MCFG Bulletin August 2003

PAGE 2 - OCTOBER 2001
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
MINNESOTA COMMERCIAL
FLOWER GROWERS
50 YEARS OF SERVICE
BULLETIN
50+ YEARS OF SERVICE
VOLUME 52, ISSUE 2 - AUGUST 2003
Fundamentals of Flowering in Plants:
Juvenility in Seed-Propagated Annuals and How Supplemental
Lighting Can Affect It
John Erwin, Neil Mattson and Ryan Warner
Department of Horticultural Science, University of Minnesota
Part I
Introduction:
This article is the first of
a series presenting new
research results on effects
of daylength and
supplemental lighting on
flowering of seedpropagated bedding plants
that we are conducting at
the University of
Minnesota. This project
started years ago when a
greenhouse grower called
and wanted to know how to
have flowering petunias
during February and March
in the Seattle area. A
couple of months after that
I was asked how to delay
flowering in summer
INSIDE THIS ISSUE:
Fundamentals of
Flowering in Plants,
Part I
1-4
Fundamentals of
Flowering in Plants,
Part II
5-8
Common Problems In
Northern Poinsettia
Production
9-10
2003 Bedding
Plant Conferences
11-12
germinated pansies in
Minnesota for sales in
Texas in the late summer
and fall. The answers were
not ‘clear cut’ and we
realized how little we know
about what causes flowering
in common bedding plants.
The last time a series of
experiments was done to
really look at what causes
bedding plants to flower
was done during the late
1950’s and early 1960’s!
Information on what
makes bedding plants
flower would allow you to
control flowering to
schedule flowering
accurately as we do with
many potted plant crops
such as chrysanthemum and
poinsettia. Because we
didn’t have the answers for
these growers and the
importance of this
information to the North
American greenhouse
industry, we started a longterm research study
examining how daylength,
supplemental lighting and
temperature affect flowering
of many bedding plant
species and how to
precisely control flowering
of bedding plants.
This article and the next
will focus on how light
intensity, or irradiance,
affects flowering of bedding
plant species. The
following articles will show
how daylength affects
flowering of common and
uncommon bedding plants.
The results were interesting
to us and have a
tremendous amount of
application for the industry
(especially those who grow
bedding plants from seed).
Juvenility versus
Maturity:
Plant flowering is
controlled by internal and
external signals. With any
seed-propagated bedding
plant there is an internal
signal that determines when
a plant is mature and
capable of flowering or
responding to a flowering
stimulus such as daylength.
For instance, there is a
period with animals after
birth when they are
incapable of reproducing.
Similarly, plants are often
unable to flower
immediately after
germinating. The period
after germination when a
seedling is not capable of
flowering or responding
to a flowering stimulus is
called the ‘juvenile
period’. When a seedling
is capable of responding
to a flowering stimulus
we say it is ‘mature’.
How do we know
when a seedling has
changed from the
juvenile to the mature
phase? Unfortunately,
with most seedpropagated annuals, there
is no visible change in
how the plant looks when
it changes from a juvenile
to a mature plant.
Probably the best
indicator we have is the
number of leaves that a
plant has.
Much work has been
conducted using leaf
number as a measure of
plant age. Fortunately,
we find that the leaf
number when a plant
changes from a juvenile
to an adult plant can be
the same over time but
will change with the
Continued on page 2
PAGE 2 - AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
50+ YEARS OF SERVICE
Continued from page 1
species and environment. Recent
articles on perennials by researchers
at Michigan State University have
shown that seed-propagated perennials
must have a certain number of leaves
on a plant before they are able to
respond to a cooling treatment
(vernalization) which is necessary
for flowering with many perennials.
The same is the case for many seedpropagated annuals; they must have
a certain number of leaves before
they can respond to an external
treatment.
Breeding of seed-propagated
flowering crops has emphasized
earliness of flowering over the years
to reduce the time to flower.
Reduced time to flower means
reduced production costs to the
grower. Because of this, there has
been progressive selection for
reduced juvenile period length by
breeding companies. In other words,
many of the traditional bedding plants
have few leaves they must unfold
when they change from juvenile to
adult plants and can flower.
For this reason, many traditional
types of bedding plants need to unfold
few leaves before they can sense a
stimulus. Often new types of bedding
plants such as violas, and Wave type
petunias have not been heavily bred
and can have a longer juvenile period.
For instance, the new petunia type
‘Purple Wave’ is much closer to the
wild germplasm and has a longer
juvenile period. Note in Figure 1 that
the plants did not perceive the 1 week
of long-days (stimulates flowering
in petunia) until after 2 weeks, i.e.
Purple Wave petunia has a juvenile
period that lasts about 2 weeks
compared to the very short juvenile
period for ‘White Storm.’
