Rhododendron Hardiness Along
the Northern Edge: The Ontario-Quebec Situation
Toronto, Ontario, Canada
Montreal Botanical Gardens
Montreal, Quebec, Canada
Cold winters, fluctuating winter temperatures, unreliable snow cover, hot summers, four-to five-month-long growing seasons with late spring and early fall frosts, and mostly alkaline soils pose a challenge to growing rhododendrons from the Niagara Peninsula to Montreal. This continental zone is located in a narrow belt, north of Lake Erie and Ontario, along the St. Lawrence River to Montreal. It features hardiness rating H2 (-15°F, -26°C) from Vineland to Toronto and H1 (-25°F, -32°C) from Toronto north and east to Montreal with dormant season climatic conditions, as defined in (5).
Hardiness Locations in Table I
Rhododendrons are not indigenous to this zone. During the early 1930s, brother Marie-Victorin, the founder of the Montreal Botanical Gardens (MBG), and Henry Teuscher, the designer of the gardens, chose a location in the arboretum section of the gardens to initiate an ericacetum. The first rhododendrons were planted at that time. Several Ghent azaleas were introduced from 1951 to 1953. They continue to bloom profusely. In 1958, a section of the arboretum nursery was allocated for the testing of rhododendrons and other ericaceous plants.
Rhododendron culture in Ontario was introduced during the early 1940s by the late Leslie Hancock and others. The Hancock family still operates the Woodland Nursery in Mississauga, Ontario.
The collection at Vineland, Ont., was established during the same period by Agriculture Canada. In 1973, the Edwards Garden collection in Toronto was initiated by the Rhododendron Society of Canada (RSC) - Toronto Region. Several of the founding RSC members (Fig. 1) initiated rhododendron gardens in Oakville, Ontario, during this period.
Figure 1. Seasoned ARS-District 12 members: Richard Birkett (right), and Jim
Lunny (left) in Jim's garden, Oakville, Ontario. Many elepidote and semi-
persistent-leaf Tsutsusi azaleas are located in front of a coniferous shelter "b"
type, preventing low angled winter sun from shining on the leaves. This coniferous
strip is situated west and south of the rhododendrons showing in the photograph.
Photos by the authors
In 1972, the idea for an ericacetum was transformed into reality by Director Andre Champagne of the MBG, with input from Rudy Behring and Leslie Hancock, member and president, respectively, of the Rhododendron Society of Canada. The Leslie-Hancock Ericacetum (Figs. 2 and 3) was officially inaugurated on June 6, 1976. On May 26, 2001, the Leslie-Hancock Ericacetum celebrated its 25th birthday.
The Orono, Ont. data locations are from Dave Hinton's Rhododendron Woods (1975), and Heinz Ruckdaeschel's rhododendron plantations (1991), located nearby. Both locations are situated on well drained sandy loam soils, ideally suited for the culture of rhododendrons. The Brampton location is home of Ralph and Carol Hansen's collection. Many members provided data from their gardens, and such data has been incorporated with the nearest climatic column in Table I.
Figure 2. Entrance to the Jardin Leslie-Hancock Ericacetum of
the Montreal Botanical Gardens.
A coniferous windbreak, showing in the background, was established in 1974. By March,
the area is usually covered with a two-foot layer of snow. Orientation: north (top left corner),
east (top right corner), south (bottom right) and west (bottom left).
Bed Preparation and Hardiness
Because of the severity of our winters, it is extremely important to provide a well drained growing medium with a friable soil structure, which will assure vigorous growth and hence increase the survival of rhododendrons.
The Oakville and Orono locations are on sandy loams, easily improved for rhododendron culture. The other locations are on heavy textured, alkaline clays and glacial clay loams requiring much work to create suitable conditions for rhododendrons. Invariably collections on poorly prepared soils show inferior cold-hardiness, as compared to the collections on sandy loams, or well prepared beds.
