododendron catawbiense 'Grandiflorum' than slow freezing, rapid thawing or slow thawing. The following table details the results of Havis's work. His results show clearly that extremely rapid temperature drops result in permanent injury to plant cells.

Treatment % Injury
Freeze Thaw  
Slow Slow 0
Rapid Slow 100
Rapid Rapid 95
Slow Rapid 0

        Flower buds are most sensitive to low temperatures followed by stem and leaf buds. Actually, if the stem is injured, then the other parts regardless of hardiness, are rendered biologically non-functional. It is worth emphasizing the sensitivity of flower buds to low temperatures. When breeding rhododendrons for colder climates hardiness should probably be the overriding concern. There are those who would argue for clearer reds and good yellows. The first priority should be a plant with good flower bud hardiness in the range of minus 20 to minus 30 degrees F.
        Stems are much hardier than roots with an average difference of perhaps 40 degrees F. Steponkus (Cornell University) determined killing temperatures for Pyracantha and reported the shoots were killed at -15 F., old roots at 2 F. and young roots at + 22 F. Root killing temperatures for selected rhododendrons are presented in Table 1.

Table 1: Root killing temperatures of container grown ornamentals in winter storage.*
  Killing temp
degrees F
Species or cultivar
R. prunifolium +20
R. 'Hino Crimson' +19
R. Exbury hybrid +17
R. schlippenbachii +15
R. 'Purple Gem' +15
R. 'Gibraltar' +10
R. Hinodegiri' +10
R. carolinianum 0
R. catawbiense 0
R. 'PJM' -9
* From Gouin, Coop. Ext. Serv., U. Maryland, HE 102-76

        Pellett has measured temperature differences between plant tissue and ambient air. On cloudy days there was no difference. However on sunny days differences as great as 25 degrees F. were noted. These types of differences would occur in evergreen rhododendrons. If cloud cover or a cold front moved in rapidly the tissue temperatures could drop sufficiently fast to result in intracellular freezing (always lethal). If this type of occurrence was repeated over the winter months the cumulative effects of dead cells would show up as browned leaves.
        Bark splitting occurs on large trees as well as rhododendrons. Multiple factors are probably involved but one major contributor is the temperature difference between the air and the cambial tissue. Pellett has recorded differences as great as 40 degrees F. The warm cambial tissue would have expanded and a sudden dropping of temperature could result in contraction which might cause bark splitting. Plane trees and Norway maples are very prone to this. Take a look in your neighborhood. The injury usually occurs on the south or southwest side where the trunk is exposed to the warm sun and the greatest temperature differential would occur.
        Another important consideration in growing rhododendrons or any plants that might require winter protection is site placement. Pellett oriented wooden fences, in North-South and East-West directions. Shrubs were then planted around the fences and tissue temperatures recorded. Where temperature fluctuations were the greatest between ambient air and tissue, the most injury occurred. The southern exposure was worst and the northern was best with east and west intermediate.
        Many people are into container growing and, unfortunately, are not familiar with the risks. Container-grown plants need protection during the winter months for roots are much more sensitive to cold than above-ground plant parts. Pellett measured the soil temperature in a container and at a depth of 3" below the field soil surface. The unprotected container temperature was +5 F. while the field soil was +21 F. Obviously many of the roots in the container medium would be killed. Refer back to the table of root killing temperatures for an idea of which species or cultivars would be injured. Storing plants in a poly-house is a common nursery practice. For the amateur, heavy mulching or the use of a cold frame might suffice.
        Desiccation injury can be serious on broadleaf plants. During the winter of 1977-78 the ground was frozen in the Midwest from December to March and snow cover was insufficient. The broadleaf plants lost moisture but were unable to replace it since the soil was frozen. No matter how much water is in the soil, if it exists in a frozen state, plants cannot absorb it. The winter of 1978-79 in the Boston area could prove to be similar. I have noticed considerable winter desiccation on Mahonia, Berberis (evergreen types), Prunus laurocerasus 'Schipkaensis', Buxus, Ilex, Gable hybrids and others. A gardener should keep records on this type of injury and note how the degree (severity) varies from year to year depending on weather conditions.
        In terms of hardiness, laboratory testing can accurately approximate the killing temperatures of flower buds and stem tissues. The Arnold Arboretum is currently cooperating with the University of Minnesota Landscape Arboretum. Briefly, stem or bud tissue is collected in January or February (it must be done at this time for plants develop their greatest cold handiness). The pieces are exposed to gradually decreasing temperatures and samples are removed at regular intervals. These plant parts are then examined for the degree of injury by various visual and/ or analytical procedures. There is usually a fairly distinct break between the temperature that caused no injury and the temperature that induced permanent injury. A temperature difference of 2 to 5 degrees may not sound like a great deal but to a plant it represents the difference between survival or death.

Editor's Note: also see article this issue "Azaleas and Their Cold Tolerance," by Dr. Harold Pellett.

