JARS v54n2 - Estimating Hardiness Using Cold Duration and Extent of Shelter

Estimating Hardiness Using Cold Duration and Extent of Shelter
Willem A. C. Morsink
Toronto, Canada

Members of ARS District 12, in eastern Canada, occasionally receive inquiries about cultivars suitable for northern rhododendron fringe zones, e.g., USDA Zones 5 and 4. Dave Hinton maintains that his Orono, Ontario, Rhododendron Woods are different from Edwards Gardens and other Toronto area gardens because of a variety of factors that influence hardiness. So, what are these differences?

The ARS has in the past used "H" hardiness ratings based on average mean daily minimum temperatures (Fahrenheit) to indicate differences among species and cultivars. Unfortunately, several 30-year-old specimens of Rhododendron catawbiense 'Boursault', rated as ARS-H2, hardy to minimum -15°F (-26°C), were seriously injured at Edwards Gardens in Toronto during the prolonged winter of 1994/1995, even though the minimum temperatures never approached the -15°F (-26°C) figures for H2. Some other H2 cultivars did not fare well at milder Vineland, Ontario, where several H3 rated cultivars thrive.

Although still widely used, the "H" rating system is not the same as the current ARS definition of hardiness, provided by Herbert Spady in 1994 (12). This definition expresses hardiness as a temperature range through which damage to flower bud, leaf, or plant may be experienced depending on factors such as unseasonable cold temperatures, duration of cold, exposure, site, snow cover, and other variable factors.

ARS "H" ratings, however, can still be an important aspect of the information for a specific species or cultivar, if the minimum threshold temperatures are replaced by an estimate of cold duration and extent of shelter as indicated in the title of this article. Once determined, such an "H" rating remains for the specific plant, while the climatic zones vary over time.

Nancy Traill illustrated this phenomena of wandering climatic zones in a presentation to our Toronto Chapter in 1996. Toronto experienced Zone 4a to 8a conditions during a 155-year period from 1841 to 1996. During the most recent 78 years (1919-1997), no Zone 4a or 4b conditions occurred in Toronto. Did it get warmer, or is this the growing heat island effect of Toronto? Zone 5a occurred three times out of 78 years, Zone 5b eight times, Zone 6a fourteen times, Zone 6b twenty-seven times, Zone 7a sixteen times, Zone 7b six times and Zone 8a four times. So, which is a safe USDA hardiness zone recommendation for rhododendron gardeners in Toronto?

Observations on Hardiness
Hardiness varies not only among cultivars and species but for a particular cultivar or species throughout the dormant season depending on its physiological growth phase. This situation was outlined by Richard H. Houghton, Jr., who used the example of an imaginary hybrid "X" (6).

Various environmental conditions that affect plant hardiness were fully explored by Raulston (10); damage may occur if:
1.  Plant has hardened off but the temperature falls below a minimum
2.  Plant has not hardened off and there occurs early frost damage
3.  Plant has become physiologically active and there is spring frost damage
4.  Swings of warm/cold temperatures which affect state of hardening.

Russell Gilkey proposed a ranking for flower bud damage because of temperature swings in Tennessee (5). Ouellet and Sherk also noted various factors that may affect hardiness (9). They produced a mathematical model to estimate the suitability of Canadian localities for winter survival of woody ornamentals; factors included were low winter temperatures, frost-free period, summer and winter precipitation, summer high temperatures, snow depth, and wind speed. They proceeded to construct indices of winter hardiness and the Canadian Plant Hardiness Zone Maps. These zones somewhat, but not completely, resemble the USDA zones in Ontario.

Because hardiness involves many factors (5), (8), (9), (10), (12) and because Raulston's four hardiness situations outlined above may occur within a dormant season, I devised a climatic factor, the "F/Mdds", which quantifies duration of cold for a dormant season for locations. So what is a Mdd?

Minimum Dormant Degree Days (Mdds)
The number of minimum dormant degree days (Mdds) (8) - the counterpart of growing degree days (3) - which can be determined for a locality, can be used to estimate Normal Minimum Climatic Conditions to be expected over a period of time. "Normal" is the term commonly used for values of climatic elements averaged over a fixed, standard, 30-year period of time. The temperature data for Figure 1 are from the 30-year normals (1961-1990) provided as Ontario Climate Normals by Environment Canada, 1993.

