JARS v38n3 - Program to Evaluate Rhododendrons for Landscape Purposes

Program to Evaluate Rhododendrons for Landscape Purposes
Clive Justice, Vancouver, B.C.

A Visual Assessment & Classification By ARS Chapters & Chapter Districts

The major attraction of the rhododendron as a garden plant has been centered on the individual richness and spectacular nature of the flowers of the species and hybrids in this large plant genus. This fleeting glimpse of paradise each variety displays is usually two weeks in duration but it has had a tendency to overshadow the value of the rhododendron as a structural and visual landscape element in the garden for the remaining 50 weeks of the year.
Put simply, we have a great deal of documentation of the shape, size and colour of rhododendron flowers along with their blooming times but we lack the same degree of documentation of the visual qualities and character of the plant which bears such a beautiful bloom.
To select rhododendrons for a design for a private garden or public park, a landscape architect or garden designer wants to know more of the visual characteristics of a particular hybrid or species than the colour of the flowers. Anyone who has experienced a landscape of rhododendrons arranged only in accordance with flower colour will know what a visual disaster this kind of landscape arrangement can be.
A landscape architect would want to know whether the rhododendron is a tree, a shrub or a ground cover. We assume he or she would know that rhododendrons are woody plants, that some are deciduous (the azaleas) but most are evergreen. Further our designer would want to know what shape and habit the plant assumes. If a tree, whether small or medium, single stem or multi-stem, narrow or upright, spreading or round-headed. If a shrub, we can assume that most rhododendrons will fall into this landscape plant material classification, whether it be small, medium, or large, compact or open, upright or spreading. If our landscape architect wanted a ground cover using rhododendrons he would want to know if the selection was creeping, mounding or arching in habit of whether it grew flat, was low or high growing.
Whether tree, shrub or ground cover our garden designer/landscape architect would also wish to know if the rhododendron has a coarse, medium or fine textured appearance. As well, the foliage, is it shiny, dull or a colour other than green? e.g., blue grey or golden variegated? With this kind of information about each rhododendron, our landscape architect or garden designer can select the appropriate rhododendrons to help create a landscape with year-round visual optimum values.
Many of us gain this knowledge over many years experience of observation and use. I'm sure many who want to use rhododendrons in our gardens would benefit if the rhododendrons for each hardiness zone or chapter region were classified according to their landscape value or the visual effect they produce.
There has been a considerable amount of writing on aspects of landscaping with rhododendrons: chapters in David Leach's; "Rhododendrons of the World," Harold Clarke's earlier; "Getting Started with Rhododendrons," along with articles in the journals and year books of the ARS. Most recently George Ring and Kendall Gambrill have compiled regional ARS Chapter ratings lists in the Society's publication, "American Rhododendron Hybrids," edited by Meldon Kraxberger. The chapter by the late Bob Bovee in ARS 1961, "Rhododendrons for your Garden," described plant habit and texture of the species grown in American gardens along with hardiness ratings, the ARS combined truss plant ratings and other pertinent cultural information for each of the species. This was a milestone effort by Bob and the ARS committee (the late Rudolph Henny, the late George Grace, Cecil Smith, Howard Slonecker and Landscape Architect, Ruth Hansen).
One of the best chapters in "Rhododendrons for your Garden" described or evaluated each rhododendron species under four descriptive terms, 'very compact', 'compact', 'open' and 'leggy'. This observable characteristic of the plant, presumably for a plant grown uncrowded, is one of the basic form classifications so necessary in choosing a rhododendron plant for landscape arrangement. However, this is only one element of a landscape evaluation that will enable the genus and its hybrids to be classified by the visible plant characteristics of form, shape, colour, texture and habit.
With a wide range of these usual elements noted, the rhododendron becomes truly part of the plant material palette for the landscape architect, garden and park designer, home owner and rhododendron enthusiast. Here is all the information necessary to arrange by size, group by colour, mass for texture, combine with other plant species, and to use as an accent or specimen that will create a year round landscape.
