JARS v47n2 - The Maine Way with Rhododendrons

The Maine Way with Rhododendrons
Paul E. Cappiello
Orono, Maine

In the early 1960s a major horticultural undertaking was initiated on the campus of the University of Maine. The brainchild of the late Lyle E. Littlefield, associate professor of horticulture, the goal of the project was simple to grow, evaluate, and display woody landscape plants with potential for use in U.S.D.A. cold hardiness zones 5 and colder.
From its inception until Professor Littlefield's retirement in 1986, the plant collection grew to include over 2,000 taxa from Abelia to Zelkova and occupied over six acres in the northeast corner of campus. In 1990 the garden was dedicated the Lyle E. Littlefield Ornamentals Trial Garden in honor of its founder.
While the collection has always been diverse, there developed over the first two decades a few major collections. The largest of these were Malus (170 taxa), Syringa (130 taxa), Magnolia and Rhododendron . Over the years the plants in the entire collection were evaluated for flower and fruit characteristics, insect and disease susceptibility, winter hardiness and overall performance. The results of these evaluations eventually led to the publication of several Cooperative Extension Service booklets including "Crabapples for Maine" and "Woody Ornamental Plant Hardiness Trials" (currently being revised and updated). Although these publications are somewhat dated now, they continue to serve as valuable resources for landscape horticulturists throughout the eastern and northern U.S.
Beginning in 1988 a major renovation of the facility began as did a redirection of the ongoing evaluation work. Over the years, the University of Maine Test and Demonstration Garden, as it was originally named, had evolved into a park for passive recreation as well as an exquisite collection of hardy woody plants. As the garden has been expanded over the last four years separate demonstration and evaluation areas have been developed. The public spaces have been redesigned with the help of students in the UM Landscape Horticulture programs. Much of the construction has been done by students as well. Today, while the renovation is still in progress, the garden sees several thousand visitors annually from young school children to horticultural professionals. During August of 1992, the garden served as the site of the annual meeting of the Northeast Regional American Association of Botanic Gardens and Arboreta.
At the present, the garden has been expanded to over 11 acres and the collection has increased substantially. Herbaceous perennials have been added both for evaluation purposes as well as for added color for midsummer attraction. A formal entrance garden was designed and is in the midst of installation (a three year project) and has already been the site of a dozen weddings during the summer of 1992.
In the area of plant evaluation, the work continues to follow a path similar to the original program. The major goal is to evaluate and display as wide a range of hardy plants as possible. The major collections have been maintained and continue to grow at a modest rate as only new introductions from reputable breeding and/or collection programs are added.
The Magnolia and Rhododendron collections have grown more rapidly and will continue to be highlighted, as the second goal of the project is to concentrate on groups of plants with great merit but which are sorely underutilized. Although calling rhododendrons underutilized in some parts of the country would be ludicrous, in Maine it is truly the case. One only needs to drive through any Maine town in May or June and the lack of variety is obvious. When it comes to magnolias the case is not one of a lack of variety but one of almost complete absence from the landscape.
Now considering the vehicle for this article, the Rhododendron collection certainly warrants further discussion. When the renovation work began in 1988 what existed was about 15 accessions and a long list of plants to get. Of course we all have similar long lists and that in itself is no great surprise, but what this list did supply was a note of intent to further pursue this group of plants.
Table 1 offers a list of all the taxa in the Rhododendron collection along with their hardiness ratings and flowering times. The hardiness ratings are from 1 (no evidence of any winter injury) to 5 (plant completely killed). The flowering times are when the plant was judged to first be at peak flower. To put the data in perspective, the garden sits in U.S.D.A. hardiness zone 4a. In the last 15 years it has seen in excess of -30°F four times and less than -40°F once. The site is on the highest portion of the campus grounds and is especially windy all winter long. The soils vary from outwash, gravely sand to heavy clay; it just depends where the spade lands.