        Drainage is of paramount importance for successful landscape establishment of most rhododendrons. There are a few species (R. viscosum, R. atlanticum, R. arborescens, R. canadense) which occur naturally in wet areas. On the University of Illinois campus, attempts to establish mass displays have failed because proper drainage was not provided. A noted horticulturist once made the comment that with some plants the hardiness can be improved by one zone if proper drainage is supplied. It may not be a case of improving hardiness but allowing a particular plant to grow vigorously thus resisting various stresses (such as cold). Inadequate drainage results in reduced aeration which may affect respiration and other aerobic processes. If the roots suffer, the above ground parts also show stress symptoms.
        Rhododendron nutrition and culture have been debated for years and if you have a successful formula then stay with it. The usual recommendation concerns low pH ( 4.5 to 5.5) and organic or ammonia nitrogen sources. Carlson's Gardens Catalog states that "fertilizing is better left undone than overdone. Cottonseed meal is one of the safest fertilizers because it is slow acting. Fertilizers for acid-loving plants are good if used in moderation." In general ericaceous plants have low nutrient requirements compared to most ornamental plants. Rhododendrons may produce optimum growth when the tissue nitrogen levels range from 1.50 to 1.75 percent of dry weight. Cotoneaster and Pyracantha might require 3 to 3.3 percent for maximum growth. In simplest terms, rhododendrons do not need as much fertilizer as other plants. In my research, the ammonium form of nitrogen has proven extremely toxic to rhododendrons and other plants when applied in high concentrations (100 ppm NH4-N at every watering). The crux with ammonium fertilization is to keep the levels low. Many people have described symptoms of "fertilizer burn" which, in fact, was an expression of ammonium toxicity. The ammonium ion can severely impair metabolic processes if it accumulates to any degree in plants. At the low pH ranges which are recommended for rhododendrons the ammonium form of nitrogen predominates. If the pH rises above 5.5, a conversion of ammonium to nitrate (NO3) occurs because of the presence of certain bacterial species which catalyze this reaction. This chemical change is influenced by pH which in turn affects microbial populations and species. Nitrate (NO3) can serve as an effective nitrogen form for rhododendrons. The pH increases if a nitrate fertilizer [(KNO3 or Ca(NO3)2)] is used. The most important aspect of using NO3-N is to provide an available iron form such as a chelate or sequestered iron. Nitrate is less toxic than NH4 and will promote good growth provided the other nutrients are supplied in an available form. I have grown 'PJM' in container culture with NO3 or NH4 nitrogen supplied at 75 and 150 ppm. Tremendous growth resulted from the NO3 fertilization while toxicity and reduced growth resulted from the NH4-N. The crux in this work was a supply of all essential elements in an available form.
        The word mycorrhizae often surfaces in relation to ericaceous plants. There is little doubt that they play a prominent part in plant survival under low fertility situations. I have collected roots from Vaccinium corymbosum, high-bush blueberry, in the wild and noted extensive mycorrhizal infections. However, plants (rhododendron, leucothoe, blueberry) growing under high nitrogen nutrition failed to show any infection. Mycorrhizae are fungi which live in a sort of symbiosis with plant roots, to the mutual benefit of host plants and fungus. Anyone fascinated by this subject should consult the following reference:
Harley, J. L. 1969. The biology of mycorrhiza. Leonard Hill, London, England. 334pp.
Mycorrhizae are probably most important in mineralizing certain essential elements, especially phosphorous. It is obvious that plants can survive without mycorrhizae. You might dig one of your rhododendrons and examine the roots. If the young roots appear swollen or flattened then there is a good chance the roots are infected. For further proof find a microscope and investigate.
        Salt damage can be another problem with rhododendrons. In general, rhododendrons are very susceptible to excess soil salinity and will exhibit severe leaf burn. Salt levels above 2 to 3 m mhos (about 1200 to 1900 ppm) will induce damage. This is part of the reason for recommending low levels of fertilizer. The fertilizer you apply [(NH4)2SO, Ca(NO3)2NH4O3, Peter's, Rapid-Gro] is formulated as a salt. Too much will induce injury. This is another reason for recommending something like cottonseed meal because of its slow availability. It should be mentioned that the injury results from an osmotic effect. This means that water may not move into the plant even though there is ample water in the soil. The salt (fertilizer) which is dissolved in the soil water causes the osmotic effects.
        Companion plants for rhododendrons should be of interest and for that reason I  have chosen to discuss a few. Rhododendron gardens can be somewhat bland when not in flower.

