August 2019 - 27

RESEARCH
short-day conditions naturally occur
in the U.S. during September.
However, flower initiation in
many short-day plants, including
poinsettia, chrysanthemum,
kalanchoe and schlumbergera, is
delayed by warm temperatures,
especially during the night period.
Research at University of Florida
by Rebecca Schnelle and Jim
Barrett found that flower initiation
in poinsettias was delayed when
average daily temperature (ADT)
was 80° F for more than a week
during the short day period,
compared with flowering when ADT
was 70 to 75° F. Rob Berghage
and Royal Heins at Michigan
State University found that night
temperatures (NT) above 73° F also
delayed flower initiation in poinsettia.
Breeders are actively selecting
genotypes that have reduced
sensitivity to heat delay. Technical
specialists base their evaluation
of cultivars on observations of
flowering time in multiple trial
locations around the U.S.
Genotypes that flower early in the
fall are also selected by breeders.
Early flowering results from a
combination of a plant initiating
flowers under a short critical night
length (early in the fall), and also
a short response time from flower
initiation to open blooms (the flower
development period when bract
expansion occurs). However, " earlyseason "
varieties are not necessarily
less sensitive to warm temperature.
For example, 'Early Prestige Red'
is an early flowering variety that is
sensitive to heat delay.
CLIMATE VARIABILITY AND
TRENDS
We analyzed historical fall weather
conditions in Florida to illustrate
how climate is changing, and to
show how temperature data can be
interpreted in terms of heat delay
risk. Climate data were downloaded
from the Florida Automated
Weather Network FAWN (https://
fawn.ifas.ufl.edu) for five outdoor
weather stations around the Florida
peninsula. The main University of
Florida campus is in Alachua County.
Air temperatures collected at
2 meters outdoors were analyzed
from 2004 to 2018.
We used two indicators of heat
delay risk of poinsettia for climate
between Sept. 1 to Oct. 1 each
year to evaluate risk of heat delay.
These were:
(A) Number of days when
average daily temperature
(ADT) was 80° F or above,
and
(B) Number of days when the
minimum night temperature
(NT) was 74° F or above.
The 2018 season was at high risk
for heat delay in terms of both ADT
and NT in all locations in Florida
(Table 1). Every year, Homestead,
Florida, shows high risk conditions.
There is an increasing risk of
heat delay when several days of
Dramm ColorStorm Professional Hose
™
High Visibility
Now Make
Your
Own Lengths
Dramm Press
Bright Yellow For a Reason
Fittings &
Ferrules
Brass
Swivel
Bulk Rolls
Crush Proof Fittings
Sure-Grip™
Dramm ColorStorm Professional
Rubber Hose.
Durable * Flexible * Visible
Because quality in hand watering
starts at the tap.
Kink Resistant
Hot Water to 160°
®
Integrated Plant Health
information@dramm.com
www.dramm.com
920.684.0227
GREENHOUSE PRODUCT NEWS 27
http://https:// http://fawn.ifas.ufl.edu http://www.dramm.com
August 2019

Table of Contents for the Digital Edition of August 2019
