Return to METRIA 11

FLowering Sequences and Duration of Peris Clones in Zone 8

Sven Svenson1 and Hannah Mathers2

North Willamette Research and Extension Center, Department of Horticulture, Oregon State University, 15210 NE Miley Road, Aurora, OR 97002-9543.

1Assistant Professor and 2Assistant Professor. Current address: Department of Horticulture, The Ohio State University, 2001 Fyffe Court, Columbus, OH  43210-1096.  The authors thank the following for assistance with data collection and processing and site maintenance: Jan Hettick, Chris Brands, Trudy Headley, Pat Kilmer-Kramer, Cheryl Denton, Kathy Sanford, Alison Henderson, Claudia Beville, Thirza Collins, Beth Mills, Neil Bell, and Bonnie Coy.

A paper from the Proceedings of the 11th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Gresham, Oregon, August 23-24, 2000, and cosponsored by the Landscape Plant Development Center.

Abstract

The objective of this study was to determine the flowering sequence and duration of Pieris clones in Oregon (USDA hardiness zone 8). Beginning in 1998, we recorded the date of first open flower, and the date when 50% of the flowers had turned brown or aborted. A Bloom Time Index was calculated based on flowering sequence relative to the first clone to bloom each year, and clones were placed into six BTI categories with significantly different average BTI. The clonal sequence of BTI was consistent for 1998, 1999 and 2000. 'Pygmaea’ was the first clone to flower (and had the longest flowering duration), and was the basis for calculating the BTI for all other clones.  There were 5 clones that flowered very early (BTI=13.7+1.9), 19 that flowered early (BTI=24.6+1.0), 13 that flowered early/middle (BTI=30.7+1.1), 18 that flowered late/middle (BTI=38.9+1.2), 14 that flowered late (BTI=53.0+1.0), and 5 that flowered very late (BTI=62.3+2.2). Very early and early selections were all Pieris japonica clones. P. japonica var. yakushimensis clones generally flowered late or very late. Clones with pink flowers generally flowered very early or early. Very late and late flowering Pieris had significantly shorter flowering duration compared to other categories, and very early flowering clones had significantly longer flowering duration compared to other categories.

Keywords

Pieris floribunda, Pieris formosa var. forrestii, Pieris japonica, Pieris japonica var. amamiana, Pieris japonica var. koidzumian, Pieris japonica var. yakushimensis, bloom time index.

Pieris are broadleaf evergreen shrubs used in landscapes for the pitcher-shaped white, pink or maroon flowers in spring, the colorful young growth of some cultivars, and the colorful young flower buds in winter. Depending upon species and location, common names vary from Andromeda to Fetterbush to Lily-of-the-Valley Shrub. Pieris floribunda is native to the eastern United States; Pieris formosa is native to the Himalayas, Nepal, Burma, Vietnam and Southwestern China; Pieris japonica is native from the southern Japanese Islands north through Taiwan and eastern China. Pieris phillyreifolia, another eastern U.S. native, was not included in this study (Griffiths, 1992).

Correct taxonomic names and correct spellings for many cultivars remain unclear. For this report, species designations rely on the most current taxonomic updates and clonal spellings common to the United States and Europe. The majority of the clones are selections of Pieris japonica, and the remainder of the clones represent a mixture of species and species hybrids, although hybrids involving species native to the United States are rare (Jaynes and Ticknor, 1984)

When studied for cultivation as a flowering potted plant, Pieris japonica var. yakushimensis 'Debutante’ had an optimal temperature of 17 C under short days (less than 12 h photoperiod) for flower bud development (Sytsema and Ruesink, 1996). Flower buds become dormant after formation, but endodormancy can be broken by chilling and application of gibberellic acid. Chilling hours required to break dormancy for each Pieris clone remain unknown.

Knowledge of the flowering sequence and duration of different clones of Pieris would allow for planting landscapes with a collection of clones to provide an extended flowering period, and would provide increased knowledge needed for future breeding. Based on a review of the literature, the flowering sequence and duration of Pieris clones have not been reported.  The objective of this study was to determine the flowering sequence and duration of Pieris clones in Oregon (USDA hardiness zone 8).

Materials and Methods

Clones of Pieris established at the North Willamette Research and Extension Center were used for this study. The collection (now over 180 selections) was initiated with 30 plants in 1959 by Dr. Robert L. Ticknor (Ticknor, 1988), and provided the germplasm from which Dr. Ticknor bred or selected 'Valley Rose,’ 'Valley Valentine,’ and 'Valley Fire.’ Except for 'Daisen’ and 'Shojo,’ all plants have been established at the site for at least 10 years (some for 35 years). 'Daisen’ and 'Shojo’ were planted in spring of 1997.