Light Intensity (Irradiance)
Affects Earliness of
Flowering:
Many environmental factors affect
how long the juvenile period is in
seed-propagated plants. Perhaps the
Figure 1a and b. The effect of providing a single week of continuous lighting
(long-days induce flowering in petunia) to Petunia x hybrida ‘White Storm’ (a) and
Purple Wave (b). Note that White Storm was sensitive to getting the single week of
continuous lighting during the first week (week 0-1) and Purple Wave was not
sensitive to the week of continuous lighting until weeks 2-3. This showed that the
juvenile period for White Storm is less than that for Purple Wave.
most important environmental factor
that affects juvenile period length is
light. When we talk about light, we
can talk about light color (light
quality), duration (photoperiod or
daylength), light intensity
(irradiance), or total amount per day
(light integral). Here, I am going to
be talking about primarily light
intensity and amount of light per day
(irradiance and light integral).
Together, these two light factors
quantify how much light is available
for photosynthesis instantaneously
(irradiance) or over a 24 hour period
(light integral).
We have known for some time
that the amount of light a plant gets
per day (light integral) affects how
soon some seed-propagated annuals
flower. For instance, there is a ‘ruleof-thumb’ that every day that you
provide supplemental lighting to a
seed geranium crop early after
germination reduces time to flower
by one day. There has been
incredibly little information on how
supplemental lighting affects
flowering of other bedding plant
species even though plug growers
Continued on page 3
50+ YEARS OF SERVICE
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
Continued from page 2
commonly use supplemental lighting to
improve plug quality. Often this
supplemental lighting does increase
plant mass – which is a determinant of
plant quality – but we do not know the
ultimate effect of these lights on
flowering. I think that some growers
just assume that adding lights results in
earlier flower. This is not the case for
some crops!
Because there is no term to describe
how irradiance or light integral affects
flowering, we made two new terms to
describe these two responses:
facultative irradiance and irradiance
indifferent response groups.
1) When a plant has a facultative
irradiance response, extra lighting
reduces the leaf number below the
first flower, i.e. flowering occurs
earlier developmentally.
2) When a plant has an irradiance
indifferent response, extra
lighting has no effect on the leaf
number below the first flower, i.e.
extra lighting does not hasten
flowering developmentally.
There is some confusion with how
lighting affects earliness in flowering
with many crops. Remember that
when you add lights, you heat the
plants because of the infra-red light
emitted by most lamps. With many
plants the earlier flowering after adding
lights is simply because plants are
warmer and grow faster, and therefore,
flower earlier. This can be the case
with earlier flowering after adding
lights over irradiance indifferent plants
where leaf number below the flower is
unaffected. For instance, in Figure 2 a
and b you can see that although leaf
number was not greatly affected by
adding supplemental lighting, time to
flower decreased on the irradiance
indifferent plants Black-Eyed-Susan
Vine (Thunbergia) and Mexican
Sunflower (Tithonia) as light intensity
increased from 0 to +150 umol m-2 s-1
(+750 footcandles) supplemental high
pressure sodium lighting. In contrast,
with facultative irradiance plants,
plants flower much quicker because
they have fewer leaves below the flower
and they are growing faster (leaves
unfolding faster) because of heating
from the lamps. For instance, in
Figure 2 a and b you can see that
time to flower was decreased from
78 to 57 days and 92 to 75 days on
the facultative irradiance response
plants Sweet Pea (Lathyrus) and
Tweedia as light intensity increased
from 0 to +150 umol m-2 s-1 (+750
footcandles) supplemental high
pressure sodium lighting.
AUGUST 2003 - PAGE 3
In most cases, heating with lamps
is pretty expensive and not a very
cost efficient way to speed up plant
development! Therefore, unless you
need more plant mass on a seedling
it may not be economical to add
supplemental lighting to irradiance
indifferent bedding plant seedlings.
Table I shows what irradiance
response group many bedding plants
Continued on page 4
Table 1. Lighting classification of different seed-propagated bedding plant species
with respect to flowering.
Facultative Irradiance Response
Irradiance Indifferent Response
Catananche caerula ‘Blue’
(Catananche)
Ageratum houstonianum ‘Blue Danube’
(Ageratum)
Lathyrus odoratus ‘Royal White’
(Sweet Pea)
Amaranthus hybridus ‘Pygmy Torch’
(Amaranthus)
Linum perenne
(Flax)
Ammi majus
(Ammi)
Tweedia caerula ‘Blue Star’
(Tweedia)
Asperula arvensis ‘Blue Mist’
(Asperula)
Convolvulus tricolor ‘Blue Enchantment’
(Dwarf Morning Glory)
Centaurea cyanus ‘Blue Boy’
(Bachelor’s Buttons)
Cosmos bipinnatus ‘White Sensation’
(Cosmos)
Cobea scandens ‘White’
(Cup and Saucer Vine)
Gazania rigens ‘Daybreak Red Stripe’
(Gazania)
Collinsia heterophylla
(Collinsia)
Limnanthes douglasii
(Fried Eggs, Meadow Foam)
Dolichos lablab
(Hyacinth Bean Vine)
Nemophila maculata ‘Pennie Black’
(Five-spot)
Eschscholzia californica ‘Sundew’
(California poppy)
Nicotiana alata ‘Domino White’
(Flowering Tobacco)
Ipomopsis rubra ‘Hummingbird Mix’
(Standing Cypress)
Origanum vulgare
(Oregano)
Limonium sinuata ‘Heavenly Blue’
(Statice)
Silene armeria
(Sweet William Catchfly)
Mathiola longipetala ‘Starlight Scentsation’
(Stock)
Mina lobata
(Mina Vine)
Oenothera pallida ‘Wedding Bells’
(Sundrops)
Phacelia campanularia
(California Bluebell)
Sanvitalia procumbens
(Creeping Zinnia)
Thunbergia alata
(Black-Eyed-Susan Vine)
Tithonia rotundifolia ‘Sundance’
(Mexican Sunflower)
PAGE 4 -AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
50+ YEARS OF SERVICE
Continued from page 3
100
92
90
82
80
78
72
Days To Flower
fit into. The next article will have a
similar table with many more bedding
plants that we have studied. With
irradiance indifferent plants, extra
lighting did not affect leaf number
below the flower. In contrast, with
facultative irradiance group plants,
extra lighting reduced the leaf number
below the first flower on these plants
which reduced the time to flower
dramatically.