In Montreal, located on the alluvial clay deposits of the St. Lawrence River, large beds are excavated to a depth of 60 cm (24 in) and re-filled with appropriate soil. A raised bed of 20-30 cm (8-12 in) is then created on top. An appropriate soil consists of a blend of equal parts sandy loam, shrimp compost consisting of a blend of peat moss and shrimp residue (pH 5.5), organic soil, sphagnum peat moss and sulphur (applied at a rate of 1000kg/ Ha = 800 lbs/acre). Even when sulphur is reduced to its finest form, it often takes two to three years before noticeably affecting the pH. Every spring, an acid fertilizer with high nitrogen content is applied in low amounts, with fine sulphur incorporated into the fertilizer. Sulphur and composted spruce needles are also added to the bottom of each planting hole. As late as possible every fall, compost is spread over the beds. Every second year, soil analyses are performed and any anomalies are immediately corrected to keep the pH between 4.5 to 5.5.
These detailed soil preparations and maintenance procedures most likely explain, in part, why rhododendron culture in Montreal has been so successful, despite its relatively colder climatic condition.
The second important factor is the establishment of windbreaks, which also proved a variety of winter-sun shelter situations indicated as a, b, and c in Table 1. These shelter situations are outlined under the section "Shelter."
Rhododendron growers usually indicate a midwinter temperature rating based on flower-bud hardiness, because flower buds of rhododendrons are less hardy than vegetative buds and leaves, and most of us are interested in the flowers. A cold ranking method of damage to flower buds was presented by Russell Gilkey (2). Researchers (4, 8, 11) used leaves instead to measure hardiness, because leaves are readily available, and leaf-hardiness correlates reasonably well with flower-bud hardiness as shown in Fig. 5 of reference (4). Sakai and others (8), reported that leaves of tender species Rhododendron griersonium and R. barbatum, were killed at -18°C (0°F) in midwinter conditions, while acclimated R. maximum and R. brachycarpum can survive midwinter temperatures to -60°C (-75°F); their flower buds could withstand temperatures down to -30°C (-25°F).
They (8), also noted that rhododendron taxa with the greatest midwinter hardiness do not necessarily possess superior hardiness earlier and later in the dormant season, and Herbert Spady (10) noted that: "damage to flower bud, leaf or plant may be experienced through a range of temperatures."
Raulston (7) explored plant hardiness and noted that damage may occur: 1) if the plant hardened off but temperatures fall below a midwinter threshold value for the plant, 2) the plant is not hardened off by midwinter and suffers frost damage, 3) plant becomes physiologically active after midwinter and suffers early spring frost damage, 4) swings of warm and cold temperatures affect the state of hardening during midwinter when frost recurs.
Burke et al, 1976 (1), noted three aspects of winter hardiness, i.e. timing of fall cold hardening, maximum midwinter hardiness and ability to resist rapid de-hardening in early spring or during warm periods in the winter. For these reasons hardiness of rhododendrons are rated in this article for dormant seasons, not just for only a minimum threshold temperature.
I expressed "coldness" or "hardiness" = H for locations with a climatic factor, "Mdd's" for dormant seasons (5), and added a column in this article for degrees Celsius:
|H = LOCATION COLDNESS (HARDINESS)|
C or F
|H1 = to||
|H1 = -25 F (-32 C)||4b/5a|
|H2 = to||
|H2 = -15 F (-26 C)||5b/6a|
|H3 = to||
|H3 = -5 F (-20 C)||6b/7a|
|*The January Temperature is the normal minimum for the coldest month, not the extreme low temperature used in the USA.|
|**Mdd's are a "measure of coldness" for the DURATION of a dormant season. Mdd's are expressed as "minimum dormant degree days" below 41 degrees Fahrenheit = 5°C, for locations.|
About the Process of Cold Hardening
After shoot growth is completed in late summer, a first stage of hardening is initiated at a critical day length specific to each cultivar or species, while temperatures are above 5°C (41°F). It involves an accumulation of starch and other metabolic compounds and a decrease of cell water content (3, 6, 12). Short days in the fall function as an early warning system in nature (3, 12). The increase in frost hardiness during the early first stage of acclimation is relatively minor. But it may be significant since just a few degrees tolerance to frost can make the difference between life and death during fall.
