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Hardy Species Survey Explanation & Caveats

by Joe Bruso

Feedback Questions for All Massachusetts Chapter Members

If you have grown this species outdoors: Species Name________________

1. Do you still have it? __Yes __No

2. How long have you had it? __years __1-5 __6 or more

3. Has it flowered for you? __Yes __No

4. Is it a reliable performer for you? __Yes __No

5. Comments on cultivars of species you grow?

Member Name______________________Address____________________

How we obtained Survey data

Survey data were obtained via three means:

  • Hardiness Survey Questionnaire sent to Massachusetts Chapter members. We particularly targeted members known to the Species Study Group to be growing a number of species.
  • "Species in our Midst" questionnaire. Each month, the Species Study Group publishes in the Massachusetts Chapter Newsletter a "species profile" titled "Species in our Midst". At the end of each profile is a brief questionnaire, which we ask members who have grown this species to fill out and return.
  • Species Group consensus experience. This was a consensus rating by Species Group members of species we have had personal experience with, as well as knowledge of others' experience.

Hardiness Survey Questionnaire

We created a database with the names of all non-vireya Rhododendron species. The source of all names was The Genus Rhododendron: Its classification & synonymy by David Chamberlain et. al.

Hardiness ratings were added for each species whenever possible (for some species we found no source for ratings). The starting point for these ratings was Greer's Guidebook to Available Rhododendrons by Harold Greer. These ratings appear in the leftmost Hardiness column, under "Greer".

We amended or confirmed Greer's ratings using information from the following additional sources:

  • The Encyclopedia of Rhododendron Species by Peter & Kenneth Cox.
  • Azaleas by Fred C. Galle.
  • Species Group members' experience.

These ratings appear in the right-hand Hardiness column under "Other".

From this compilation of data we extracted and produced a comprehensive list, which we call our "Master List", of species that could reasonably be expected to survive, if not thrive in New England. That is, they have a hardiness rating of -5° F or lower. Using this list, we created the Hardiness Survey Questionnaire.

Definitions for selected fields on the Survey Responses detail section

Name – We've consolidated our data at the species-level. Note that there can be considerable variation in hardiness among the subspecies, varieties etc. within a species. For example, minus ssp. chapmanii is borderline hardy in New England whereas minus ssp. minus, Carolinianum group is very hardy in New England. There can also be great variation in hardiness among plants within the same species/subspecies, which have a different geographic origin, or are from different elevations. Plants from higher latitudes and/or elevations are often hardier than their lower latitude/elevation relatives. For example, viscosum from Maine is much hardier than viscosum from Florida.

Type – indicates the broad group in which each species belongs. We chose to use less technical terminology than classification by sub-genus, section and subsection. Values:

  • Azalea, Deciduous
  • Azalea, Evergreen
  • Elepidote – without scales
  • Lepidote – with scales
  • Other – other than the above (a few odd-balls)

Hardiness: Greer/Other - Described in previous section.

Group Rating - A consensus rating of the Species Study Group. Values:

  • A = Recommend. These are the "Good Doers" which should do very well in many, if not all New England zones. They can be expected to look healthy and to bloom.

  • B = Conditionally Recommend. These are species for which the data are not as clear-cut. They may do very well for some growers, but not for others. Some may survive, but may not thrive without special treatment of some sort (for example, winter protection, or constant moisture).

  • C = Not Enough Data. Some of these species may some day be determined to be "Good Doers". We do not currently have enough data to make a conclusion about these. This rating was given to most species for which we had three or fewer observations, or for which we felt additional experience was needed.

  • D = Not Recommended. These species have proven themselves to be "reliably unreliable" in New England. Some have never been successfully grown. Some may survive, but will not thrive unless given special conditions and/or care. Only experienced growers looking for a challenge should expend effort on these.

Still Own? – Is the plant still alive in the garden.

Years Owned – Mostly as-of 1997 when the Hardiness Survey questionnaire was distributed.

Flowered? – Has the plant ever bloomed.

Healthy? – Does the plant look good and grow well.

Zone – New England zones for which we have data range from 4A to 7A.

  • Zone 4A has an average minimum temperature between –25° F and –30° F.
  • Zone 4B has an average minimum temperature between –20° F and –25° F.
  • Zone 5A has an average minimum temperature between –15° F and –20° F.
  • Zone 5B has an average minimum temperature between –10° F and –15° F.
  • Zone 6A has an average minimum temperature between  – 5° F and –10° F.
  • Zone 6B has an average minimum temperature between     0° F and  – 5° F.
  • Zone 7A has an average minimum temperature between  + 5° F and     0° F.


  • From the data we obtained and our best judgement, species listed on the "A" list can be expected to do better in New England than species on the "B" list. Species in the "B" list can be expected to do better than those on the "D" list. However, "your results may vary". Growers’ personal habits, skills, microclimates, the source of the plant, and other factors may cause one to succeed where others have failed, and vice versa.
  • Hardiness ratings for some species may be understated for one or more of the following reasons:
    • Hardiness may vary among subspecies, varieties etc. of a species.
    • Plants originating at higher altitudes or more northerly latitudes can be hardier.
    • Some species’ ratings are based on the lowest temperatures recorded in relatively mild climates. They have not yet been adequately tested in New England's coldest zones.
  • Winter hardiness is not the only factor in success. Some species survive cold New England winters only to die in the heat and dryness of summer.

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