Who better than the fifty-two chapters and the many individual members of ARS to evaluate and document the landscape values for the species and hybrids of this plant genus for which we have such a passion. This wealth and depth of knowledge and experience is now continent wide extending into almost every hardiness zone and climate in North America.
In addition there are already several software programs available, that, once the information is gathered, coded and entered in the computer, can provide us with printouts of information on rhododendrons by colour of foliage, size, leaf shape, habit, etc. It's obvious that a whole dossier of information, history, cultural, genetic - even regional phenology for each hybrid can also be stored and retrieved. This information on each rhododendron species or hybrid can be programmed in a myriad of ways with the objective of increasing our knowledge and understanding. We can develop new avenues for breeding and the means to exploit more fully the potential of the genus as a landscape material.
In order to achieve this narrower objective of classifying and documenting rhododendrons as visual landscape plant material it is proposed to employ some of the criteria used to classify and rate other kinds of plant material used in landscape design.
Gertrude Jeckyll in her many books wrote copiously on how, where and what plants to use for gardens and landscapes. However, for the evaluation of plant material for use in North American landscaping and garden making; A.D. Taylor's 1920 classic reference work the "Complete Garden" contains the more pragmatic, practical and easily accessible lists of plants by texture, colour, form, habit, region, with special uses; such as hedges, ground covers, etc. While the writer enjoyed reading Miss Jeckyll's books and trying to remember and use her combinations and arrangements when preparing a planting plan without having to reread the book each time, Taylor's lists were always kept handy for reference. These too lacked the form to evoke a mind picture that comes with personal familiarity with each plant one uses in a garden 1 . Up until now this experience and judgment one could only get after 20 to 30 years experience of growing, observing and using plants in different ways and under different conditions.
However, most of us can only have experience with the use of a restricted range of plants. As well, we share a common tendency to use in our own gardens or to recommend to others only those plants with which we are familiar or those whose names we remember. We often miss the more fitting or more appropriate selection because we lack knowledge of it.
A system of evaluation, storage and retrieval of the landscape uses and qualities for even a portion of the 1000 registered American hybrid rhododendrons, the close to 1500 hybrids listed from the U.K., Holland, Germany and Sweden and the 500 species of rhododendrons, is exciting to contemplate. Just to be able to have access to the judgment and experience of others and to eliminate the time consuming drudgery and effort of remembering, recalling or searching through many sources would free one to think more creatively and artistically. In addition, the ultimate objective for all of us, whether landscape architect, horticulturist or just plain rhododendron lover is to gain a fuller deeper appreciation and understanding of this marvelous genus of plants.
Having justified and documented the reasons and detailed the advantages for a landscape evaluation of rhododendrons, it remains only to describe a proposed system for achieving this objective.
It is suggested that a Landscape Evaluation Committee be formed in each ARS Chapter, whose task would be to rate the rhododendrons growing in the area encompassed by their chapter membership. Each Chapter Landscape Evaluation Committee would first determine the USDA Hardiness Zones of their chapter district area. Evaluation cards would be supplied by the National body with a preprinted system of rating on them. Each chapter committee would then start the evaluation by recording the name of the rhododendron and the USDA Hardiness Zone or zones of the chapter area. Numbers on the card would be circled corresponding with the visual assessment rating of each of the landscape values of each 7-13 year old rhododendron hybrid or species to be rated. First would be the type of landscape material, whether the rhododendron viewed is a: 1. tree, 2. shrub, or a 3. ground cover. If a tree, one of three for size: small, 8-15 foot height; medium, 15-30 foot height; large, 30 feet or over. It is fairly obvious that one does not find a rhododendron 30 feet or over in a 7-13 year old plant; perhaps not even a medium size tree should be rated for rhododendrons 15 years old or more. However, it's possible to buy and use rhododendrons that are trees - tree moving equipment is now so common almost anyone can obtain an instant tree for his garden - so why not a tree rhododendron? The R. arboreum hybrid (Sir Robert Peel) is used as a street tree in Rotorura, New Zealand, and 30 foot R. cinnabarinum trees in Windsor Great Park are used as a forest. These are just 2 examples of rhododendrons as trees with landscape uses. Further rating of trees are type and shape - 2 types; single stem or multi-stem, 3 shapes; upright, round-headed or horizontal branching.
