JARS v38n3 - Corolla Thickness of Rhododendrons

Corolla Thickness of Rhododendrons
Pat Halligan, Langley, WA

Last year I decided to try to make crosses with a small flowered azalea which has the peculiarity of retaining its flowers for months in good condition. I needed to find a somewhat larger flowered azalea with flowers of very heavy substance. I tried to compare a large number of azaleas for floral substance by feel, but was unable to come up with any consistent ranking. A clone which felt good one day would feel different the next. Any comparisons which were not done side by side were hopeless. Anything influencing the turgor of the flowers, such as recent lack of rain or a hot day had a drastic effect on the feel of the flower. I needed a way to measure floral substance which was objective, could be used for comparisons remote in space or time, and would not be so completely influenced by turgor.

Corolla lobes were measured with a Sears micrometer #40707 (price $13.99) with 0.001 inch (1 mil) graduations. This micrometer had a friction clutch with a light setting, minimizing crushing of the petals, and ensuring that all measurements would be made with approximately the same pressure. Flowers were measured in two places on each lobe, one on the midvein halfway between the base and tip of the lobe, and the other on the lamina halfway between the midvein and edge (Figure 1). In this report the term lamina is defined as that part of the corolla lobe which is lateral to the midvein. All petals on a flower were measured and the results were averaged separately for lamina and midvein measurements. The notation used in this report is lamina/midvein in mils (thousandths of an inch) thickness. Thus 5/9 means that the average lamina thickness was 0.005 inch and the average midvein thickness was 0.009 inch.

Figure 1
Figure 1. Dissected corolla showing locations of measurements (circles) and terminology
used in this report. The midvein measurement was located halfway between the base of the
lobe and its tip. The lamina measurement was located halfway between the midvein and edge
of the lobe at the level of the midvein measurement. If the lobe was not wide enough, then
the lamina measurement was taken over the entire width of the lamina to one side of the
midvein. Shaded area indicates thicker areas of the corolla.

Flowers of R. 'Goldstrike' and R. nuttallii , including the base of the lobe at the midvein, at the middle of the tube, and near the base of the corolla. Flowers of R. japonicum and R. 'Double Beauty' were measured during four stages of development: expanded bud, partially open flower, fully open flower, and dehisced corolla. A survey was made of a number of rhododendron flowers as well as a few other types. These measurements were taken on fully expanded turgid flowers which were in good shape.

Corolla thickness varied over different parts of the corolla (Figure 2). The midveins were thicker than the laminae. The upper lobe was the thickest, and the lower lobes were the thinnest. The corolla was thickest near its base and thinnest at the tips and edges of the lobes, with a continuum between. The pattern for R. nuttallii was similar to that of R. 'Goldstrike', with measurements of 10/14/21/29/38 mils for lamina, midvein, base of lobe, mid tube, and base of corolla respectively.

Figure 2
Figure 2. Dissected corolla of R. 'Goldstrike' showing variations in
thickness over various parts of the flower. Thicknesses are in mils. Stippled
areas represent approximate distribution of areas thicker than 15 mils.

Each cultivar had its own pattern of thicknesses. Some of the popular elepidote hybrids showed exaggerated differences between midvein and lamina, and between the different lobes. The midvein thickening also varied in width. Rhododendron 'Belle Heller' was observed to have a narrow midvein, while some other hybrids were observed to have very wide midvein thickenings in the flare area of the upper lobe. The opposite could be seen in some of the lepidotes and azaleas, where the midvein was only slightly thicker than the lamina.
The width of the midvein thickening was not measured, although its width does contribute to floral substance. I observed that the midvein thickening tended to be wider in flowers with thick laminae. Thus a flower with a measurement of 7/20 would likely have a narrow midvein thickening, while one with a measurement of 12/20 would likely have a wider midvein thickening.