The study site has the following characteristics: latitude 45 degrees, 17 minutes north and longitude 122 degrees, 45 minutes west; elevation 150 feet above sea level; average last freeze date is April 17; average first freeze date is October 25; Clackamas County, Oregon.  The weather records for this site can be viewed on the world wide web (http://mac1.pn.usbr.gov/agrimet/ — select the Aurora, OR location). The AgriMet network weather station is located within 120 yards of all the plants used in this study, and within 40 yards of the majority of the plants. All plants used for this study were growing under full sun conditions in a Willamette silt loam soil. Depending on location, some of the plants receive either early morning or late afternoon shading from nearby taller specimens.

Beginning in 1998, we recorded the date of first open flower, and the date when 50% of the flowers had turned brown or aborted (end of useful flowering duration, and the time when many landscapers will remove flower racemes or 'deadhead’). Following a procedure similar to den Boer (1995) for crabapples, we calculated a Bloom Time Index based on flowering sequence relative to the first clone to bloom each year.  The BTI for a particular Pieris clone represents the average number of days after the first Pieris clone has its first open flower. Similar to Warren (1987), we separated the clones into six BTI categories: very early, early, early/middle, late/middle, late, and very late.

The BTI, flowering duration for each clone and within BTI category were analyzed for significant differences using SAS ANOVA, with SAS GLM and least squares means used for the unbalanced, within-category dataset.

Results and Discussion

The BTI and flowering duration of Pieris clones was significantly different (P<0.001, ANOVA F-test). The least significant difference mean separation procedure developed 28 different overlapping groups. Pieris were placed into BTI categories based on breaks in average BTI. Based on the weather pattern for a particular year, the clones located near the beginning or the end of a category break could easily have a BTI outside of the designated category.

The clonal sequence of BTI was consistent for 1998, 1999 and 2000. Actual BTI and duration of flowering was not the same in all three years as weather patterns varied. Compared to BTI, flowering duration was more variable.

'Pygmaea' (sometimes listed as 'Congesta Linearifolia') was the first clone with an open flower, and had the longest flowering duration (Table 1). The BTI of all other clones was determined relative to the flowering date of 'Pygmaea.' Our 29-year old specimen of 'Pygmaea’ is 4.5-ft tall. Newly planted, smaller specimens of the same clone often aborted flower buds during winter, and flowered for a shorter period of time. In 1997, the large 'Pygmaea’ specimen flowered irregularly through June, for a flowering duration of nearly six months (Svenson, unpublished). Since 'Pygmaea' started flowering on January 15, 23, and 18 for 1998, 1999 and 2000, respectively, the actual flowering date of all other clones we studied can be predicted by adding about 19 days to the listed BTI.

Other very early clones started flowering 14 to 18 days after 'Pygmaea' (Table 1). With the exception of 'Pygmaea' and 'Variegata,' the two oldest clonal selections of Pieris japonica, all of the very early flowering Pieris have pink ('Flamingo'), pink blush ('Valentine's Day'), or pink-bicolor ('Christmas Cheer') flowers. All very early varieties were clones of Pieris japonica

Early flowering clones started flowering 20 to 27 days after 'Pygmaea' (Table 1).  Similar to the very early category, 63% of the early Pieris had pink or pink-bicolor flowers. Many of the clones popular with the U.S. nursery industry were placed in this category including: 'Snowdrift,' 'Dorothy Wycoff,' 'Daisen,' 'Spring Snow,' 'Valley Rose,' 'Purity,' and 'Valley Valentine.' All of the named early varieties were clones of Pieris japonica. Our data is not consistent with the observations by Van Gelderen (1979) that 'Purity’ flowered 3 to 4 weeks later than “most other cultivars.”

Early/Middle category Pieris flowered from 28 to 33 days after 'Pygmaea' (Table 2). Only 25% of the early/middle Pieris had pink or pink-blush flowers. Some of the clones popular with the U.S. nursery industry were placed in this category including: 'Karenoma,' 'White Cascade,' and 'Mountain Fire,' and 'Shojo.' Pieris floribunda 'Karenoma' was the first clone to flower that was not a selection of Pieris japonica. Kruse (1987) noted that Pieris floribunda flowered 2 to 3 weeks later than Pieris japonica. Dirr (1990) noted that 'Shojo' was said to have the darkest red flowers of any Pieris, but our specimens were lighter in color and rapidly faded to white compared to 'Flamingo,' 'Wada,' or 'Valley Valentine.'