If we generalize the leaf shapes of rhododendrons they all fall roughly into 5 groups. Whether the plant is a tree, shrub, or ground cover, we can assign each rhododendron to a foliage group as follows:
1. Small leaved: triflorums, Kurume azaleas, R. augustinii and any of the Glen Dales are examples of this leaf size.
2. Narrow and willow-like: R. caucasicum and hybrids, R. yakushimanums and hybrids. 'Elizabeth' is a typical hybrid with this leaf form. R. makinoi is not typical but is included here.
3. The rounded heart-shaped leaf of R. williamsianum, R. wardii and exemplified by 'Unique' and 'Bow Bells.'
4. The traditional rhododendron leaf shape in which the great bulk of garden rhododendrons fall is exemplified by the iron clad, R. fortunei , R. catawbiense and ponticums .
5. The large leaved - mostly species - R. sinogrande , R. falconeri , R. calophytum with leaves large and crinkly.
A simple but valuable visual characteristic is whether a rhododendron is evergreen or deciduous. If deciduous - whether:
1. Red fall colour
2. Yellow fall colour
3. No fall colour
Both deciduous and evergreen leaves of rhododendrons have a certain colour quality while green. These qualities can be rated for visual landscape uses as:
1. Shiny
2. Dull
3. A colour other than the usual green.
There are 3 parts to this category:
A. Variegated yellow (Either edge or spotted it gives approximately the same visual landscape effect.)
B. Blue-green
C. Silver-green
New growth, though fleeting like flowers, along with coloured indumentum are added bonuses. However, these 3 qualities should be subsidiary and down-played in the rating of rhododendrons for their landscape effect.
All rhododendrons, trees, shrubs or ground covers have a visible texture - so within the 5 foliage groups there are 3 valuations of texture - this is the texture of the plant itself (not the size of the leaves). We have 3 values for texture; fine, medium, and coarse. It's possible though not usual to have a fine textured large leaved rhododendron. This texture evaluation should be made viewing the plant from a distance of 10-25 feet or the distance most plants are seen in a landscape or garden. It's not the close-up hand lens texture of the botanist or enthusiast, but the human scale texture of the plant as a garden element that we evaluate.
When a 7-13 year rhododendron is visually classified as a shrub, there are three heights:
small, 1-3 feet,
medium, 3-6 feet and
large, 6-9 ft. in height.
There are 3 shapes:
1. Upright
2. Round, equal height or spread
3. Spreading, 1.5 to 2 times the height
The third landscape use classification for rhododendrons is for those that are groundcovers. In landscape, grass has been the traditional almost universal groundcover but it is far from the only one for our modern landscapes, parks, and gardens. Many rhododendrons make and are appropriate groundcovers. To use them correctly and to take full advantage they should be classified first in the 7-13 years of our base line as to size - first by height:
1. Flat, under 6 inches in height,
2. Low, 6-9 inches in height,
3. Mounding, 9-12 inches in height
Then the 3 values for spread:
1. Small, under 12 inches,
2. Medium, 12 - 24 inches,
3. Wide-spreading, 24-36 inches
This evaluation of spread would also give an indication of the appropriate spacing to achieve full coverage for mass planting of the selected groundcover rhododendron.
To complete the landscape assessment for each rhododendron the flower colour is recorded either by the information already in the computer from the ARS Registry, or programmed descriptions of hybrids and species taken from the many various sources already in print. If this flower colour information was in the computer in the Nickerson or RHS horticultural colour chart designations our landscape assessor need only circle these numbers in the card. However, it may be simpler to record flower colour in a generalized fashion under 10 colours with 2 or 3 shades for each:
1. Red, 3 shades; dark, mid, light
2. Pink, 2 shades; light and dark.
Time of flowering would be recorded by designating which of the 52 weeks of the year the bloom usually occurs for the chapter region; e.g., weeks 17, 18 & 19 would be the 4th week of April and 1st and 2nd weeks of May.