Table 1. Flower substance with age of flower. Average thickness of corolla lobes measured on lamina and midvein (L/M) in mils. Five outermost petals were measured in R. 'Double Beauty'.
Age of Flower Flower Substance (L/M)
R. 'Double Beauty' R. japonicum
Expanded Bud 6.3/9.9 5.7/9.8
Partially Open 6.5/10.3 5.7/10.1
Fully Expanded 6.5/10.9 5.5/10.3
Dehisced Corolla 6.8/10.6 5.0/9.1

Corolla thickness changed slightly during the development and decline of the flower, for those cultivars which were measured (Table 1). Open flowers which have not yet wilted can probably be compared without fear of error due to differences in developmental stages. The dehisced flowers of R. japonicum were collapsed, which probably explains the lesser measured thicknesses in Table 1.
Measurements of a number of species and hybrids can be seen in Table 2. Elepidote hybrids with superior substance were R. 'Trude Webster' (9/31) and R. 'Virginia Richards' (12/29). Azaleas with good substance were R. 'Eikan' (13/21) and R. 'Linda R' (12/18). Lepidotes with good substance were R. cinnabarinum (14/25) and R. baileyi (16/22). In contrast were the elepidotes R. roxieanum (4/7) and R. macrophyllum (5/9), and many lepidotes and azaleas with thicknesses below 7 mils.

Table 3. Average flower thicknesses of various rhododendron groups using the sampling given in Table 2. Averages expressed as in Tables 1 and 2. Ratios are of lamina thickness divided by midvein thickness expressed as percentages.
Rhododendron Average (L/M) Ratio (L/M)
Group (mils) (%)
Elepidote Hybrids 7.9/18.2 43%
Elepidote Species 7.0/13.7 51
Lepidote Hybrids 5.5/8.7 63
Lepidote Species 5.8/9.2 63
Evergreen Azaleas 6.5/10.2 64
Deciduous Azaleas 5.8/10.8 53

The average flower thickness of elepidotes was greater than that of other groups of rhododendrons (Table 3). This was due to slightly thicker laminae and much thicker midveins. The ratio (L/M) compares the relative thicknesses of laminae and midveins. This ratio is much greater in the lepidotes and evergreen azaleas than in the elepidotes and deciduous azaleas (Table 3). Lepidotes lack prominent midveins, and those with thick midvein areas also have thick laminae (Table 2). In the lepidotes, both hybrids and species had laminae almost two thirds as thick as the midveins. The elepidote species had midvein thickenings about twice as thick as their laminae, but the elepidote hybrids had midveins much more than twice as thick. Flower thicknesses of rhododendrons were comparable to those of several other cultivated plants (Table 2). Some plants such as orchids had members with flowers much thicker than those of rhododendrons.
Petal thickness of diploid lepidotes in Table 2 averaged 5.6/9.0 while that of polyploid lepidotes averaged 7.5/12.5. Although a trend could be seen toward thicker petals in polyploids, it was by no means consistent. For example the diploids R. nuttallii (10/14) and R. edgeworthii (9/19) had thicker flowers than the polyploids R. russatum (3/5) and R. rubiginosum (5/7). Lepidotes with thick flowers often belonged to the Maddenii Series and to the related Series Edgeworthii and Cinnibarinum (average 10.3/17.3). Some others (Subseries Baileyi and Series Glaucophyllum) also had species with thick flowers. Most of the other lepidotes had thin flowers.