Figure 2a and b. The effect
of increasing irradiance
(light intensity) from 0 to +150
umol m-2 s-1 (+750 footcandles)
supplemental high pressure sodium
lighting on time to flower (a) and
leaf number below the first flower
(b) of the facultative irradiance
response plants Sweet Pea
(Lathyrus) and Tweedia and the
irradiance indifferent plants (BlackEyed-Susan Vine (Thunbergia) and
Mexican Sunflower (Tithonia).
Ambient light integral (day light)
70
75
69
68
64
60
60
57
62
57
50
46
41
43
40
39
30
20
10
0
Lathyrus
Tweedia
Thunbergia
Tithonia
Crop
was 12-13 moles per day with an
average irradiance (light intensity)
of 500-606 umol m-2 s-1 (2,500-3030
footcandles) during the course of
this experiment.
Acknowledgements: The authors would
like to express their appreciation to the
Richard E. Widmer Research Fund,
F.I.R.S.T., and the Gloeckner Foundation
for their generous support of this research.
Maximize Your Earning and
Learning Potential...
2004
Enjoy three days of learning and networking with other green industry professionals by
attending the 2004 Minnesota Green Expo at the Minneapolis Convention Center on January 7-9.
Benefit from a full schedule of educational programs, and over 800 booths in the trade show.
Speakers include:
CINDY ASH – American Phytopathological
Society (APS), MN
Seasonal Plant Problems
MACK COOK – Cook Water Farms, MN
Aquatic Plants
JOHN ERWIN – University of Minnesota
Growth Regulators on Perennials,
Sanitation and Weed Control in the
Greenhouse, Preparation of Landscape
Planting Beds
JOHN FRIEL – Yoder Green Leaf Perennials,
Pennsylvania
Marketing and Merchandising
Perennials, New Perennials
ED GILMAN – University of Florida
Environmental Horticulture
How Tree Biology Should Drive Urban
Forestry and Arboricultural Practices,
Influence of Nursery Production Systems
on Transplanting and Establishment,
Designing Sites to Fit Desirable Trees,
Reduce Pest Problems with Plant
Selection, Planting, and Management
Practices
CHRIS HIGGINS – Olympic Horticultural
Products, Texas
Chemical Controls, and Management
Strategies for Insects and Diseases
DWIGHT HUGHES – Hughes Nursery, Iowa
Systems for Success for Landscape
Installers and Nursery Growers
LAURA JULL – University of Wisconsin –
Madison
Heat Tolerance of Woody Plants, New and
Unusual Conifers
BILL MCCURRY – McCurry Associates, New
Jersey
Garden Center Marketing and
Communication
GREG MEWS – Farmer Seed and Nursery, MN
Tie-In Marketing
KERSTIN OUELLET – Pen and Petal Inc.,
California
Container Garden Ideas, Container Garden
Production
Watch your MNLA News for more details about educational programs and the trade show. New information will be available soon at www.minnesotagreenexpo.com
50+ YEARS OF SERVICE
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
AUGUST 2003 - PAGE 5
Fundamentals of Flowering in Plants:
Supplemental Lighting and Earliness of Flowering
John Erwin, Neil Mattson and Ryan Warner
Department of Horticultural Science, University of Minnesota
Part II
Introduction:
In Part I, we discussed how light
intensity (irradiance) and
supplemental lighting can affect
flowering of seed-propagated bedding
plants. This article is the second
article to focus on the same topic and
provides additional information on
how the need for supplemental
lighting varies in different parts of the
country. In addition, how species
differ in how much light they can use
is presented. Lastly, how high day
temperatures can affect
photosynthetic rate and plant quality
is discussed. Future articles will
show new results on how daylength
affects flowering of many bedding
plants.