A second stage of hardening is induced by temperatures between 0 to 5°C (32-41°F) and temporary periods of frost. This is the onset of the dormant season (5), and Mdd's are accumulated when the mean minimum temperature for an autumn month (September, October or November) is shown to be below 5°C (41°F) for those months.
Temperature conditions during the second stage of hardening induce a considerable change in metabolism of cells, when starch is converted back into various sugars. Sugars together with amino acids and other substances play a major role in increasing the concentration of cell fluids and thus they depress the freezing point of water in the cells. This allows the super-cooling of cell water several degrees (-7 to -13°C for leaves) below the freezing point of water (11). During the second stage of frost hardening in the fall, the cell membranes of hardy rhododendrons become increasingly permeable to water movement into or out the cell.
Plants that are exposed to short days during fall, while temperatures remain above 5°C (41°F), only reach the first stage of acclimation (12). Such a situation occurred at the Montreal Botanical Gardens three years ago (1997-1998). The weather was so warm during the fall, causing a second late leafing and burst of flowering on early spring flowering rhododendrons such as Rhododendron 'Olga Mezitt' and R. 'PJM Elite'. These shrubs were not prepared for the real cold that came at the end of December; these are considered normally reliably hardy in Montreal, but they did not bloom well the following year. The buds did not fully open the following spring, and the new shoots were small and lacked vigor.
Rapid Cooling, Winter Burn and Injury
Salisbury (9), noted that most acclimated species which survive freezing temperatures do so by super-cooling of cell water, followed by dehydration of the cells within the leaf. During this process of freezing starting at temperatures just below freezing, super-cooled water diffuses from within the cells to crystallize as ice crystals in the spaces between the cells. However, if plants are cooled rapidly or if their permeability to water is low (as in tender and/or non-acclimated species), the cells will maintain cell vapor pressure by intracellular freezing, rather than through dehydration. Thus, the internal ice crystals formed during rapid cooling are small and are likely to enlarge during warming and may cause rupture of cells (freeze-thaw damage).
In situations of rapid cooling in the laboratory (20°C/hour), ice can be observed within the cells. These fast rates had been observed in nature when evergreen foliage of arborvitae (Thuja occidentalis) is heated by the sun during subzero temperatures and little air movement. As soon as the sun rays depart, the temperature drops rapidly at 10°C per minute from +2 to -8°C) and further down to the actual freezing air temperature. This rapid cooling causes winter burn (12). Winter burn can also be observed on rhododendron leaves, which are exposed to winter sun heating during wind-still conditions and very cold temperatures. The curled-up leaves rapidly unfold in response to temperatures rising well above freezing inside the leaf. As the sun disappears the temperature drops extremely fast in response to low air temperatures anywhere from -10 to -30°C, causing winter-scald and death of rhododendron leaves.
Evergreen rhododendrons grown in continental climates, as in Ontario and Quebec, must be provided with winter shelter from direct exposure to the winter sun to prevent sun-scald and death of leaves from rapid heating and cooling during periods of freezing temperatures. Winter sun shelter is a survival requirement in addition to providing shelter from desiccating winds. For this reason the following shelter requirements were created to estimate injury or lack of injury in relation to a shelter situation:
a) OPEN-WITHIN A TREED URBAN NEIGHBOURHOOD WITH WINDSHELTER. There is no screen, hedge or shelter to protect rhododendron leaves from winter sun burn during months of December to April.
b) SEMI-OPEN SITE, WITH WINTER SHADE, Fig. 3. The site is like (a), but with winter shade/shelter protection from sun rays from December to March. (Winter sun shelter protects leaves, buds and or twigs from heating/freezing injury).
c) SEMI-OPEN CONIFEROUS TREE CANOPY OVERHEAD, Fig. 4, providing for sun and shade (dappled light), throughout the year. To estimate outdoor dormant season injury the following injury ratings were created:
INJURY RATINGS. Injury would be to leaves, leaf/flower buds, and/or, twigs above December-March snow or ground level:
No injury above snow or ground level (a, b, c without number)
Figure 3. Montreal Botanical Gardens, showing "b" type of shelter.