When a person "feels" the substance of a flower, he is "measuring" the stiffness of the corolla lobes. Factors which influence this stiffness include thickness of the corolla, turgor, shapes of the surfaces (Fluted or corrugated surfaces will be stiffer due to mechanical considerations, like corrugated versus flat sheet aluminum, or an I-beam versus a rod of the same weight.), and inherent stiffness (Wood is a lot stiffer than potatoes of the same shape and size.). A small flower will feel stiffer than a larger one of the same thickness due to mechanical considerations (length of lever arm).
"Floral substance" which is characterized as a desirable quality which enhances bloom longevity, reduces marginal burn and makes the flower look and feel more substantial is probably a combination of petal thickness and inherent stiffness. In measuring the thickness of corolla lobes with a micrometer there is some compression of floral tissues in the micrometer. Although I attempted to minimize this crushing, the measurements in this report include both a factor of thickness and a factor of inherent stiffness. Most flowers of thickness of 5 mils or more tended to be compressed only slightly, while those of lesser thickness tended to be compressed more, especially lepidotes with "mealy" flowers of very little substance.
A comparison between flower thicknesses of species and hybrids of the same groups reveals some possible trends that have been followed by hybridizers (Table 3). Elepidote hybrids differ from the species in that the midveins are much thicker in the hybrids. The laminae are almost unchanged. Apparently growers and hybridizers have selected for thick midveins, but not for thick laminae. Thick midveins make a flower feel thick and stiff, and hybridizers have taken advantage of the genetic sources of this trait. Lamina thickness has apparently been largely ignored. Elepidote hybrids with thick laminae are R. 'Medusa' and R. 'Virginia Richards'. An elepidote species with thick laminae is R. beanianum , and I suspect that some other species of the Neriiflorum Series, perhaps R. dichroanthum , might also be found to have thick laminae.
Thickness of midvein, width of midvein thickening, and thickness of lamina all contribute to floral substance. Although all three probably have some common genetic bases, they probably also have independent controls. Thus by manipulating each of these three factors, a flower with maximum and uniformly heavy substance could be produced.
My comparisons of different groups of plants are based on small samples and should be interpreted with caution, since most species were left unmeasured. I feel that the selection of hybrids in this report reflects those plants which have become accepted by hybridizers and the public, since most were measured at commercial nurseries, and many are newer hybrids.
The measurements presented in this report should be taken with a grain of salt. Petals, flowers and plants vary. I would suggest that based on my experience in measuring flowers of the same cultivar at different times and places, the degree of error was greatest on flowers with thin laminae and very thick midveins. Those flowers with midveins only slightly thicker than their laminae tended to have much more consistent measurements.
After having tried measuring floral thicknesses for only a short time, I can claim that it has already influenced my own breeding program. It has helped me make choices between plants, and it has pointed out some "sleepers" to me. The micrometer is inexpensive and easy to use. I mounted mine on a small block of wood so I could use both hands to manipulate the flower and turn the micrometer. Be sure to zero the micrometer each day you use it, since the zero setting seems to vary from day to day.

