JARS v46n1 - Rhododendron Species Hardy in Southern New England

Rhododendron Species Hardy in Southern New England
J. Powell Huie
Westport Point, Massachusetts
Reprinted from the Summer 1991 issue of the Rhodi-review

One of the projects of the Species Study Group of the Massachusetts Chapter was a survey made in 1988 of the rhododendron species growing in our gardens in southern New England. The purpose of the survey was to develop general guidelines for ourselves and those other gardeners interested in planting species not only in the immediate area covered by the survey, but outside the area and having similar growing conditions.

As shown on the accompanying map, data was obtained from 16 sources indicated by small circles and located in three arbitrarily chosen areas: Area A limited by minimum temperatures of approximately -1°F, Area B from -1°F to -6°F, and Area C from -6°F to -15°F. The locations of these sources are listed in Table I.

Table I
Area A: Minimum temperatures approximately -1°F
1. Martha's Vineyard, MA
2. Brewster, MA
3. Westport Point, MA
Area B: Minimum temperatures approximately -1°F to -6°F
1. Duxbury, MA
2. Newton, MA
3. Braintree, MA
4. Newton, MA
5. Marshfield, MA
Area C: Minimum temperatures approximately -6°F to -15°F
1. Concord, MA
2. Longmeadow, MA
3. Concord, MA
4. Roxbury, CT (not shown on map)
5. Wellesley, MA
6. Belmont, MA
7. Longmeadow, MA
8. North Andover, MA

Some consideration was given to using the U.S.D.A. zones of hardiness, but the idea was discarded because it has been to a certain extent outdated by more recent weather observations. For example, according to the U.S.D.A. map, much of southernmost southeastern Massachusetts would lie in Zone 7, although temperatures of 0°F have been recorded in the last 10 years. Locally, in low lying areas with relatively poor air circulation, temperatures have been observed well below the 0°F mark.
It is standard procedure to rate the hardiness of rhododendrons according to their resistance to minimum temperatures, although many other factors can affect the hardiness of the plant. In much of southern New England, last December had the lowest average temperature on record. In the area in southern Massachusetts where I live, the ground generally does not freeze until after New Year's Day. In 1989, after an unusually warm October and November, the temperature dropped 50° F in less than two days; the ground froze the first week in December, and remained frozen until late February. Fortunately, throughout December and until mid January we had an unusual three to four inch snow cover. Even though the average temperature was the lowest on record, the minimum was about 5°F, well above the 0°F we generally have in January. In January and early February of the following year, the average temperatures were well above normal, but in late February and early March they dropped to near the zero mark, which is much lower than generally experienced at that time of year. Some of the rhododendrons and other plants which until the winter of 1989-1990 had easily survived 0°F either died or were badly damaged, most likely from the sudden drop in temperature in early December.
Another factor affecting the hardiness of species in southern New England can be the lack of moisture in winter. We can cope with dry weather in the summer by irrigating, but this is impossible during subfreezing weather. This points up one of the problems of growing dwarf plants, which with their small, shallow root balls, have difficulty surviving the winter drought because it is not possible to give them the same thick mulch of leaves that we have around the larger rhododendrons. In their native habitat high in the Himalayan mountain ranges or the mountains of western China, the low growing alpine rhododendrons in winter are covered by a protective blanket of snow. Furthermore, the summer temperatures in the higher elevations are much below those experienced in southern New England, and to successfully grow these plants it may be necessary during this period to provide additional shade (more than 50%), good air circulation, and ample moisture.
Although the rating of hardiness using a minimum temperature serves as a useful guideline, the true hardiness of a plant can be determined for a particular locality only by growing it for a number of years. In a letter to the Rhododendron Species Foundation, a copy of which is in their January 1990 newsletter, Nick Nickou of Branford, Conn., makes the point that a rhododendron species plant should be at least 10 years of age before rating its hardiness. During this period it will have experienced most of the extremes of winter and summer weather conditions. I certainly agree with his argument and accordingly have used it in listing those rhododendrons which can be considered hardy in zones A, B, and C. Although some of these listed species are undoubtedly natural hybrids, the true species itself may be planted with some confidence if given the proper care. My experience has been that in acquiring species, one is more likely to get a true species from the Rhododendron Species Foundation, which specializes in the propagation and distribution of these plants.

In Table II is a list of those rhododendron species now growing in Area A. The numbers after each species refer to the specific location in which they are grown in that area. The numbers followed by asterisk indicate those species which are at least 10 years of age. There are 63 different species in this category. Another 26 have survived without protection for more than three years.
Table III lists those species which are being cultivated without protection in Area B. Nineteen are at least 9 years of age and 39 are more than 3 years of age, but less than 10 years.
Moving now to Area C, Table IV, you will note that 37 different species are more than 10 years old, and 10 are from 3 to 10 years. In all areas many more are being tested, but it is premature to even suggest which of these species may be hardy.
In general, I would suggest the following to those interested in growing rhododendron species:
1)  Select those species having a hardiness rating five degrees or more below the minimum temperature experienced at your locality. This will give the plants some cushion against unusual conditions such as those at the beginning of the winter of 1989-1990.
2)  Obtain young plants and grow them with protection for two years or more until they have developed a relatively large root ball, and then move them to the permanent site.
3)  Avoid older plants, particularly if they are pot bound, and if they have been used as stock plants for cuttings. They are more expensive and the mortality rate is much higher.
4)  After getting started raising species, you might wish to try some of the less hardy species. However, on occasion be prepared to lose older plants which had previously given every indication of being hardy in your locality.