Orientation: south (top left corner), west (top right corner), north
(bottom right), and east (bottom left corner). Coniferous trees are
providing winter sun shelter from the East through the south and west
sides of the rhododendrons located in the foreground of the photo-
graph. From left to right R. 'Fundy', R. 'Russel Hermon', and R.
brachycarpum (alpine form). Note, that if the orientation of this
photograph would be reversed, with north in top left, and south at the
bottom right this figure would become a type "a" open-shelter,
because there exists no nearby coniferous shelter, hedge or fence at
the bottom right. Direct winter sun rays could reach the rhododendrons,
from the bottom right, reversed south side, of the figure.
Figure 4. Rhododendrons situated below a semi-open coniferous shelter type "c", Montreal
Gardens. Orientation: due south (top of photograph), west (right side), north ; (bottom), and east
(left side of photograph). From left to right, R. 'White Peter', R. 'La Bar's White' x R. 'Crest',
R. 'Spring Parade', and R. 'Vivacious' x R. 'Nova Zembla'.
Experience gained by individuals and organizations with the hardiness of rhododendrons growing in this "northern-edge-region" for the last thirty years enables us today to gather and combine this information. The Vineland, Oakville, Toronto, Brampton, Orono, MBG and north data columns in Table I show an increasingly colder dormant season climatic sequence. The climatic data used are 30-year normals from 1960 to 1990 available from Environment Canada. Injury /shelter estimates for rhododendrons are shown for each of the seven data columns.
|Table1. Estimating Rhododendron Hardiness|
Vinl=Vineland, Oakv=Oakville, TOcent=Toronto Central,
Brampt=Brampton, Orono=Orono, MBG=Montreal Botanical Gardens, North=Locations far
north 200-300 km of Toronto
? = likely occurs in climatic location _ = no record available * Name not registered
Extreme Minus F
January Mean F
Annual Mdds F
Extreme Minus F
January Mean F
Annual Mdds F
|R. catawbiense||b||_||b||c||_||c||_||'Canary Islands'||?||?||?||?||c1||b||_|
|R. degronianum ssp. heptamerum||b||c||c||_||_||c||_||'Crete'||?||b/c||_||_||c2||a||_|
|R. degronianum spp. yakushimanum||b||b/c||b||c||_||a/c||_||'Finlandia'||?||c||b||_||_||_||_|
|R. 'Ken Janeck'||b||b/c||c||_||c||b||_||'Hawaii'||b||?||b||_||_||_||_|
|R. 'Mist Maiden'||b||b/c||b||_||c||a||_||'Janet Blair'||b||?||b/c||c||c||c1||_|
R. degronianum ssp. yakushimanum
|'Wyandanch Pink'||b||b/c||c1||c||_c||_||'Spring Parade'||b||?||?||_||_||c||_|
|'County of York'||b||c||c||_||_||_||_||Others|
|'Katherine Dalton'||b||_||b||_||_||_||_||'Album Novum'||b||?||?||_||_||_||_|
|'Jolly Red Giant'*||_||b||c||c||_||c||_||'Calsap'||b||b||c||b/c||c||c1||_|
|'Rudy Behring'*||_||_||c||_||_||_||_||'Dorothy Amateis'||b||?||?||_||c5||_||_|
|R. smirnowii x R. fortunei||_||b||b||_||_||c||_||'Haaga'||_||_||?||_||c||_||_|
|Ironclads & Old Standards||'Hellikki'||_||_||?||_||c||_||_|
|'Album Elegans'||b||b/c||c||b||_||c||_||'Hello Dolly'||?||b/c||_||_||_||_||_|
|'Boule de Neige'||b||b/c||c||c||c||_||_||'Mrs P. den Ouden'||b||?||?||_||_||c||_|
|'Catawbiense Grandiflorum'||b||b/c||c||c1||c1||c1||_||'Paul R. Bosley'*||b||?||?||_||_||c||_|
|'Cunningham's White'||b||c||c||b1||c5||_||_||'Patty Bee'(dwarf)||/||/||a||_||_||_||_|
|'English Roseum'||b||b||c/b1||b2/c||c1||c1||_||'President Lincoln'||b||?||?||_||_||_||_|