Late/Middle category Pieris flowered 35 to 47 days after 'Pygmaea' (Table 2). Less than 11% of the late/middle clones had pink-blush flowers. Many clones popular with the U.S. nursery industry were placed in this Category including: 'Forest Flame,' 'Firecrest,' 'Red Mill,' 'Brouwer's Beauty,' 'Little Heath,' and 'White Caps.' 'Forest Flame' and Firecrest (P. formosa var. forrestii x P. japonica), and 'Brouwer's Beauty' (P. floribunda x P. japonica) were the first named species hybrids to flower, and P. formosa var. forrestii and P. japonica var. amamiana were the first species varieties to flower.

Although still in containers, our specimens of Pieris japonica var. koidzumiana and its named hybrids with 'Forest Flame' ('Rosary Bell' and 'Spring Bell') from the Ofuna Botanical Garden in Japan (courtesy of Katsuhiko Oishi) would also be placed in the late/middle BTI category.  Ticknor (1988) noted that 'White Caps' was one of the last P. japonica selections to flower, and our data are consistent with this observation with the exception of a few dwarf forms.

'Brouwer's Beauty' is reportedly seed and pollen sterile (Jaynes, 1975), but Ticknor in 1993 and Svenson in 1996 have collected seed and grown seedlings from a specimen of 'Brouwer's Beauty.' The seedling grown by Svenson had an early/middle BTI, flowering before the seed source plant (data not shown).

Late flowering Pieris clones started flowering 49 to 59 days after 'Pygmaea' (Table 3). All late flowering clones had white flowers. We have listed 'Mouwsvilla' (HavilaTM) and 'Flaming Silver' as separate clones, but some have argued that these are two different names for the same clone. Neither BTI nor their flowering duration were significantly different between 'Mouwsvilla' and 'Flaming Silver' (Table 3), nor was there a difference in the date when vegetative shoot growth was initiated (Svenson, unpublished). Pieris x 'Valley Fire' offers the same bright red new growth with white flowers as 'Mountain Fire' and 'Forest Flame,' but at a later BTI.

Very late flowering Pieris clones started flowering 62 to 68 days after 'Pygmaea' (Table 3).  All very late clones had white flowers. Only two named clones were categorized as very late, 'Prelude' and 'Bolero.' The latest flowering Pieris in our study was NA40209C, a Skip March (U.S. National Arboretum) seedling selection of  P. japonica var. yakushimensis from Yakushima Island made from seed collected above 5000 ft elevation (Ticknor, 1988).

Very late and late flowering Pieris had significantly shorter flowering duration compared to other categories, and very early flowering clones had significantly longer flowering duration compared to other categories (Table 4).  It remains to be determined if there is genetic linkage between BTI and flowering duration, or if flowering duration is controlled by night temperatures or other factors.

There was a general trend for pink-flowered selections to have shorter BTI.  Principle component analysis or other procedures may reveal if there is linkage between flowering time and flower color.  Our hybrid of 'Valley Valentine' (maroon flowered) and 'Spring Snow' (white flowered) flowered early/middle with white flowers when both parents flowered early, suggesting that the relationship between flower timing and flower color may be rather complex.  Both parents of 'Valley Valentine,' 'Flamingo' and 'Valley Rose' (Ticknor, 1988) flowered very early or early.  The parents of 'Valley Rose,' 'Flamingo' and 'Deep Pink' (Ticknor, 1988), also flowered very early or early (data for 'Deep Pink' not shown).  There is no indication that a hybrid between 'Valley Valentine' and 'Spring Snow' should flower later than early.  Similarly, 'Valley Fire' flowers late, while its parents, P. formosa var. forrestii and 'White Caps' (Ticknor, 1988), both flower late/middle.

All of the “semi-dwarf” Pieris selections named from the seed collected by Robert de Belder on Yakushima Island in 1970 (clones of Pieris japonica var. yakusimensis; Bond, 1982) were late or very late bloomers including: 'Debutante,' 'Nocturne,' 'Chaconne,' 'Sarabande,' 'Cavatine,' and 'Prelude.'   Many other dwarf and semi-dwarf clones were also late or very late bloomers including: 'Pygmy,' 'Cupid,' 'Sinfonia' and 'Bolero.'  Except for 'Pygmaea,' all other dwarf or semi-dwarf forms flowered late/middle including 'Brookside Miniature,' 'Little Heath,' and 'Gavotte.' Principle component analysis or other procedures may reveal if there is linkage between dwarf shoot forms and flower timing.