Mdds are calculated for a location using monthly means of daily minimum temperatures below 42°F (5.5°C) (8), i.e., in Table I, the difference between 42°F (5.5°C) as selected by Boughner (3) and the mean daily minimum temperature for January (17.8F) equals 24.2F; times 31 days yields 750 Mdds. Mdds from January plus February plus March plus April plus November plus December equals 2693 Mdds for the Toronto location. Mdds incorporate duration of exposure to low temperatures during a dormant season, and they enclose early fall and late spring frost when the plant is going into or coming out of its optimum state of dormancy (Table 1).

The downtown Toronto location in Table 1 and the Toronto Islands experience the "city heat island effect" which moderates the minimum winter temperature values. The downtown Toronto value is similar to the milder Vineland/ Pelee Island values. Lake Ontario also moderates winter minimums; this moderating effect diminishes rapidly away from the lake shore areas, and the Mdd values change rapidly from 3,000 Mdds at the lake shore to 4,000 Mdds north of Metropolitan Toronto.

TABLE 1. A sample calculation for downtown Toronto. University of Toronto-Bloor Street location, 17.8F/2693 Mdd:
Month Mean Day/Min° F Diff.42° F Days Mdds
Jan. 17.8 24.2 31 750
Feb. 19.0 23.0 28 644
March 27.7 14.3 31 443
April 38.3 3.7 30 115
May 48.7 31
Oct. 45.0 31
Nov. 35.8 6.2 30 186
Dec. 24.1 17.9 31 555
=2693 Mdds
Please note that the "F" temperatures shown above and as used in Figure 1 are normal averaged daily minimums calculated for the coldest month; the "F"s are not the extreme low 'F' temperatures used in the ARS "H" and USDA zone systems.

The Proposal
I present here winter injury/shelter "H" ratings in relation to a dormant season climatic estimate for locations - the "F/Mdd." The averaged mean daily minimum temperature "F" for the coldest month, January or February, is shown with Mdds (minimum dormant degree days) for locations. Mdds combine duration and intensity of normal minimum temperatures below 42°F (5.5"C) during the dormant season (8).

The proposed F/Mdd factor for "H" ratings are: H1=4F/4500Mdd; H2=13F/3500 Mdd; H3=20F/2500 Mdd; and H4=27F/1500Mdd.

The proposed injury/shelter ratings are: a=no injury/open garden; b=no injury/sheltered; c=occasional injury/sheltered; d=frequent winter injury/sheltered; e=overwinters below snow cover.

Hardiness reporting would be based upon the above injury/shelter ratings with the criteria that the plant should have lived for a minimum of five years in a location.

Plotting F/Mdds for Locations
In Figure 1, I plotted the mean daily minimum temperature for the coldest month (Jan. or Feb.), which includes occasional very low minimum temperatures, against quantities of Mdds from 2500 to 4500, to show the relative positions of rhododendron/azalea collections in Ontario. Figure 1 shows a diagonal line of locations moving from the right bottom with the coldest winters (Ottawa) to the left top (Vineland, Harrow and Pelee Island) with milder winters.

Fig 1. Climatic (F/Mdd) 
conditions for selected cities (letters) and rhododendron/azalea collections.
FIGURE 1. Climatic (F/Mdd) conditions for selected cities (letters) and rhododendron/azalea collections.
(Letter/numbers) in Eastern Canada.  B=Brampton; C=Comwall; H=Harrow; M=Midhurst(Barrie);
N=Niagara Falls; P=Pelee Island; R=Richmond Hill; TI=Toronto Island.

Letter/numbers indicate climatic positions of gardens:
O1. Paul Jaques Boult garden, 4.8F/4300 Mdd, Ottawa; ref.(2).
M2. Montreal Botanical Gardens, 8.2F/4100 Mdd; ref.(2).
U3. Heinz Ruckdaeschel plantation, 10.5F/4100 Mdd, Uxbridge;
O4. Dave Hinton's Rhododendron Woods, 11.7F/3900 Mdd, Orono;
G5. Hansen's Rhododendron Garden, 12.1F/4OOO Mdd, Georgetown;
T6. Edward Gardens, 13.3F/3400 Mdd, Central Toronto;
H7. Royal Botanical Gardens, 15.8F/3000 Mdd, Hamilton;
T8. Nancy Traill garden, 15.5F/3000 Mdd, Toronto shore (13);
M9. Mississauga/Oakville gardens, 16.0F/3000 Mdd; ref.(14).
V10.Vineland and Niagara Peninsula, 17.6F/2800 Mdd, ref.(4, 14).
H11. Halifax (NS)-15.7F/3300 Mdd.
S12. St. John (Nfld)-15.5F/3400 Mdd, ref.(1).