Impact of Light
on Flowering:
Increased lighting, or irradiance,
can reduce the length of the juvenile
period with some species, thereby
reducing the time to flower. We do
not really understand how this occurs.
Is juvenile period length reduced by
supplemental lighting because there is
more food, or photosynthates,
available for flower induction?
Photosynthesis is the process where a
plant utilizes sunlight and carbon
dioxide in the air to make sugars, or
food. Does supplemental lighting
alter the hormonal balance in plants?
Answers to these questions could help
us to decrease, or increase, juvenile
period length through breeding efforts
and environmental treatments.
Juvenile period length varies in
bedding plants. The juvenile period
of bedding plants can be nearly nonexistent or be as long as months with
some of the perennial species we
grow in the industry. Additional
lighting is most effective in reducing
juvenile period length on those
species that have a longer juvenile
period such as perennials, seed
geraniums, and ‘Purple Wave’
petunias.
The juvenile period of most
common bedding plants (herbaceous
annuals in temperate climates) is
between 1 and 2 weeks. We
measure the ‘maturity’ of a plant by
counting the number of leaves below
a flower. In many cases, bedding
plants are mature when we can see 3
unfolded leaves on a seedling (1-2
weeks after germination). We
mention this because the length of
the juvenile period of a crop gives
us some indication of the maximum
amount we could shorten production
time! For instance, if the juvenile
period on a ‘White Storm’ petunia is
nearly nonexistent, then providing
supplemental lighting will reduce
crop time minimally. In contrast, if
the juvenile period of ‘Purple Wave’
petunias is 2 weeks, then it may be
possible to reduce crop time by a
maximum of 2 weeks by shortening
the juvenile period using lighting.
Classification of Lighting
Responses with Flowering:
We identified two new terms to
describe how irradiance and/or daily
light integral affect flowering:
facultative irradiance and irradiance
indifferent response groups.
1) When a plant has a facultative
irradiance response, extra
lighting reduces the leaf number
below the first flower, i.e.
flowering occurs earlier
developmentally.
2) When a plant has an irradiance
indifferent response, extra
lighting has no effect on the leaf
number below the first flower, i.e.
extra lighting does not hasten
flowering developmentally.
Many of the bedding plants we
grow did not flower earlier
developmentally when supplemental
lighting was added to ambient light
(St. Paul, Minnesota (fall and spring))
in the range of light levels we studied
(6.5 – 13.5 moles per day). Any
affect of lighting on these crops was
restricted to possible increases in
plant size or mass through increased
photosynthesis. Therefore, at total
light levels at or above 6.4 moles per
day crops such as dill, cosmos,
gomphrena, lobelia and zinnia will
not flower earlier if supplemental
lighting is provided (irradiance
indifferent) (Table 1). In contrast,
crops such as snapdragon, cleome,
hibiscus (perennial), petunia, lavatera
and blue salvia (Figure 1) will flower
earlier as light intensity increases
above 6.4 to 29.2 moles per day
(facultative irradiance response)
(Table 1).
Facultative irradiance plants occur
in most photoperiodic response
groups studied. We will discuss
which plants fit into which
photoperiodic groups in a future
article. However, as an example for
now, the facultative long-day plant,
and day neutral plants Blue Salvia
and Cleome, respectively, are
facultative irradiance plants and will
flower more quickly as total daily
irradiance increase to from 6.4 to
Continued on page 6
PAGE 6 -AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
Table 1. Lighting classification of different seed-propagated bedding plant species
with respect to flowering. Ambient daylight varied from 6.4 – 13.9 moles of light per
day. Total light varied from 6.4 – 29.2 moles of light per day.
Facultative Irradiance Response
Irradiance Indifferent Response
Antirrhinum majus
(Snapdragon)
Anethum graveolens ‘Mammoth’
(Dill)
Centranthus macrosiphon
(Long-Spurred Valerian)
Calendula officinalis ‘Calypso Orange’
(Calendula)
Cleome hasslerana ‘Rose Queen’
(Cleome)
Carpanthea pomeridiana
‘Golden Carpet’
Hibiscus moscheutos
(Rose Mallow; Swamp Mallow)
Celosia plumose ‘Flamingo Feather Purple’
(Plumed Celosia)
Lavatera trimestris ‘Silver Cup’
(Lavatera)
Cosmos bipinnatus ‘Diablo’
(Cosmos)
Linaria maroccana
(Toadflax)
Dianthus chinensis ‘Ideal Cherry Picotee’
(Dianthus)
Salvia farinacea ‘Strata’
(Blue Salvia)
Dimorphotheca sinuata ‘Mixed Colors’
(African Daisy)
Gomphrena globosa ‘Bicolor Rose’
(Globe Amarath)
50+ YEARS OF SERVICE
Continued from page 5
29.2 moles per day within their
prescribed photoperiod.
Figure 1. Effect of delivering long day
conditions to Petunia x hybrida ‘Purple
Wave’ flowering as a traditional night
interruption (left) (2 umol m-2 s-1 (10
footcandles); 10 pm to 2 am)) or as a day
extension with high pressure sodium
supplemental lighting (right). Light integrals
for each treatment were 11.4-12.4 (left) and
24.4-25.4 (right) moles per day.