Flowers of late and very late clones were very susceptible to browning from a late frost.  These clones are best grown under light shade where overhead tree canopies can provide some protection from clear-sky frosts.

Our selections of 'Spring Snow', and 'Scarlett O'Hara' that were growing under natural shade from tree canopy cover started flowering 7 to 12 days later when grown under shade (data not shown).  All clones produced more flowering racemes the following spring when seed pods were removed (dead heading) in early summer, an observation that is consistent with the comments of others (Bond, 1982).

An interesting flowering effect can be created by randomly removing about one-third of the flower buds in the late Fall. This allows the disbudded shoots to initiate vegetative growth the following spring while the other shoots are still in full bloom. For clones with colorful new growth, the combination of flowers and colorful new growth can make a nice seasonal landscape feature. The procedure has been effective with 'Forest Flame,' 'Firecrest,' 'Mouwsvilla,' and 'Valley Fire,' but not with 'Scarlett O'hara' or 'Red Mill.' Since many Pieris clones do not produce new vegetative growth on flowering shoots (Bond, 1982), late Fall disbudding assures some new shoot growth each year.

Clones that are species hybrids tended to flower late/middle or later (for example: 'Brouwer's Beauty,' 'Firecrest,’ 'Forest Flame' and 'Valley Fire'), which is consistent with some expectations from earlier interspecific hybridization (Jaynes and Ticknor, 1984).

The 74 clones represented in this study include most of the selections grown by the U.S. nursery industry.  A few popular clones have not yet been evaluated including: 'Balls of Fire,' 'Bert Chandler,' 'Blush,’ 'Charles Michael,’ 'Crispa,' 'Grayswood,’ 'Henry Price,’ 'Jermyns,’ 'March Magic,’ 'Millstream,’ 'Pink Delight,’ 'Red Head,' 'Red Volcano,' 'Rowallane,’ 'Select,' 'Temple Bells,' 'Tilford,' and 'Wakehurst.'

This is the first report of the flowering sequence and duration for available Pieris clones. Flowering may differ in different climates.  As additional data is collected, a prediction model based on degree-days may be useful in predicting the flowering start date and duration of selected Pieris clones. In USDA hardiness zone 8, careful Pieris cultivar selection for landscape use could provide a flowering season of 4 to 5 months or longer.

Based on the flowering response of small and large specimens of 'Pygmaea,' the size and age of the specimen may influence the BTI and the flowering duration. The BTI and flowering durations developed from this study require verification before being considered generally reliable.

Literature Cited

Table 1A. Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering very early. Means and standard errors.

Clone Very Early BTI Flowering Duration
(number of days)
'Pygmaea' 0.0±0.0 100.0±6.0
'Christmas Cheer' 14.3±3.8 67.0 ±5.6
'Variegata' 15.7±2.6 69.7±7.8
'Valentine’s Day' 16.0±1.7 63.0±14.5
'Flamingo' 18.0±2.3 64.7±13.7

Table 1B. Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering early. Means and standard errors.

Clone Early BTI Flowering Duration
(number of days)
'Snowdrift' 20.3±0.8 71.3±12.8
'Wada' 20.7 ±0.7 66.0±13.8
'Dorothy Wycoff' 23.0±3.0 67.7±8.7
T36-82A 23.3±4.7 64.3±7.1
'Daisen' 23.3±11.7 49.0±11.0
VVx[Vx(WxCC)] 24.0±7.6 55.0±7.5
PI418531 24.0±2.5 77.3±7.4
'Spring Snow' 24.1±5.6 52.0±9.3
'UNH' 24.3±5.2 68.7±10.3
'Valley Rose' 25.0±3.1 61.0±5.8
'Stockman' 25.3±5.3 67.0±12.5
T44-82V 25.3±6.1 55.7±3.3
NA40868B 26.3±4.1 50.0±5.6
'Purity' 26.3±2.6 67.7±7.8
'Wil-Chris 34' 26.7±6.6 55.7±0.9
NA45364(2) 26.7±3.8 55.0±7.6
'Valley Valentine' 27.5±3.1 58.5±9.5
NA45364(1) 27.7±4.6 51.0±5.0
'Benihaja' 27.7±4.6 52.7±4.2

Table 2A. Bloom Time Index (BTI) and flowering duration of selected
Pieris
clones flowering early/middle. Means and standard errors.