Table 1. Rhododendrons in the Maine Collection
Name Hardiness 2 Flowering Date
aberconwayi 5 -
arborescens 1 6/20
atlanticum 2 6/12
atlanticum 1 6/12
bakeri 1 6/15
bakeri 1 6/15
barbatum PI 307331 5 -
brachycarpum ssp tigerstedtii 1 6/8
calendulaceum 1 6/2
calendulaceum 2 6/2
calendulaceum 1 6/2
calendulaceum 1 6/2
campanulatum PI 307332 5 -
canadense 1 5/29
canadense 1 5/29
canadense var albiflorum 1 5/29
carolinianum , white flowered 1 6/2
'Catawbiense Album' 1 6/8
'Cunningham's White' 1 6/5
'Nova Zembla' 1 6/9
'Roseum Elegans' 1 6/9
dauricum 1 5/8
dauricum, white flowered 1 5/6
dauricum 'Madison Snow' 1 5/6
'Weston's Aglo'* 1 5/22
'Alexander' 3 6/5
'Anna's Cheer'* 2 6/18
'Anna's Dream'* 2 6/18
'Apricot Surprise' 1 6/2
'April Snow' 1 5/14
'Big Deal' 1 5/21
'Bixby'* 4 6/1
'Black Satin' 1 5/22
'Boule De Neige' 1 6/2
'Desmit'* 1 -
'Frank Abbott'* 1 6/8
'Gable's Pioneer' 5 -
'Gallant Warrior'* 1 6/10
'Gibraltar' 1 5/28
'Golan Heights'* 1 -
'Golden Gala' 1 6/9
'Golden Lights' 1 6/1
'Golden Showers' 1 7/14
'Hidden Treasure'* 1 6/23
'Homebush' 1 5/27
'Irridescent'* 1 6/22
'Jane Abbott Peach'* 1 6/1
'Jane Abbott Pink'* 1 6/3
'Joe Paterno'* 1 6/10
'Jolie Madame' 1 6/10
'Keowee Sunset'*** 1 -
'Klondyke' 1 6/1
'Weston-Lemon-Drop'** 1 7/12
'Weston-Lollipop'** 1 6/22
low red frilled (Weston's) 1 6/8
'Majesty'* 4 6/4
'Marie Hoffman'* 1 6/2
'Mary Kittel'* 1 6/2
'Merley Cream'* 2 -
'Milestone' - 5/15
'Molly Fordham'* 1 5/22
'Mrs. J.A. Withington III'** 1 5/19
'Narcissiflora' (Ghent azalea) 1 6/6
'New Patriot' 2 5/15
(Mollis hybrid x R. prinophyllum ) 1 6/1
'Olga Mezitt' 1 5/22
'Orchid Lights' 1 5/30
'Weston-Parade'** 1 7/3
Unnamed azalea, pink #7 1 7/10
'Pink And Sweet'* 1 6/30
Pink Discovery'* 5 -
Pioneer Silvery Pink'* - 5/19
PJM group - 5/10
'PJM Victor' - 5/18
'Purple Gem' 2 6/3
'Purple Pajamas'* - 5/18
'Rio' - 6/11
'Rosy Lights' 1 6/1
'Silvery Pink'* - 5/25
'Spicy Lights' - 6/1
'Thunder' 1 5/18
'Toucan' 1 6/4
'Victoria's Consort' 1 5/19
'Viscosepala' 2 6/15
'Weston's Pink Diamond' - 5/14
'Wally (Vallya)'* - 5/24
Weston's Innocence'* - 6/22
'White Lights' - 6/1
'Windbeam' - 5/29
impeditum - 5/29
keiskei 5 -
kiusianum - 6/2
lepidotum PI 307344 5 -
'Ponticum Roseum'
(syn. 'Maximum Roseum')
1 6/20
degronianum ssp heptamerum NA 45155 5 -
molle? 15159034 5 -
mucronulatum 'Cornell Pink' - 5/13
mucronulatum , dwarf form - 5/18
prinophyllum 1 -
prinophyllum 1 -
dauricum 'Hokkaido' 5 -
rupicola 1 6/2
schlippenbachii 1 5/16
schlippenbachii , white flowered 1 5/16
serpyllifolium PI 274546 5 -
tschonoskii NA 44806 5 -
vaseyi 1 5/18
vaseyi 'White Find' - 5/22
viscosum 1 7/12
viscosum hybrid, pink flowered 1 7/18
viscosum hybrid, yellow flowered 2 7/29
weyrichii 5 -
yakushimanum 'Mist Maiden' 1 6/8
2 hardiness ratings defined as follows:
1)no winter damage.
2)occasional tip die-back or flower bud kill.
3)plant consistently experiences tip die-back, occasional severe winter
4)plant consistently experiences severe winter damage.
5)plant killed.