Gardens 11 and 12 experience wetter seasons and significant winter snow cover.

Injury/Hardiness "H" Ratings as a Rhododendron/Azalea Attribute
A winter injury/shelter rating should be estimated for each cultivar growing in specific locations for the duration of a dormant season. It is important to attach an injury rating to rhododendrons and azaleas as a plant attribute just as are flower colour, plant habit and so on. An injury/hardiness rating, once determined, remains as an attribute of that cultivar.

Below I provide an estimated comparison of F/Mdds to ARS "H" ratings, minimum temperatures and USDA zones as follows:

New H rating Jan*/Mdds Old ARS ratings USDA Zones
H1 = 4 F/4500 H1 -25°F (-32°C) 4b/5a
H2 = 13F/3500 H2-15°F(-26°C) 5b/6a
H3 = 20 F?/2500 H3 -5°F (-20°C) 6b/7a
H4 = 27°F?/1500 H4+5°F (-15°C) 7b/8a
*Mean daily minimum "F" temperature for the coldest month.

Combining New "H" Ratings with Injury F/Mdd Ratings
Elepidote large-leaved Rhododendron 'Janet Blair' flourishes in Edward Gardens, Central Toronto (13.3 F/3400 Mdds), with excellent winter sun and wind shelter. It deserves a "b" rating for this location; it is rated "d" (injured or killed under 11.7 F/3930 Mdd conditions at Orono, Ontario.

Proposed hardiness H2-"b" (H2-13F/35OO Mdd), for 'Janet Blair', is listed as H2 (7),and Zone5b (11). Elepidote small and persistent leaved (evergreen) azalea 'Maybelle' earned the "b" rating at the Traill garden (15. 5F/3000 Mdd) with winter sun and wind protection (13). It dies out and it is ranked as "d" at Orono (11.7 F/3900 Mdd)

Proposed hardiness H2-b, or H2-c (H2=13F/3500 Mdd) for 'Maybelle' is listed as Zone 6a (11).

Cultivar Rhododendron catawbiense 'Boursault' may have to be rated as H2-c (H2-13F/3500 Mdd), because of its 1995 dieback in Edwards Garden, central Toronto.

Applying the Proposal
Nancy Traill provided a descriptive hardiness-type evaluation of evergreen azaleas in southern Ontario (13). I applied some of this information together with other hardiness data, e.g., James Lounsberry (7) and information from Edwards Gardens to demonstrate dormant season rhododendron/azalea injury/shelter estimates a, b, c, d, and e under five f/Mdd headings in Table 2.

There exist no 20F/2500 Mdd locations in Ontario; Vancouver, B.C., with 33F/700 Mdds would be well within an H4 (27F/1500 Mdd) climatic condition. Hagerstown, Pennsylvania, with 22F/ 2050 Mdds and Boston, Massachusetts, with 23F/1950 Mdds would fall well within the H3 (20F/2500 Mdd) condition; Richmond, Virginia, with 27F/1500 may represent a USA H4 (27F/1500 Mdd) borderline condition. Norfolk, Virginia, has an estimated value of 32F/800 Mdds and falls well within the H4 (27F/ 1500 Mdd) climatic condition. All values for the USA cities are estimates only from the lower average temperature shown for January by Reiley (11). Such values must be re-calculated by members in the USA.

TABLE 2. Preliminary Injury/shelter ranking of selected cultivars for selected locations in Ontario.
Ottawa Orono Toronto Vineland
Edwards Lake
(*F) 4.8 11.7 13.3 15.5 17.6
Name (Mdd) 4300 3930 3400 3000 2800
R. 'Nova Zembla' H1-b e b b b a
R. cat. ' Boursault' H2-c e c c b b
R. cat. 'Album' H1-c e c b b b
R. 'Janet Blair' H2-b ? d b b b
R .'Scintillation' H2-c e d c b b
R. 'Herbert' H2-b ? d b b b
R. 'Maybelle' H2-b ? d ? b b
R. 'Ramapo' H1-e e b b b a
R. 'P.J.M.' H1-b/e e b b a a
R. schlippenbachii H2-b/c d b/c b b a
(*F) = mean daily minimum for coldest month in location.
a=no injury/open location; b=no injury/sheltered; c=occasional bud, leaf, or twig injury/sheltered; d=frequent winter damage/sheltered; e=survival under snow cover.
Note: the column of letters (mostly e's) below 4300 Mdd refers to Ottawa with snow cover providing protection most years (2).