As an example let's rate a rhododendron in the writer's garden as it would appear on the computer card and what would be printed out if one programmed for a printout of the landscape description (evaluation) of this rhododendron hybrid.
With black pencil in hand we note the following: R. 'Olympic Lady' Vancouver Chapter Area USDA Zone 7 2
Landscape type 1 2 3
Size 1 2 3
Foliage Group 1 2 3 4 5
Foliage Type 1 i, ii, iii, 2 i, ii, iii
Texture 1 2 3
Type 1 2 3
Flower Colour 1 i, ii, iii, 2 i, ii, iii, 3 i, ii, iii etc.
Flowering Time weeks 16 & 17
A printout of the information would record - rhododendron 'Olympic Lady' in USDA Hardiness Zone 7 is a medium sized 3 to 6 foot high round shrub with heart-shaped or rounded shiny green foliage, coarse in texture (it has a loose open habit of growth) with light pink flowers from April 15th to May 8th. The dossier on 'Olympic Lady' would be complete if the printout could also include Registry description from the journal, the description in Kraxberger's, "American Rhododendron Hybrids," etc. - all now possible with computer technology.
The writer believes that the time for the landscape evaluations of rhododendrons, on a chapter, chapter district and hardiness zone basis is overdue. An initial action should commence immediately by way of a modest pilot project with perhaps some ARS research funds involving 3 widely dispersed chapters; West Coast, East Coast and South Midwest to describe and test the evaluation system elements, examine the criteria and determine the costs of a part or fully computerized system and more importantly, to develop a simple and easy method of landscape evaluation that will enable consistent and accurate data recording by each evaluator.
The writer, having had a long association with the genus, is willing to be involved in such a project. Hopefully there are some of you who will join in this worthy endeavor for the mutual benefit of ourselves and all who would landscape with this great diverse genus of plants.

Bowers, Clement G., Rhododendrons and Azaleas . Macmillan, New York, 1945
Clarke, Harold J., Getting Started With Rhododendrons And Azaleas . Doubleday And Company, Inc., Garden City, New York. 1960
Jekyll, Gertrude, Colour In The Flower Garden . Country Life Ltd., London, 1908
________________, Wood And Garden , Country Life Ltd., London, 1899
Krussman, Gerd., Rhododendrons Ward Lock Ltd., London, 1970
Leach, David. Rhododendrons Of The World . Charles Scribners Sons, New York, 1961
Plant Form Studies . Environmental Design Press, Reston, Virginia, 1975
Plant Sorting Systems . Landscape Architects Microcomputer User Group Newsletter, (LA-MUGS) Iowa State University, Ames Iowa.
Taylor, Albert, The Complete Garden . Garden City Publishing Co., Inc. New York, 1920
Wakefield, Geoffrey R., Rhododendrons For Every Garden . W. H & L. Coningridge Limited London, 1970
Wade, Keith L., Phenology Of Cultivated Rhododendrons Technical Bulletin #10 The Botanical Garden, University Of British Columbia, Vancouver, 1979
Van Veen, Ted. Rhododendrons In America . Sweeny, Krist & Dimm Inc., Portland, Oregon 1969
American Rhododendron Society. Rhododendrons For Your Garden ARS 1961

1 In today's landscape architecture office, Taylor has been largely supplanted by a trio of books by Donald Wyman - "Trees for American Gardens," "Shrubs & Vines for American Gardens" and "Ground Covers."
2 While the writer's garden is in Zone 8, the Vancouver Chapter Zone is Zone 7. In selecting plants for landscape use it is usually best to select plants one zone lower than the garden. In assessing plants the same conservatism should apply. Frozen and dead plants are usually not very good looking landscape elements of a garden.