The Rhododendron collection consists of a number of separate subgroups which include North American native species, introduced species, and improved hybrids. The major emphasis of the collection is obviously on cold hardy varieties which represent as wide a range as possible of rhododendron characteristics. The hardiest lepidotes and deciduous species and hybrids make up the bulk of the collection.
Of the entire collection, the earliest to flower are Rhododendron dauricum and its varieties. Opening in late April to early May with either lavender or white flowers, these begin a Maine rhododendron flowering season which does not end until late August or even September. Of the R. dauricum accessions, the whites flower a day or two earlier than the straight species. R. dauricum , white flowered form, and R. dauricum 'Madison Snow' open on the average during the first week of May and from our evaluations the cultivar shows little difference from the species. The last of the collection to flower are R. viscosum and some of its hybrids which routinely offer peak display in late July but occasionally last until late August. During the summer of 1991, several were still hanging on in the first week of September. Possibly this benefit of the cool Maine summers can do a little bit to offset the trials and tribulations of the state's infamous winters.
One of the largest groups of improved hybrids in the collection are those developed and introduced by Weston Nurseries of Hopkinton, Mass. A cooperative evaluation project was initiated between the University of Maine and Weston Nurseries in the summer of 1989 and all the plants included have been evaluated for three winters.
One of the Weston groups consists of the ever popular PJM group. While some rhododendron enthusiasts consider the PJM group to be too common to be considered for a true connoisseur's collection, for those of us in the far north, it serves both as a valuable and dependable standard for many gardens as well as an excellent bench mark with which to compare other varieties. The group includes the most adaptable and toughest evergreen cultivars on the market today. It performs admirably from deep shade to full sun and even does well on moderately dry soils. In fact, there are several specimens on the UM campus which sit on a bright sunny site on the south side of a building. These plants are exposed to temperatures of -25°F and lower and a substantial amount of deicing salts from a nearby sidewalk. Even under these trying conditions, the plants never show more than some temporary leaf rolling. In Orono, Maine, the PJM group reliably flowers the last week of April through the second week of May depending on exposure.
While the PJM group has gotten much press and can be found in most northern landscapes, many of the other closely related lepidotes are much underutilized and worthy of mention. 'Thunder' is one of the darkest, producing fairly deep purplish flowers around the 18th of May on a plant with a narrow, upright form. On the other end of the flower color spectrum but opening a few days earlier is the double white flowering 'April Snow' which tends to be deciduous in Orono and unfortunately seems to be the most tasty to deer. About one week later to flower is one of the best in the entire group. 'Molly Fordham'* produces mid May flowers which want desperately to open a slight pink but fade rapidly to almost pure white. It has spectacular foliage throughout the season and produces a reliable and heavy crop of flowers.

R. 'Molly Fordham'
'Molly Fordham'*
Photo by Paul E. Cappiello

'Olga Mezitt' gets the vote for one of the toughest rhododendrons ever. I witnessed several plants installed in a landscape in Urbana, III., late one fall in a site which was completely exposed to full sun and the full wrath of the northwest winter winds. The low temperature during the ensuing winter was about -26°F and the plants were never mulched. The next spring, the plants produced a spectacular display of pink flowers in such abundance that I have not seen duplicated on any cultivar since. Considering the harshness of the site and the natural alkaline soil pH of the area, one would have to travel long and far to beat this one. In the garden, 'Olga Mezitt' flowers around the third week of May.