Table 2. Average corolla lobe thicknesses of various rhododendrons in mils. Measurements were taken on the laminae and midveins (L/M) as explained in text. A listing of some other types of flowers is included for comparison.
Elepidote Species
arboreum campbelliae 5/11 Dr. Stocker 7/15
arboreum cinnamomeum 6/11 Elisabeth Hobbie 8/20
arboreum 'Sir Charles Lemon' 8/14 Elizabeth 8/15
argyrophyllum 7/14 Ethel 9/16
beanianum 11/15 Etta Burrows 9/14
callimorphum 7/16 Fabia x bureavii 8/17
campanulatum 6/13 Flower Girl 9/18
campylocarpum 8/16 Friedman 5/12
floccigerum 9/15 Gartendirektor Glocker 6/13
fortunei 7/16 Gomer Waterer 6/22
galactinum 6/12 Hello Dolly 7/16
gymnocarpum 6/11 Hotei 9/28
haematodes 8/19 Janet Blair 6/16
irroratum 7/14 Jean Marie de Montague 9/20
litiense 7/14 Lem's Cameo 7/15
macrophyllum 5/9 Loderi King George 9/21
makinoi 10/17 Loderi Pink Diamond 9/21
neriiflorum 6/13 Loder's White 7/17
orbiculare 7/12 Lori Eichelser 9/15
praevernum 10/20 May Day 9/26
puralbum 7/14 Medusa 11/21
roxieanum 4/7 Molly Ann 7/14
smirnowii 6/16 Moonstone 8/16
souliei 7/13 Mrs. Betty Robertson 8/19
thayeranum 6/14 Mrs. Furnival 6/17
vernicosum 7/12 Mrs. G.W. Leak 9/25
wardii 7/13 Nugget 7/14
williamsianum 8/13 Pink Pearl 8/27
yakushimanum 6/12 Red Majesty 9/18
Red Wax 9/16
Elepidote Hybrids Rosamundi 5/11
Ruby Bowman 7/17
Alice 6/19 Scarlet Wonder 8/20
America 6/17 Scintillation 8/22
Arthur J. Ivens 7/13 Stardust 10/20
Baden Baden 9/20 Starlet 8/14
Belle Heller 5/19 Temple Belle 7/12
Black Prince 8/18 Treasure 9/15
Blue Peter 5/18 Trude Webster 9/31
Brickdust 9/16 Unique 9/20
Crest 7/15 Unknown Warrior 7/21
Cynthia 6/19 Virginia Richards 12/29
Debbie 7/13 Voodoo 7/19
Diane 10/23 Vulcan 6/14
Lepidote Species Lepidote Hybrids
augustinii 4/8 Anna Baldsiefen 2/3
baileyi 16/22 Blue Diamond 3/5
campylogynum 8/12 Blue Tit 3/6
carolinianum 4/8 Bric-a-brac 7/11
ciliatum 8/11 Candi 5/8
cinnabarinum 14/25 Chikor 3/6
concinnum pseudoyanthinum 4/6 Chink 5/7
dauricum 3/4 Cilpinense 5/10
dauricum album 4/5 Cream Crest 5/7
edgeworthii 9/19 Crossbill 5/6
flavidum 5/5 Dora Amateis 6/10
glaucophyllum 6/10 Else Frye 10/15
hanceanum 'nanum' 4/7 Goldstrike 10/14
hormophorum 6/7 Lady Berry 11/18
impeditum 3/5 Lady Chamberlain 12/17
intricatum 3/5 Lucy Lou 5/9
keiskei 'Yaku Fairy' 3/5 Mary Fleming 4/7
lepidotum 3/5 Mood Indigo 3/5
leucaspis 5/9 Patty Bee 5/8
luteiflorum 14/20 Quaver 6/10
lutescens 4/5 Racil 5/8
mucronulatum 4/5 Sapphire 3/5
nuttallii 10/14 Seta 7/10
oleifolium 5/9 Shamrock 4/5
oreotrephes 6/10 Snow Lady 5/9
pemakoense 6/10 Trewithen Orange 10/14
racemosum 2/3 Windbeam 3/6
rigidum 5/9 Wyanokie 4/7
rubiginosum 5/7 Yellow Hammer 5/8
russatum 3/5
sargentianum 4/6
triflorum 4/8
tsangpoense 8/17
Evergreen Azaleas Deciduous Azaleas
Aladdin 7/11 Ballerina 7/16
Double Beauty 6/11 Flamingo 8/14
Eikan 13/21 Golden Sunset 6/12
Geisha 5/8 japonicum 6/10
Girard's Crimson 8/12 luteum 4/9
Glamour 6/11 Old Gold 7/11
Hexe 6/9 Orangeade Pink 8/15
Hino Crimson 6/7 Ruffles 7/12
kaempferi 6/8 prinophyllum 2/4
kiusianum 4/6 Renne 6/13
Ledifolia Alba 6/9 schlippenbachii 6/9
Linda R. 12/18 vaseyi 3/4
Redwing 10/17
Purple Splendor 4/7 Other Rhododendrons
Roberta 9/13 Brilliant (Ledudendron) 6/11
Rosebud 5/6 Hardijzer's Beauty (Azaleodendron) 5/7
Sherwood Orchid 4/8 Pink Delight x jasminiflorum (Vireya) 7/9
Stewartstonian 4/7 wilsoniae (Choniastrum) 6/10
Sweet Briar 5/7
Tradition 4/6
Vuyk's Scarlet 7/13
Other Flowers
Apple 5/6 Magnolia liliflora nigra 16/30
Cattleya hybrid (Orchid) 24/30 Magnolia wilsoni 13/18
Daffodil 6/9 Odontoglossum hybrid (Orchid) 16/31
Fuchsia petals 6/9 Tulip 6/13
Fuchsia sepals 14/16

I wish to thank Sky Nursery, Garden Valley Nursery, Baker and Chantry Orchids, and Meerkirk Garden for giving me permission to measure their flowers.

Ammal, E.K.J., I.C. Enoch, and M. Bridgewater (1950) Chromosome numbers in species of Rhododendron . Rhododendron Yearbook, Royal Horticultural Society, 5: 78-91.