Flowering
Sequence and Duration of Pieris
Clones in Zone 8
__________
1Assistant Professor and 2Assistant Professor and
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.
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. koidzumiana, 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:
Bond, J.
1982. Pieris – A survey. The Plantsman 4:65-75.
den Boer, J.H.
1995. Blossom times. Malus 9(1):16.
Dirr, M.A.
1990. Manual of Woody Landscape
Plants. Stipes Publishing Company,
Champaign, IL.
Griffiths, M.
1992. Index of Garden Plants. Timber Press, Portland, OR.
Jaynes, R.A.
1975. 'Brouwer's Beauty' Pieris, a new interspecific hybrid. HortScience 10(2):185-186.
Jaynes, R.A. and R.L. Ticknor. 1984.
Interspecific crosses among American and Asiatic Pieris species. HortScience 19(3):436-437.
Kruse, K. 1987. Pieris floribunda. Amer. Nurs. 166 (12):126.
Sytsema, W. and J.B. Ruesink. 1996.
Forcing Pieris japonica
‘Debutante.’ Scientia Hort.
65(2-3):171-180.
Ticknor, R.L.
1988. Breeding new Pieris
cultivars. Comb. Proc. Intl. Plant
Prop. Soc. 38:161-163.
van Gelderen, D.M.
1979. Pieris. Dendroflora 15/16:36-44.
Warren, K.
1987. Crabapple bloom
sequence. Malus 2(3):7.
Table 1.
Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering very early or
early. Means and standard errors.
________________________________________________________________________
Flowering
duration
Clone BTI (number
of days)
________________________________________________________________________
'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
'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
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 2.
Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering early/middle or
late/middle. Means and standard errors.
________________________________________________________________________
Flowering
duration
Clone BTI (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
‘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
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
________________________________________________________________________
Table 3.
Bloom Time Index (BTI) and flowering duration of selected Pieris clones flowering late or very
late. Means and standard errors.
________________________________________________________________________
Flowering
duration
Clone BTI (number
of days)
________________________________________________________________________
'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
'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.
________________________________________________________________________
BTI Flowering duration
category BTI (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
________________________________________________________________________