R. 'Olga Mezitt'
'Olga Mezitt'
Photo by Paul E. Cappiello

Finally, the best foliage award among the Weston introductions has to go to 'Desmit'.* Although this has only produced flower buds this past summer (probably because of sitting in too dense shade), the foliage is so exceptional that it could be grown exclusively for the effect. The R. ferrugineum parentage is evident in the narrowly pointed dark green leaves, and they remain so throughout the season. After three years the plant is approximately two feet tall and two and half feet wide.
The second group of Weston introductions tested over the last several years are the midsummer flowering deciduous hybrids. In Orono, these begin flowering in late June and continue until late August, and include a tremendous range of flower and foliage characteristics. Most produce heavy crops of brightly colored flowers with elegant fragrances. In addition, many provide an excellent addition to the fall color palette and some show excellent mildew resistance.

R. 'Weston's Lemon Drop'
'Weston's Lemon Drop'
Photo by Paul E. Cappiello
R. 'Weston's Lollipop'
'Weston's Lollipop'
Photo by Paul E. Cappiello

Among the earliest of these midsummer flowering hybrids is the brilliant white 'Weston's Innocence'*. This is a compact, spreading plant with nice deep mahogany fall foliage which remains essentially mildew free. The cultivars 'Weston-Lollipop',** 'Weston-Parade',** and 'Pink And Sweet'* are all excellent pink flowering varieties which are quite similar in all respects except plant shape and the color of the flowers' throats. Their flowering times vary about one week from the earliest 'Weston-Lollipop'** to the latest 'Weston-Parade'.** All three produce heavy loads of pink/red flowers from late June through late July. Each has a sweet, spicy fragrance which is sufficient to fill the air of any garden. Of the three, 'Weston-Parade'** is the more upright grower and has the deepest colored flowers.
'Weston-Lemon-Drop'** is one of the latest of these hybrids to flower, producing yellow flowers from mid July to mid August, on an upright plant. While the flowers are not quite as showy as some of the others, the spicy fragrance is quite effective. In addition to its late flowering, one of the best characteristics of this cultivar is the foliage. All summer long, the leaves remain a deep, glossy green, and in the fall, the mildew free leaves develop a deep burgundy/red and retain their glossy nature. Adding together the flowering time, fragrance, and the foliage characteristics, this has to be rated as one of the very best of the summer flowering varieties.
While the above summer flowering cultivars are but a few of this group, they represent some of the best of a tremendously useful series of rhododendrons. They thrive in full sun and require little care. In addition, they can tolerate a wide range of soil moisture regimes from quite moist to moderately dry. There are few gardens that would not benefit from the addition of one of these exceptional plants.
In addition to the Weston introductions, the Maine program has included extensive evaluations of native North American species. At present, there is a minimum of three separate collection sources for each of the following species: R. arborescens , R. calendulaceum , R. canadense , R. prinophyllum , R. vaseyi , and R. viscosum . In addition, there are two different accessions of R. bakeri and R. atlanticum . Of all of these accessions, only one accession of R. calendulaceum and one R. atlanticum have experienced considerable winter damage over the last several years. While it is not necessary to expound on the exceptional virtues of these species, this should serve as a reminder that while many of us have gotten caught up in the hybridization/introduction mania, mother nature has done an exceptional job of providing spectacularly showy, disease resistant, adaptable specimens.

R. canadense
Rhodora ( R. canadense ), Pushaw Bog
Photo by Paul E. Cappiello

Finally, the collection continues to grow. We are adding new plants all the time and plan to continue the evaluation work indefinitely. There are exciting new varieties arriving as I write this article, and, hopefully, future columns will include the performance of these specimens. In the meantime, anyone interested in visiting the garden or obtaining more information on the evaluation program is invited to do so. We are always looking to engage new cooperators and would welcome any inquiries.
As a final note, I would like to thank Mr. Roger Luce of Butternut Hill Gardens, Newburgh, Maine, for his assistance with this work and for supplying many of the plants in the Maine collection.

Paul Cappiello is an assistant professor of landscape horticulture at the University of Maine in Orono. He directs the renovation, daily management, education programs and collections at the Lyle E. Littlefield Ornamentals Trial Garden at the University of Maine. His research includes cold hardy companion plants for rhododendrons. Professor Cappiello received his Ph.D. in 1988 in horticulture at the University of Illinois.

Editor's Note: * Name not registered but not in conflict with a registered name. **Known in the New England nursery trade by illegitimate names 'Lollipop', 'Parade' and 'Lemon Drop', respectively. ***Names reserved, unregistered.