|'Gomer Waterer'||b/c||c||c||b||_||_||_||'Queen Anne's'||b||?||b||_||_||_||_|
|'Ignatius Sargent'||b||_||b||c||_||_||_||'Red Brave'*||?||?||b||_||_||_||_|
|'Lee's Dark Purple'||b||?||b||b/c2||c5||c2||_||'Roslyn'||b||?||?||_||_||_||_|
|'Marchioness of Lansdowne'||b||?||_||_||c1||_||_||'Sandra Hinton'||?||?||b||_||c||_||_|
|'Mrs Charles S. Sargent'||b||?||b||c||_||_||_||'Solidarity'||b||?||?||_||_||_||_|
|'Mrs T.H. Lowinsky'||?||b/c||b||b||_||_||_||'Vinebelle'||b||b/c||?||_||_||_||_|
|'Roseum Superbum'||b||b/c||b||_||_||_||_||'White Peter'||?||?||_||_||c1||c||_|
|Leach||R. yak. X 'Hinton'*||?||b/c||b||_||_||_|
|'Anna H. Hall'||b||c||?||c||c1||c||_||'Yaku Picotee'||?||b/c||_||_||_||_||_|
|R. minus var. minus||_||b||_||_||_||_||_||'Toucan'||a||?||b||_||_||_||_|
|R. minus var. minus Carolinianum Group||b||b||b||c||c||c||_||'Tunis'||a||?||_||_||_||5||_|
R. minus var. minus
Car. Gr. (white)
|R. mucronulatum (dec.)||b||b||b||b/c||c||a||_||'Debutante'||a||?||b||_||_||_||_|
|R. mucronulatum (white)||?||b/a1||_||_||_||_||_||'Fireball'||a||a||?||_||_||_||_|
|Dr. Brueckner||'Strawberry Ice'||a||a/b||b||_||_||b||_|
|'Pink Pixie'*||b||?||b||_||_||_||_||'Golden lights'||?||?||b||_||_||a||?|
|'Gable's Pioneer'||a||?||_||_||_||a1||_||'Northern Hi-Lights'||?||?||?||_||_||a||?|
|'Pink Pompon'*||b||b||?||_||_||b||_||'Rosy Lights'*||a||?||a||_||c||a||?|
|'Little Boy Blue'*||a||a||?||_||_||_||_||'Spicy Lights'||a||?||b||_||c||a||?|
|'Evening Sky'*||a||a||b||_||_||_||_||'White Lights'||a||a||a||_||c||a||?|
|'Tow Head'||b||b||b||_||_||_||_||'Vineland Gold'||a||a||b||_||_||_||_|
|'April Rose'||?||b||c||_||c||a||_||Boskoop (viscosum)|
|'Mary Fleming'||a||?||c||_||_||a1/c||_||R. yedoense var. poukhanense||b||b||b||b||_||a/b||_|
|Weston Mezitt||R. yedoense
|'April Snow'||b||?||b||_||_||a||_||R. kaempferi||b||c||b1||_||_||b1||_|
|'Weston's Pink Diamond'||b||c||b||a/b||c||a||_||'James Gable'||b||b||?||_||_||_||_|
|'Purple Gem'||a||b||b1||b1||_||a||_||'Elsie Lee'||b||b||?||_||_||_||_|
|'Sassy Lassy'||b||?||_||_||_||_||_||'Helen Curtis'||?||b||_||_||_||_||_|
|R. arborescens||a||b1||_||_||b||b||_||'Phyllis Moore'*||b||b||?||_||_||_||_|
|R. atlanticum||a||a||?||_||_||b||_||'AI's Picotee'*||?||b/c||_||_||_||_||_|
|R. molle ssp. japonicum||a||a/b||b||b||c||b||_||'Hino-white'||b||?||_||_||_||b1||_|
|R. periclymenoides||a||?||b||_||_||b||b4||GENUS KALMIA|
|R. prinophyllum||?||?||b||_||_||b||_||Kalmia 'Nancy'||?||?||?||b||_||b1||_|
|R. vaseyi||a||b||?||?||c||a/b||_||Kalmia 'Sarah'||?||?||b||_||_||b1||_|
|R. viscosum||?||a||b||_||_||b||_||Kalmia 'R. Jaynes'||?||?||b||_||_||b1||_|
Evergreen elepidote rhododendrons are genetically adapted to grow in relatively shady locations, and as shade adapted plants they have a lower rate of photosynthesis than sun adapted plants. They are sensitive to excessive sunlight that may cause inhibition of photosynthesis, or photo-inhibition, as reviewed by Vainola (11). Rhododendron maximum and R. catawbiense often grow under deciduous tree cover in the Appalachian mountains of eastern USA. These species continue photosynthesis during the dormant season as long as the air temperature remains above 5°C (41°F). Bright sunlight may cause bleaching of the chlorophyll, and if low freezing temperatures occur together with bright sunlight, sunscald damage of the leaves may occur.