Styrax japonicum - Japanese snowbell. An especially dainty, 20 to 30 foot, multi-branched tree with white, bell-shaped, pendulous flowers in May-June. Gray bark is also handsome. Easily propagated from cuttings.
Parrotia persica - Persion parrotia. A small tree (30') of oval to rounded outline with lustrous foliage which changes to yellow-orange-red in fall. The bark develops a sycamorish character and is beautiful in winter.
Stewartia pseudocamellia or S. koreana - Another small to medium-sized shrubby tree with large white flowers in late July-August, deep maroon to reddish-purple fall color, and superbly mottled gray to cinnamon-brown color bark.
Fothergilla gardenii - Dwarf fothergilla
Fothergilla major - Large fothergilla. Both are superb plants, the only significant difference being size. The former grows to 3'; the latter from 6 to 10'. They make dense rounded shrubs. The foliage is a leathery dark green and turns the most gorgeous combination of yellow, orange and red. Flowers appear in white, 1", honeyscented, bottle brush masses in late April or early May. The fothergillas are tremendous plants; unfortunately they are not well known.
Clethra alnifofia - Summersweet clethra. A good native plant that seems to thrive in wet or dry soils. The fragrant, white flowers span four to six weeks from July into August. The plant will sucker and form colonies. The variety rosea has pink flowers. Softwood cuttings root easily.
Aronia arbutifolia 'Brilliantissima' - Red chokeberry.  One of the best shrubs for fall color and persistent bright red fruits. The white flowers in May are showy but not spectacular. Height varies between 6 to 9 feet. Based on observations in the fall of 1978, this was perhaps the most spectacular fall-coloring shrub in the Arnold. It is very easy to grow but again not widely known to gardeners.
Euonymus alata - 'Compacts' Dwarf-winged euonymus. This 8 to 10' high, densely branched, rounded shrub is often praised for its rich fall coloration. There is little doubt that it ranks among the best fall coloring shrubs but still is not as intense as the chokeberry. It does not sucker, is easily propagated from cuttings and has no serious insects or diseases.
Rhus aromatica - 'Gro-low' - Fragrant sumac.  The species is adapted to virtually any type of cultural condition, from railroad right-of-ways to shady woods. The cultivar 'Gro-low' is especially useful because it does not grow over 18" and forms a solid ground cover. The lustrous dark green foliage holds late into fall when it may assume reddish purple tints. It is easily propagated by softwood cuttings.
Mahonia aquifolium 'Compactum' - Compact Oregon grape. This is a beautiful broadleaf evergreen with rich bronze new growth which matures to a lustrous green. In winter the foliage assumes a purplish tint. The plant grows 1 to 2' high and about twice as wide. Cuttings should be taken in November.
Hydrangea quercifolia - Oakleaf hydrangea. This shrub might prove excessively coarse for most gardens. The large red oak-shaped leaves are leathery green and may turn a rich wine-red in the fall. The leaves are often held into November. The white flowers are borne on 8 to 12" long panicles in June-July. The flowers develop a purplish pink color with time and are effective into August-September. This species is well adapted to heavy shade.
        All plants discussed will do well under cultural conditions which promote rhododendron growth. They provide colors and textures unattainable with rhododendrons. Too often when companion plants are discussed other members of the Ericaceae come to mind. The plants mentioned are serviceable and ornamental. Who knows, some of you may become so enamored by Aronia that you join that society!

Cross, James E., 1978, The winter of 76-77. To the root of the problem., Green Scene, 7 (1): 17-19.
Govin, Francis R., 1973, Winter protection of container plants, Proc. Int. Plant Prop. Soc., 23: 255-258.
Govin, Francis R., 1976, Winter injury to container-grown plants, Proc. Better Trees for Metropolitan Landscapes, USDA Forest Service, Gen. Tech. Rep, NE-22: 179-183
Havis, John R., 1965, Desiccation as a factor in winter injury of rhododendron, Proc. Amer. Soc.,  Hort. Sci., 86: 764-769
Havis, John R., 1964, Freezing of rhododendron leaves, Proc. Amer. Soc Hort. Sci., 84: 570-574
Havis, John R., 1976, Root hardiness of woody ornamentals, Hort Science, 11, 385-386. 
Havis, John R., R.D. Fitzgerald and D. N. Maynard, 1972, Cold hardness response of flex crenata, Thumb. cv. Hetzi roots to nitrogen source and potassium, Hort. Science, 7: 195-196.
Patton, George E., 1977, Unusual winter tests borderline hardness, Green Scene, 6 1 1012.
Pellett, Harold, 1971, Comparison of cold hardiness levels of root and stem tissue, Can. J. Plant Sci., 51: 193-195.
Pellett, Harold, 1974, Sunscald injury-influence of stem size and exposure on winter temperature  of stem tissue. U. of Minn. Agr. Expt. Sta. Misc. Rept., 111, p21-22.
Pellett, Norman E, 1973, Influence of nitrogen and phosphorous fertility on cold acclimation of roots and stems of two container-grown woody plant species, J. Amer. Soc. Hort. Sci., 98: 82-86.
Steponkus, Peter L., George L. Good and Steven C. West, 1976, I Cold hardness of woody plants. 
   II. Freezing injury and cold acclimation of woody plants. III. Root hardiness of woody plants. IV. 
   Using polyhouses for protection. V. Cultural factors for over wintering. These articles appeared in 
   the American Nurseryman from August 15 to October 15, 1976. They should be read by anyone 
   interested in cold hardiness.
Studer, Elaine J., Peter L. Steponkus, George L. Good and Steven C. West, 1978, Root hardiness of container-grown ornamentals, Hort Science, 13: 172-174.

Volume 34, Number 1
Winter 1980

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