Helianthus annus ‘Vanilla Ice’
(Sunflower)
Helipterum roseum
(Strawflower)
Ipomea x multifida ‘Scarlet’
(Cardinal Climber)
Legulosa speculum-veneris
(Venus’s Looking Glass)
Leptosiphon hybrida
Lobelia erinus ‘Crystal Palace’
(Lobelia)
Mimulus x hybridus ‘Magic’
(Monkeyflower)
Mirabilis jalapa
(Four O’Clock)
Nemophila menziesii
(Baby Blue Eyes)
Nigella damascene ‘Miss Jekyll’
(Love-In-The-Mist)
Polemonium viscosum
Verbascum phoeniceum
(Mullein)
Viguiera multiflora
Zinnia elegans ‘Exquisite Pink’ and
‘Peter Pan Scarlet’
(Zinnia)
Figure 2a and b. The effect of photoperiod
(long day (LD) and short day (SD)) and
increasing light intensity (irradiance) on the
facultative long-day plants Viola x
wittrockiana ‘Delta Pure White’ (top) and
Salvia farinacea ‘Strata’ (bottom) flowering
Continued on page 7
50+ YEARS OF SERVICE
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
AUGUST 2003 - PAGE 7
Continued from page 6
Irradiance levels were ambient
daylight, or ambient plus 50, 100 or
150 umol m-2 s-1. Daily light integrals
varied from 6.4 – 20.2 moles per day
in these treatments.
Supplemental Lighting
Effects on Plant Growth:
Although some plants may not
flower earlier when supplemental
lighting is provided, there are distinct
benefits to crop quality such as
increased plant mass and/or
decreased plant height that are
beneficial to a grower. High plant
mass and short plants are usually
associated with an increased plant
quality. Internode elongation
generally decreases as light intensity
increases up to approximately 400
umol m-2 s-1 or 2000 footcandles
(unpublished data). In general, plant
mass increases as light intensity,
irradiance, increases. However, there
is a maximum light intensity that
plants can utilize. Further increasing
light intensity (irradiance) above this
maximum will not increase
photosynthesis and plant mass
further. For instance, Figure 3 shows
plant photosynthetic rate versus light
intensity of pansy, New Guinea
impatiens and raspberry. Note that
for each species there is a maximum
photosynthetic rate and that
increasing irradiance does not further
increase photosynthesis and offers no
benefit! For instance, a maximum
photosynthetic rate is achieved on
raspberry, New Guinea impatiens and
pansy with irradiance levels of 500,
570 and 630 umol m-2 s-1,
respectively.
Also note that the benefit of
increasing irradiance with each crop
differs from a standpoint of net
photosynthesis. For instance,
increasing light intensity from 200 to
300 umol m-2 s-1 results in a net 20%,
20% and 33% increase in
photosynthesis for raspberry, New
Fig. 1. The maximum rate of photosynthesis is often around 600 µmol m-2 s-1 (3000
footcandles) or less. The species were all grown at 68 ºF.
Guinea impatiens, and pansy,
respectively (Figure 3). In other
words, plant mass increased most
when irradiance increase from 200 to
300 umol m-2 s-1 on pansy compared
to raspberry and New Guinea
impatiens.
Supplemental lighting controls
should be based on irradiance at
plant level. On many sunny days
lights can be shut off because
irradiance levels exceed what the
plant can use. Lighting above the
level where photosynthesis is
maximized wastes money and adds
heat to the greenhouse! The only
way to increase photosynthesis above
the maximum rate reported is to
increase carbon dioxide
concentrations (CO2) above ambient
air levels (approximately 333 ppm).
CO2 levels less than ambient air
levels are possible in a greenhouse as
well and can limit crop
photosynthesis. For instance, CO2
levels within canopies can often drop
below ambient levels and can limit
photosynthesis. Further, when air is
not circulated in a greenhouse
photosynthesis levels can also be
decreased through pockets of lower
CO2 concentrations.
How Does Heat Stress
Affect Photosynthesis?
Stresses will reduce the maximum
rate of photosynthesis (Figure 4).
Water or high temperature stress can
reduce photosynthesis. Water stress
will result in the pores on the leaf
(stomata) closing which limits
photosynthesis by reducing CO2
intake. This is the reason why
regularly water stressed plants are
smaller in size and/or mass than
plants grown under non-water
stressed conditions. Similarly, plants
exposed to a short term high
temperature stress also have a
reduced capacity for photosynthesis
(Figure 4).
Results of Supplemental
Lights Will Vary Throughout
North America:
The benefit of adding
supplemental lighting for earliness of
Continued on page 8
PAGE 8 -AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
50+ YEARS OF SERVICE
Conclusions:
Supplemental lighting will
decrease flowering time on
facultative irradiance plants but not
irradiance indifferent plants. If a
grower’s focus is decreased internode
length, internode length will decrease
as light intensity or irradiance
increases up to approximately 400
umol m-2 s-1 (2000-2250 footcandles).
