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12 NZ TURF MANAGEMENT JOURNAL Summer 2014
TIMING
Temperature is the driving
force for all biological
activity. Consequently, the
growth, development and
reproduction of many organisms are
predictable based on temperature.
A growing degree day (GDD) is a
measure of heat above a threshold for
that day. Growing degree accumulation
reflects the number of GDDs or heat
units above a base temperature for
consecutive 24-hour days.
An important aspect of GDD is that no units
(e.g. grams, litres or hectares) are associated
with the value. Instead, the accumulated GDD
values can be correlated with an event in an
organism’s life. For example, seeded couch
(Cynodon spp.) may require approximately
, GDD at a base temperature of °C to
fully establish (>% ground cover).
EARLY HISTORY OF
TURFGRASS GDDS
The first use of GDD in agriculture was to
predict the maturity in corn. The use of GDD
in turfgrass was first proposed in a paper to
predict Poa annua seedhead emergence and
development. Then a version of this model
was used to time applications of plant growth
regulator for seed head suppression.
Subsequently, GDD models were proposed
to predict growth stages of turfgrasses and
how plant growth regulator applications
could be timed. GDD could also be used to
determine when and for how long a turfgrass
species was under high temperature stress,.
Recently, there has been renewed interest in
using GDDs to time plant growth regulator
applications on a seasonal basis (see Table ).
CALCULATING GDDS
There are ways to calculate them:
The most common is the average method
where the minimum and maximum
temperatures for a day are added together
then divided by . The base temperature,
which varies according to the plant, is
subtracted from the average. If the result
is greater than , it is added to the total
accumulating since the start date. If the
GDD is less than , it is treated as .
The base temperature can range from
-°C depending on the GDD model and
the plant being grown. Some commonly
used base temperatures would be °C
and °C.
The modified average method is used
if the minimum temperature is below
the base temperature. Then the average
method simply uses the base as the
minimum temperature in the calculation.
This method has an advantage over the
average method when temperatures
fluctuate above and below the base
temperature, as often occurs during
early spring. In such circumstances, the
average method would underestimate
the number of GDDs (as some
biological activity still does occur while
the temperature is above the base
temperature). For example, if the daily
maximum temperature was °C, the
minimum was °C and the base was °C,
the GDD would be using the average
method. Under the modified average
method, the GDD would be .
The modified average method is more
likely in spring to ensure the correct
timing of growth regulator applications
to reduce the production of seed heads in
Poa annua.
The modified sine curve method is the
most accurate means of calculating
GDD. It is based on the assumption that
daily temperature patterns are similar to
a sine curve. The area under the curve
determines the GDD. The calculation
is considerably more complex than the
previous two methods and requires a
computer programme.
This method allows for an upper
temperature limit to be set, by which GDD
are calculated to the threshold and not
above. The upper threshold concept can
be used to calculate stress degree-days
which may help define and predict the
summer stress period,.
GDD START DATE
With all GDD models, a set start date is
Working out the optimum time to plant or treat turf can be the dierence between
success and failure. Most importantly, it saves time and money.
GROWING DEGREE DAYS
and Megan Cushnahan
Business development manager
NZSTI, Palmerston North
by Karl Danneberger, PhD
professor - dept of Horticulture
& crop science
The Ohio State University, Columbus, Ohio, USA
Table 1. Plant growth regulators that can be based on accumulated
growing degree days.
Common Name Trade Name Mode of Action Absorption
Trinexapac-ethyl Primo EC, Moddus Late gibberellic acid (GA) inhibitor Foliar
Paclobutrazol GTi Shortstop, Payback Early GA inhibitor Root
Ethephon Ethrel, Ethin Ethylene Foliar
NZ TURF MANAGEMENT JOURNAL Summer 2014 13
TIMING
Better timing of interventions against pests like the Argentine stem weevil will aid success.
given (sometimes referred to as biofix). New
Zealand’s common start date is st July, but
this can vary. Early GDD models predicting
Poa annua or Kentucky bluegrass seedhead
emergence in the Northern Hemisphere
had a start date of st April and st March,
respectively. In New Zealand, these would
correspond to start dates of st October and
st September.
Recently, applications of the growth
regulator trinexapac-ethyl have been
recommended based on work at the
University of Wisconsin. They have suggested
applications being timed around GDD
(base temperature °C), with repeated
applications being made at GDD
increments.
When creating or using a GDD model,
always check the calculation method, the
temperature units (Celsius or Fahrenheit),
the base temperature and the start date used.
Note that the base temperature varies with
dierent research so you need to ensure you
are using the correct one when doing your
calculations.
