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RESPONSE OF PLANTS TO PHOTOPERIOD
PLANT TYPES BASED ON RESPONSE TO PHOTOPERIOD
short-day plant (SDP) - plants that exhibit their photoperiodic response when the 
                                             photoperiod is shorter than a critical photoperiod.

long-day plant (LDP) - plants that exhibit their photoperiodic response when the 
                                           photoperiod is longer than a critical photoperiod.

day-neutral plant (DNP) - plants that are not affected by photoperiod. 

 
Examples of Plants Based on Response to Photoperiod
Response
Short-Day Plants
Long-Day Plants
Day-Neutral Plants
Flowering
chrysanthemum
poinsettia
kalanchoe
strawberry
soybean
sedum
tuberous begonia
carnation
radish
spinach
bean
tomato
squash
rose
corn
Bulbs and 
Tubers
Jerusalem artichoke
tuberous begonia
onion
tulip
crocus
Plantlets
none
bryophyllum
spider plant
piggy-back plant
Runners
none
strawberry
grasses
Color 
Development
maple
sumac
none
coleus
foliage plants
Dormancy
dogwood
birch
none
many temperate 
trees
Cold
Acclimatization
dogwood
birch
none
many temperate
trees
Elongation of
Stems
-
most plants
-
civil twilight - reflected sky light that occurs approximately 1/2 hour before sunrise and 
                          1/2 hour after sunset.

Plants can detect civil twilight, so it must be taken into account when determining the photoperiod that plants perceive.  Plants cannot detect moonlight, so it does not effect the photoperiod plants perceive.

photoperiod - the day length a plant perceives, which will be the absolute day length (time
                           from sunrise to sunset) plus 1 hour of civil twilight. 

critical photoperiod - the photoperiod (absolute day length + civil twilight) above or below 
                                          which the photoperiodic response is turned-on or turned-off.

Each species has its own unique critical photoperiod that it "looks" for.