If increasing plant mass or
photosynthesis is your objective to
increase plant quality, increasing
irradiance over 650 umol m-2 s-1 will
likely have no effect on
photosynthesis on many crops.
Periodic stresses (water and heat) will
decrease the ability of a plant to do
photosynthesis and therefore waste
any investment you may have in
providing supplemental lighting.
Fig. 2. A two-hour exposure to 95 ºF dramatically reduced photosynthetic rate of
New Guinea Impatiens ‘Celebration Orange’. Data presented above are
photosynthetic rates measured the following day
Continued from page 7
flowering and increased
photosynthesis varies with
geographic location. Our data are
based on adding supplemental
lighting to ambient daylight
conditions in Minnesota. Ambient
daily light integral ranged from 6.4 to
13.9 moles day-1. Supplemental
lighting resulted in a variation in
actual daily light integrals ranging
from 6.4 to 29.2 moles per day.
Supplemental lighting will have a
greater effect on flowering time on
crops with a facultative irradiance
response in lower light areas of the
country than Minneapolis/St.Paul
such as western Michigan, western
New York, and/or the Seattle area. In
contrast, adding supplemental
lighting in higher light areas of the
country than Minneapolis/St. Paul
such as Atlanta and Miami may have
little or no effect on hastening
flowering developmentally and/or
photosynthetic rate because the plants
may already be receiving sufficient
light.
Table 2. Variation in total daily light available for plant growth outside of
the greenhouse.
MONTH
City
January April
July
October
5-10
3-6
25-30
15-18
40-45
24-27
15-20
9-12
Minneapolis Outside
Inside
10-15
3-9
30-35
18-21
40-45
24-27
15-20
9-12
Atlanta
Outside
Inside
15-20
9-12
35-40
21-24
40-45
24-27
25-30
15-18
Miami
Outside
Inside
20-25
12-15
40-45
24-27
40-45
24-27
35-40
21-24
Seattle
Outside
Inside
Numbers based on a FIRST research report: Light management in greenhouses.
I. Daily light integral: A useful tool for the U.S. Floriculture industry. By
James Faust, Clemson University. Inside values were calculated by taking 60%
of the outside levels which is standard for a double polyhouse.
50+ YEARS OF SERVICE
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
AUGUST 2003 - PAGE 9
Common Problems In Northern Poinsettia Production
John Erwin, Department of Horticultural Sciences, University of Minnesota
This is just a quick reference for
some of the most common
poinsettia production problems in
the cooler North American climates
such as Minnesota. These problems
probably constitute about 75% of all
the problems that growers
experience with poinsettias.
Calcium Deficiency:
Many
new poinsettia cultivars have
difficulty taking up calcium. High
temperature and/or high humidity
can decrease calcium uptake by
limiting transpiration (water loss
from the leaves). In addition, high
levels of ammonium, magnesium
and/or potassium can also reduce
calcium uptake by competing with
calcium for uptake into the root.
Preventative treatments and
solutions for calcium deficiency are
shown below:
Reduce the Possibility of Calcium
deficiency by:
1) Using fertilizers that contain
calcium.
2) Spraying calcium nitrate (or
chloride) on foliage till wet
(0.8-1.9 pounds calcium nitrate
per 100 gallons of water (1.32.6 ounces per 10 gallons))
from the 3rd week of September
to the 3-4th week of October.
Spray on a weekly basis.
Root Rot:
Root rot is a common
problem in poinsettia production
when fungicides are not regularly
used. Root rot is an infestation of
either or both Pythium and
Rhizoctonia fungi. These pathogens
are water fungi; i.e. wet conditions
promote their proliferation. Heavy
soils, soluble salts burn and/or
fungus gnat infestation increase the
incidence of root rot by providing
an environment conducive to root
rot proliferation or by providing a
point of entry for root rot fungi.
Pythium attacks from the roots tip
and moves up the root system into
the stem. In contrast, Rhizoctonia
attacks at where the stem touches
the media and moves both up and
down the stem.
Reduce the Possibility of
Root Rot by:
1) Not keeping media wet.
2) Applying fungicides for control
of both Pythium and
Rhizoctonia every 3-4 weeks.
3) Rotating fungicides to limit
fungicide resistence.
Pythium Control
Subdue
(1/2 ounce/
100 gallons)
Banrot
(8 oz/100 gallons)
Truban
(8oz/100 gallons)
Rhizoctonia Control
Cleary’s 3336
(8 oz/
100 gallons)
Banrot
(8 oz/100 gallons)
Terraclor
(8 oz/100 gallons)
Note: always check the label for
recommended rates as these rates
are based on WP formulations only.
Bract Expansion:
Bracts can
often be smaller than desired.
Leaves that will become bracts
expand during the last 2 weeks of
October and early November.
Leaf/bract expansion increases as
temperature increases to about 7476oF. Therefore, cool days or nights
will reduce bract size.
