October 2023 - 8

PRODUCTION
CREATING ARTIFICIAL
LONG DAYS
Growers may use either dayextension
(DE) or night interruption
(NI) photoperiodic lighting to create
LDs. Each method has advantages
and disadvantages but generally, they
are similarly effective. Both require the
delivery of at least ~2 µmol·m−2
·s−1
of
light when measured at plant height.
DE lighting is the delivery of light to
extend the length of the natural day.
The duration of DE lighting will depend
upon the period of darkness required
and the natural day length. For
instance, if the natural day length is
9.5 hours and a 13-hour photoperiod
is desired, lighting should operate
from dusk for ~3.5 hours.
NI lighting is the practice of
providing low-intensity light to plants
during the middle of the night. By
interrupting the dark period, plants
will perceive a " short " night (or " long "
day). It is typically delivered for 4
hours from 10:00 p.m. to 2:00 a.m.
PHOTOPERIOD FOR
YOUNG AND FINISH PLANT
PRODUCTION
A common misconception is that
you need to grow photoperiodic
crops under inductive day lengths
from the start. Given that stem
length of cut flowers is one of the
most important quality parameters
when harvested, you do not want to
provide an inductive day length to
seedlings or rooted cuttings as they
will prematurely flower. Premature
flowering can result in shorter stem
lengths than desired, small flowers,
and greatly reduced yields.
Therefore, the opposite daylength
that is required for flowering should
be provided at the young plant stage.
Let's reference Table 1, we can
see that bellflower is classified as
either an obligate or facultative LDP.
Therefore, young plants should be
propagated and grown a few weeks
after transplant under SDs to bulk
8 OCTOBER 2023 GPNMAG.COM
Table 1. Genera and common names of common specialty cut flowers and their flowering
responses to day length. Daylength and daily light integral (DLI) recommendations are
provided for young plant production and for flower induction.
GENERA
COMMON NAME
YOUNG PLANT
& 2 TO 4
WEEKS AFTER
TRANSPLANT
OBLIGATE LONG-DAY PLANTS
Asclepias
Campanula*
Centaurea
Clarkia
Matricaria
Nigella damascena
Trachelium
<15-h
Caryopteris
Celosia
Ageratum
Alstroemeria
Anemone+
Antirrhinum
Milkweed
Bellflower
Bachelor's Buttons*
Punchbowl godetia
Mayweed
Love-in-a-Mist
Blue throatwort
OBLIGATE SHORT-DAY PLANTS
Bluebeard
Cockscombs
Flossflower
Peruvian lily
Windflower
Snapdragon
Callistephus chinensis Aster
Delphinium
Dianthus
Eustoma grandiflorum
Lilium hybrid
Limonium
Matthiola
Ranunculus
Cosmos bipinnatus
Dahlia
Gerbera
Panicum
Tagetes erecta
Zinnia*
Helianthus annus*+
Anthurium
Bracteantha bracteate
Craspedia
Gomphrena
Hyacinthus
Narcissus
Tulipa
Talinum
Zantedeschia
Larkspur
Sweet William
Lisianthus
Lily (Asiatic and Oriental)
Statice
Stock
Buttercup
FACULTATIVE SHORT-DAY PLANTS
Mexican aster
Dahlia
Transvaal daisy
Witchgrass
Marigold
Zinnia
Sunflower
Flamingo lily
Strawflower
Billy Buttons
Globe amaranth
Hyacinth
Daffodil
Tulip
Flameflower
Calla
*Also reported as a facultative long-day plant or +
them up before flower induction
under LDs. Thus, by properly
managing the photoperiod during the
off-season, you can achieve desired
plant responses, such as bulking up
the plant or accelerating flowering
and reaching target market dates
more reliably.
Acknowledgements: We thank the
Association of Specialty Cut Flower
day neutral plant.
Growers, Pan American Seed, and
the American Floral Endowment
for supporting our cut flower
research.
Roberto Lopez is an Associate
Professor and Controlled Environment/
Floriculture Extension Specialist in
the Department of Horticulture at
Michigan State University.
9 to 16
5 to 10
12 to 15
Depends on the cultivar
DAY NEUTRAL PLANTS
5 to 10
12 to 15
Long-days
>12 h
12 to 16-h
or 4-h night
interruption
lighting
Short-days
<12 h
12 to 16-h
or 4-h night
interruption
lighting
12 to 15
Any photoperiod
Long-days >12-h
FACULTATIVE LONG-DAY PLANTS
<15-h
<12-h
12 to 15
Short-days
<12 h
13 to 16-h
or 4-h night
interruption
lighting
12 to 15
FLOWER
INDUCTION
MINIMUM
DLI
(mol∙m-2
∙d-1
)
12 to 15
http://www.GPNMAG.COM
October 2023

Table of Contents for the Digital Edition of October 2023