1.  Rhododendrons are not necessarily killed by low threshold temperatures; they are one of a number of winter hardiness factors.
2.  Minimum Dormant Degree Day (Mdds) bracket the dormant period to account for many winter hardiness factors.
3.  A visual/graphical separation of locations results by plotting the Mean Daily Minimum Temperature "F" for the coldest month against Mdds for the dormant season.
4.  Mdds can easily be established for locations and they represent a 30-year (1961-1990 normals) averaged minimum temperature condition for that location.
5.  Injury/shelter ratings can easily be established for rhododendrons using members' garden information.
6.  The ARS "H" ratings can be improved by substituting threshold temperatures with F/Mdds and by incorporating the injury/shelter ratings.
7.  The Mdd system is a location system, not a zone system; injury/shelter ratings for rhododendrons and azaleas are compared between F/Mdd locations.

I pondered the question of how to incorporate the ARS hardiness definition, authored by Herbert Spady (12); e.g.,"damage to flower bud, leaf or plant may be experienced through a range of temperatures." This situation was elaborated on by Richard Houghton, Jr. (6), who used the example of an imaginary hybrid X.

The ARS definition can be incorporated by using a double "H"-range-rating for a cultivar, i.e., Rhododendron catawbiense 'Boursault' would be rated with a "H-range" of H3b-H2c. H3=20F/2500 mdd and H2=13F/35OO; b=no injury/sheltered location, c=occasional injury/sheltered location.

I thank Dr. Nancy Traill, Newsletter Editor, Toronto Chapter, ARS District 12, for the zone data and my daughter, Dr. Sharon Morsink for preparing Figure 1.

1.  Boland, Todd. 1995. Rhododendrons in Newfoundland. J. Amer. Rhod. Soc. 49:4, 182-184.
2.  Boult,Paul. 1996. Meeting a challenge: growing rhododendrons in Ottawa. Toronto Newsletter, Can. Rhod. Soc. Fall 1996.
3.  Boughner, C. C.1964. The distribution of growing degree days in Canada. Can. Met. Branch, Dep. Transport.
4.  Fleming, Robert A., James Lounsberry and Nicholas Yarmoshuk. 1997. Azaleas and rhododendrons at Guelph University, Ontario. J. Amer. Rhod. Soc. 51:4, 195-196.
5.  Gilkey, Russell. 1996. Cold hardiness ranking of rhododendrons by means of flower bud damage. J. Amer. Rhod. Soc. 50:2, 100-102.
6.  Houghton, Richard H., Jr. 1995. ARS hardiness reporting criteria. J. Amer. Rhod. Soc. 49:4, 234.
7.  Lounsberry, James. 1991. Rhododendrons and azaleas. Publ. No 45, Ont. Min. Agric. Food, 18 pg.
8.  Morsink, W. A. G. 1970. A suggested frost injury rating system for clones of trees. Can. J, Botany 48(3): 493-97.
9.  Ouellet, C. E., and L.C. Sherk. 1967. Woody ornamental plant zonation: I. Indices of winter hardiness, 231-238; II. Suitability indices of localities, 339-349; III. Suitability map for the probable winter survival of ornamental trees and shrubs. Can. J. Plant Sci. 47:4.
10.  Raulston, J. C. 1996. Exploring the complexities of plant hardiness, J. Amer. Rhod. Soc. 49:4, 227-230.
11.  Reiley, H. Edward. 1992. Success with Rhododendrons and Azaleas. Portland, OR: Timber Press, pp. 285.
12.  Spady, Herbert A. 1994. Hardiness redefined. J. Amer. Rhod. Soc. 48:2, 65.
13.  Traill, Nancy H. 1996. Evergreen azaleas in southern Ontario. J. Amer. Rhod. Soc. 50:4, 212-214.
14.  Yarmoshuk, Nicholas, Lyall Crober, Bob Dickhout and Peter Phelps. 1998. Niagara '98-growing rhododendrons ON-THE-EDGE. J. Amer. Rhod. Soc. 52:1, 15-21.

Willem Morsink is a member of the ARS District 12, Toronto Chapter, and a retired manager of urban forests in Ontario.