Get bigger bracts by:
1) Some of the best quality bracts
are produced when poinsettias
are grown at constant 68oF
during the last 2 weeks of
October and the first week of
November. Growing warmer
will increase bract size still
further.
Bract Coloring: Bract coloring
occurs during the latter half of
October and beginning of
November. Red pigmentation in
bracts increases as light intensity
increases and as the average
temperature decreases. Therefore,
you will have the brightest bracts
when temperatures are dropped
after bract expansion and light
levels are high.
Brighten Bracts by:
1) Decreasing temperature the last
2 weeks of production
(beginning of November) to 5862oF.
2) Growing plants with as much
light as possible during the end
of October and early November.
Early Flower Initiation: Lateral
shoots that have few leaves <3
leaves, will have reduced leaf and
bract size when marketed, i.e.
reduced quality. Lateral shoots
must have 3-4 leaves (>1” in size)
or more when lateral shoots initiate
flowers. Most current poinsettia
cultivars initiate flowers from
September 8th-15th.
Make Sure Plants Do Not Initiate
Flower Too Early By:
1) Pinch plants no later than
three weeks prior to flower
initiation (September 8).
2) Maintain plant temperatures
at 68-74oF to encourage
leaf unfolding. This may
Continued on page 10
PAGE 10 -AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
Continued from page 9
Poinsettias are
a ‘high feed’
requiring crop.
Poinsettias are
commonly
underfed early
in development
and are overfed
late in
development.
mean that air temperatures
have to be increased to 72oF
on cloudy days or decreased
to 70oF on sunny days to
maintain the plant temperature
in the desired range.
3) If lateral shoots do not have
a sufficient leaf number on
September 1, then provide
night interruption lighting
until lateral shoots do have
the sufficient leaf number
before you allow plants to
receive short-days.
50+ YEARS OF SERVICE
Limit Late Shoot Breakage:
1) This phenomenon is due to the
environment that stock plants
are grown under. In particular,
the higher the temperature and
the lower the light that stock
plants are grown under, the
more lateral shoot breakage
will occur on cuttings produced
from the stock plants.
Therefore, purchase cuttings
from a source where stock
plants are grown in a cooler
and well-lighted environment.
2) Ring plants to support lateral
shoots if you suspect you may
have late stem breakage.
Inadequate Nutrition:
Poinsettias are a ‘high feed’
requiring crop. Poinsettias are
commonly underfed early in
development and are overfed
late in development. This results
in insufficient leaf and bract size,
poor plant color and reduced
postharvest life.
Make Sure Plants Have Adequate
Nutrition By:
1) Feeding poinsettias with
400-600 ppm nitrogen
and potassium the first
fertilization (second watering).
Reduce levels to 300-400 ppm
until recommended media
nutrient levels are achieved
thereafter. Use primarily
ammonium based fertilizers
during August (20-10-20),
ammonium plus nitrate based
nitrogen (15-5-15 Cal Mag)
during September, and primarily
nitrate based fertilizer (15-0-15)
during October.
Lateral Shoot Breakage: Lateral
shoots can break off of the mother
shoot on the bench or when plants
are moved later in the season after
lateral shoot and bract weight
increases.
Insufficient Lateral shoot
Number: Insufficient lateral
shoot number typically results from
insufficient leaf number below the
pinch, and/or excessively high day
temperatures during cutting
production resulting in few axillary
buds in the leaf axils.
Increase Lateral Shoot Number By:
1) Making sure that there are 20%
more leaves left after the pinch
than the final lateral shoot
number that is desired.
2) Purchase cuttings from a
propagator that has good day
temperature control. Inspect
cuttings when they arrive to
insure that they have axillary
buds in the leaf axils.
3) Try applying a suggested new
Florel application procedures
which have increased lateral
shoot number with some
growers. Experiment by
applying Florel (300-500 ppm)
3 days prior to pinching, pinch,
then reapply Florel (300-500
ppm) 5-7 days later.
Call If You Have Any Questions:
1) John Erwin, 612-624-9703
Erwin001@umn.edu
2) Ryan Warner, 612-624-0736
50+ YEARS OF SERVICE
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
Co-sponsored by
UNIVERSITY OF MINNESOTA
MNLA
2003
BPlant
edding
C O N F E R E N C E S
Tuesday, September 23
Holiday Inn Detroit Lakes
Wednesday, September 24
Lakeview Castle Duluth
Thursday, September 25
Rochester Community and
Technical College Rochester
Wednesday, October 1
Midland Hills Roseville
Looking for a refresher
course on bedding plant
production and greenhouse
management? Enjoy a full
day of education by the
University of Minnesota’s
professors and researchers
in four different locations.
8:30 a.m. – 9:00 a.m.
REGISTRATION
9:00 a.m. – 9:45 a.m.
GREENHOUSE COVERINGS
AND COOLING SYSTEMS
Neil Anderson, Assistant Professor,
University of Minnesota
Department of Horticultural
Science.