GDD USE IN NEW ZEALAND
Growing degree days are widely used by the
horticultural and agricultural sectors to make
management decisions, such as the likely
maturity and harvest dates for a crop. Turf
managers in New Zealand are also starting
to use GDD to schedule the application of
Reference
1. Gilmore, E.C. and J.S. Rodgers. 1958. Heat
units as a method of measuring maturity in
corn. Agronomy Journal 50:611-615.
2. Danneberger, T.K. and J.M. Vargas, Jr. 1984.
Annual bluegrass seedhead emergence as
predicted by degree-day accumulation.
3. Danneberger, T.K., B.E. Branham, and J.M.
Vargas, Jr. 1987. Mefluidide applications for
annual bluegrass seedhead suppression based
on degree-day accumulation.
4. Branham, B.E. and T.K. Danneberger. 1989.
Growth suppression of ‘Kenblue’ Kentucky
bluegrass using plant growth regulators and
degree-day application timing. Agronomy
Journal 81:749-752.
5. Danneberger, T.K., and A.J. Turgeon. 1985.
Climatic adaptability of three cool season
grasses in northeastern United States based on
growing degree-days. Proceeding of the Fifth
International Turfgrass Research Conference
5: 801-806.
6. Danneberger, T.K. and J.R. Street. 1985.
Climatic adaptability of annual bluegrass in
Ohio using growing degree-days. Ohio Journal
of Science 85(3): 108-111.
7. Kreuser, W.C. and D.J. Soldat. 2011. A growing
degree day model to schedule trinexapac-
ethyl applications on Agrostis stolonifera golf
putting greens. Crop Science 51:2228-2236.
8. Barker, G.M., and Addison, P.J. 1990. Sampling
Argentine stem weevil, Listonotus bonariensis
(Kuschel), populations in pasture; the egg
stage. New Zealand Journal of Agricultural
Research 33: 649-659.
9. Ferguson, C. M., Evans, A. A., & Barratt, B. I. P.
(1996). Phenology of Listronotus bonariensis
(Kuschel)(Coleoptera: Curculinonidae) in
Otago. In Proceedings of the New Zealand
Plant Protection Conference (pp. 270–274).
New Zealand Plant Protection Society Inc.
Retrieved from http://www.nzpps.org/
journal/49/nzpp_492700.pdf
insecticides for the management of Argentine
stem weevil (eggs require approx. GDD
above °C, to complete development) and
plant growth regulators for the prevention of
seedhead in Poa annua.
The most commonly used method of
calculation is the average method. Calculations
are relatively straightforward (although you
may want to use an Excel spreadsheet so that
a zero value is automatically returned when
calculations require).
To calculate GDD using the average
method, first choose a base temperature
to suit the turf grass being grown. This is
typically °C for cool season grasses and °C
for warm season grasses. Then obtain the
maximum and minimum daily temperatures
for the previous hours. This data should be
easy to obtain if you have your own weather
station. Alternatively, NIWA runs CliFlo,
the national climate database. It is free to
subscribe to and allows you to download
temperature data.
To calculate the GDD for a particular day
at base temperature of °C, use the following
formula:
Using the formula above, the GDD for st
July at a base temperature of °C (see Table )
is as follows:
Table 2. Growing Degree Days – at a base temperature of 4°C and 10°C,
plus accumulated GDD values, for the first week of July 2014 using
the average method. Data provided by the climate station located at
AgResearch, Palmerston North.
Date Max temp.
(°C)
Min temp.
(°C) GDD4Accum
GDD4GDD10 Accum
GDD10
1/07/2014 12.4 86.2 6.2 0.2 0.2
2/07/2014 14.6 7.4 7. 0 13.2 1.0 1.2
3/07/2014 11.1 1.2 2.2 15.4 0.0 1.2
4/07/2014 11.5 1.4 2.5 17. 8 0.0 1.2
5/07/2014 13 2.2 3.6 21.4 0.0 1.2
6/07/2014 14.9 4.5 5 .7 27. 1 0.0 1.2
7/07/2014 13.2 2.7 4.0 31.1 0.0 1.2
GOOD TIMING WORKS
GDDs are a useful tool for the Turf Manager
taking aspects of local weather into account
to predict the development of plants or
insects. Improving the accuracy of timing
for interventions (e.g. preventing seedhead
development or applying chemical and
biological controls) will help improve the
ecacy while minimising product wastage.
As more turf-related data on GDD becomes
available, its use in our industry is likely to
become more popular.
This article is based on a speech by Prof Karl
Danneberger given at the 2014 NZ Fine Turf
Seminar. Edited and added to with permission.
Source: Climate data sourced from the CliFlo database (http://cliflo.niwa.co.nz)
GDD = (. + ) – = .
GDD = (Max.temp. + Min.temp.) –