Large leaf elepidote rhododendrons also grow below a deciduous tree cover in Hendricks Park, Eugene, Oregon. They obviously photosynthesize during the cloudy, misty dormant season of Oregon with sufficient light penetrating through the leafless deciduous tree canopy. During the hot sunny growing season, incoming sunlight is moderated by the fully leafed tree canopy.
The aforementioned deciduous tree shelter situations are in sharp contrast with the semi-open coniferous shelter situation (situation "c" or "b" in this article) which is a must if H2 and H1 hardy evergreen elepidotes are to survive in the freezing and sunny dormant seasons of Ontario and Quebec. Sun heating of leaves followed by rapid freezing at sunset would cause excessive killing of leaves. Table I shows many evergreen elepidote, lepidote and some semi-evergreen Tsutsusi azaleas that can successfully be grown in coldness zones H2 and H1 if provided with shelter situations "b" or "c" Deciduous azaleas on the other hand do reasonably well in these coldness zones in relatively unsheltered "a" winter sun locations.
A semi-open coniferous tree canopy (situation "c"), preferably consisting of pines, is used by many in continental northern climates, as shown in the illustration from the Montreal Botanical garden, Fig. 3. Most urban gardens, however, are too small to use a coniferous canopy throughout the garden. This is the reason why fences, hedges or individual conifers (situation "b"), Fig. 4, are used in the small garden to shield evergreen rhododendrons from being exposed to direct winter sun from the south, southwest and southeast sides of a garden.
Deep snow cover during the dormant season from December to April would provide winter sun protection as well protection against desiccation for many of the dwarf to medium sized evergreen rhododendrons, as usually occurs in Montreal, Fig. 2. Winter snow cover, however, is extremely variable from year to year and cannot be relied on alone to provide winter shelter for small and medium sized rhododendrons. Low growing cultivar 'Elviira' may be leaf-hardy in H1 conditions when snow cover is available. It has been rated as c3 in a H1 location, because it tries to flower too early in May when early spring frost kills the flowers.
Individual injury/shelter reports by different individuals for the same cultivar may vary for H2 or H1 coldness situations, because of soil rooting depth, amount of organic matter, and soil texture. Invariably, sandy loams with a high organic content provide better hardiness performance as compared to the same cultivar growing in raised beds over unimproved heavy textured soils. The injury /shelter ratings shown in Table I can be used to indicate a hardiness rating for rhododendrons, i.e., rather than indicating 'Janet Blair' as hardy to -15°F (-26°C) a rating of H2-b, c can be shown. It indicates that this cultivar can be grown in H2 climatic locations with the provision of "b" or "c" type of shelter.
The H2-b, c hardiness rating for 'Janet Blair' will remain as a hardiness attribute of this cultivar, regardless of climate changes. When the climate changes, climatic Mdd's can always be recalculated for locations in the future.