Hear about the latest information,
as well as the "basics" to maximize
plant growth and profitability in
greenhouses. Learn about new
greenhouse products that you can
use for season extenders or yearround production of flowering
plants. Light transmission, cost,
and longevity of greenhouse
coverings will be discussed. The
need for cooling systems in
northern growing conditions varies,
depending on your crops, location,
and production cycles. View some
of the options, whether you grow
plants during the summer and/or
winter seasons.
9:45 a.m. – 10:15 a.m.
BEST PERFORMING
ANNUALS/ PERENNIALS
FOR 2003
David Zlesak, Graduate Research
Assistant, University of Minnesota
Department of Horticultural
Science.
What's new? What's hot? How did
all of those new seed and
vegetative products perform in the
garden this past season? You will
want to hear about the best
performing annuals from the 2003
season. Learn what your
customers will be asking for next
year to include in your fall orders.
10:15 a.m. – 10:30 a.m. BREAK
AUGUST 2003 - PAGE 11
10:30 a.m. – 11:15 a.m.
GROUND BED
PREPARATION
John Erwin, Associate Professor,
University of Minnesota
Department of Horticulture Science.
Learn about the issues involved with
testing, planting and fertilizing
annuals and perennials in a ground
bed. Many of our crops are sold to
landscapers for use in ground beds.
Erwin will provide answers for
preparing the bed, testing the soil,
soil amendments, and the standards
for ground bed soil for annuals.
11:15 a.m. – 11:45 a.m.
FERTILIZER SELECTION
Charlie Rohwer, Graduate Research
Assistant, University of Minnesota
Department of Horticulture Science.
Rohwer will help you select the
appropriate fertilizer for your
specifications by discussing the
fertilizer types, advantages, and
disadvantages. You may be
surprised with the fertilizers that are
good for your needs, and those that
are not!
11:45 a.m. – 12:30 p.m. LUNCH
12:30 p.m. – 1:15 p.m.
SELECTING
GREENHOUSE MEDIA
John Erwin.
Different greenhouse media are best
for different crops. Which ones are
best for containers, geraniums, and
flatted bedding? Which ones are
best for hanging baskets? These are
important questions that many
growers ask. Erwin will help you
identify media that are best for you.
Feel free to bring along a
greenhouse media analysis and
receive answers from Erwin.
Continued on page 12
PAGE 10 -AUGUST 2003
UM / MNLA MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
Continued from page 11
1:15 p.m. – 2:00 p.m.
PESTS AND DISEASES
AFFECTING YOUR
OPERATION –
John Erwin and Neil Anderson.
Erwin and Anderson will review
each pest, what pesticides work the
best, and how to develop a
pesticide rotation schedule. This
will be a practical and hands-on
presentation. Participants will
break into teams and each team
will develop a control strategy.
2:00 p.m. – 2:15 p.m.
BREAK
2:15 p.m. – 3:00 p.m.
DO’S, DON’TS, AND
ANSWERS FOR SHIPPED
PLUGS AND NEW VARIETIES
Tami van Gaal, Wagner’s
Greenhouses.
There are many new seed
propagated products out there, but
how do you use these new products?
Can any of them be combined with
vegetative products in mixed
containers and baskets? What
container sizes work best for each
product? In addition to answering
these questions, Van Gaal will talk
about treating plugs that just arrived
and how to hold them if needed.
50+ YEARS OF SERVICE
For a registration
form, call
651-633-4987
or see
www.mnla.biz.
MINNESOTA COMMERCIAL FLOWER GROWERS BULLETIN
MNLA
CREATING &
CARING FOR
YOUR ENVIRONMENT
Minnesota Nursery &
Landscape Association
Published by the University of Minnesota Extension
Service and Horticulture Department
in cooperation with the
Minnesota Nursery & Landscape Association
WE’RE ON THE WEB
Minnesota Nursery &
Landscape Association
www.mnla.biz
U of M. Hort. Dept.
www.florifacts.umn.edu
The Minnesota Commercial Flower Growers Bulletin
is compiled and edited by John Erwin, Associate
Professor, Greenhouse Crop Physiology and Extension,
Department of Horticultural Science, University of
Minnesota. Feel free to call with suggestions and/or
comments (numbers below).
Issued in furtherance of cooperative extension work
in agriculture and home economics, acts of May 8 and
June 30, 1914, in cooperation with the U. S.
Department of Agriculture. The University of
Minnesota, including the Minnesota Extension Service,
is committed to the policy that all persons shall have
equal access to its’ programs, facilities and employment
without regard to race, religion, color, sex, national
origin, handicap, age, veteran status or sexual
orientation.
The Bulletin is published under the auspices of the
Commercial Flower Growers Committee of the
Minnesota Nursery and Landscape Association.
©2003 MNLA. 651-633-4987
Send comments to:
Department of Horticultural Science
1970 Folwell Ave. • St. Paul MN 55108
Phone: 612-624-9703 • Fax: 612-624-4941
Email: erwin001@tc.umn.edu