Clone Early/Middle BTI Flowering Duration
(number of days)
'Karenoma' 28.7±1.5 71.3±13.5
'Wil-Chris 12' 29.0±3.5 66.0±7.2
'Bovee Pink' 29.7±8.0 54.7±7.1
'White Cascade' 30.0±5.5 68.3±9.4
'Valley Valentine' x 'Spring Snow' 30.0±2.7 63.0±9.9
T43-82A 31.0±2.9 59.7±10.7
T40-82I 31.0±2.9 59.7±11.1
'Mountain Fire' 31.0±2.9 55.3±9.4
'Shojo' 31.2±4.7 53.2±12.7
'Iseli Cream' 32.7±6.9 32.3±6.4
T40-82A 33.0±6.0 54.3±14.8
'La Rocaille' 33.7±1.5 72.7±10.9
'Grandiflora' 33.7±7.8 63.7±7.7

Table 2B. Bloom Time Index (BTI) and flowering duration of selected
Pieris
clones flowering late/middle. Means and standard errors.

Clone Late/Middle BTI Flowering Duration
(number of days)
'White Water' 35.0±0.6 61.6±11.4
'Brookside Miniature' 35.0±6.7 40.0±9.5
'Coleman' 36.0±7.6 54.0±14.4
'Forest Flame' 36.7±4.3 66.7±13.7
T44-82O 37.3±9.0 58.3±15.3
'Firecrest' 37.7±5.2 50.7±11.8
P. formosa var. forrestii 37.7±5.2 68.3±8.0
'Red Mill' 37.7±5.2 77.3±12.9
NA40868A 38.0±3.5 64.3±9.8
'Brouwer’s Beauty' 39.0±4.5 65.7±14.2
NA40868D 39.0±4.5 56.3±10.4
'Little Heath' 40.3±5.8 55.0±14.7
'Gavotte' 41.0±8.5 35.3±9.8
T44-82Q 41.3±8.9 59.0±22.1
T44-82S 41.3±8.9 52.3±17.9
NA40269A 41.3±6.8 72.0±14.2
P. japonica var. amamiana 43.7±5.0 43.7±10.5
'White Caps' 46.7±3.3 54.7±5.7

Table 3A. Bloom Time Index (BTI) and flowering duration of selected
Pieris
clones flowering late. Means and standard errors.

Clone Late BTI Flowering Duration
(number of days)
'Debutante' 49.0±3.5 44.0±11.4
'Nocturne' 50.0±4.0 62.7±9.4
T44-82U 50.0±4.0 44.7±10.7
'Chaconne' 50.0±4.0 52.0±14.7
'Compact Crimson' 50.0±4.0 44.3±3.9
'Flaming Silver' 50.0±4.0 52.7±7.1
'Ticknor’s First' 51.7±5.2 60.7±10.5
'Havila' 52.7±1.4 42.7±12.2
'Sarabande' 52.7±3.7 44.7±8.1
'Pygmy' 54.3±3.0 9.7±1.2
'Valley Fire' 57.0±4.0 64.3±6.4
'Cavatine' 57.3±3.7 52.0±11.0
'Cupido' 58.0±4.2 51.3±12.0
'Sinfonia' 58.7±5.6 34.3 ±10.6
Clone Very Late BTI Flowering Duration
(number of days)

Table 3B. Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering very late. Means and standard errors.

'Prelude' 62.0±6.0 49.0±9.9
'Bolero' 62.0±2.9 58.0±4.7
NA40269B 63.0±7.0 52.0±4.5
T64-88 67.0±2.9 46.0±12.1
NA40209C 67.7±5.8 45.7±5.9

Table 4. Bloom Time Index (BTI) and flowering duration of Pieris clones averaged within BTI categories. Means and standard errors.

Clone BTI Flowering Duration
(number of days)
Very early 13.7±1.9 71.6±4.9
Early 24.6±1.0 59.6±1.9
Early/middle 30.7±1.1 60.2±2.6
Late/middle 38.9±1.2 56.5±1.3
Late 53.0±1.0 47.1±3.0
Very late 62.3±2.2 51.1±2.9
     
Significance
(PR>F, GLM F-test): 
0.001 0.001

Return to METRIA 11


Web Crafter: Anne S. Napier ~ Email: anne_napier@ncsu.edu

Format updated July 24, 2009