Growing Degree Days Page Added

A Growing Degree Day s page for ornamental growers has now been added to the Ornamental Growers' IPM Update. The page can be found on the right hand column of this blog.  Click on the page title "Growing Degree Days Current Information 2008.”  Then click on the link.  The page looks like this:

For Current Growing Degree Day information, click on this link: http://www.personal.psu.edu/rab147/blogs/growerdegreedays/

What are Growing Degree Days?

Plant and insect development is temperature dependent, not calendar dependent.  After the winter dormancy period, temperatures rise and plants and insects develop.  Growing Degree Days are a measure of accumulated heat.  Growers and landscape technicians can use the Growing Degree Day (GDD) measurement of heat to predict which insects will be active at which point.  For example, at 90 GDD, Eastern Tent Caterpillars are active.  This is the time to scout for Eastern Tent Caterpillars and to make management decisions.

Phenology is the study of periodic occurrences in nature and their relation to weather.  The study of phenology is made by observation.  When does Eastern Redbud bloom?  When do gypsy moth eggs hatch?  Because the development of plants and insects is related to the accumulation of heat, these questions can be answered in GDD units.  {When does Eastern Redbud bloom? = 90GDD; When do gypsy moth eggs hatch? = 90GDD}  However, growers can skip the GDD calculations by using straight phenology.  When the Eastern Redbud is blooming, it is time to scout for gypsy moth larva (caterpillar).  

By using a GDD and phenology pest management system combined with scouting, growers and landscape technicians may improve pesticide application efficacy and reduce pesticide usage when compared with a calendar-based pest management system. 

How are Growing Degree Days Calculated?

In a 24-hour period, the minimum and maximum temperatures are recorded.  Then the following formula is used to calculate the GDD for that time period:

       Temp Min + Temp Max   - Base Temperature =GDD

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What is the base temperature?   It is the temperature below which plants and insects do not develop.  Base temperatures vary by species.  For some species of plants and insects, the base temperature is 30 degrees F, for others it might be 50 degrees F.  We will use a base temperature of 50 in our calculations, because the majority of the pests that we are looking at do not develop below 50 degrees F. 

For Example, if the high was 65 and the low was 48:

 48 + 65   - 50 =6.5 GDD for that day

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We began measuring GDD in mid March in Erie County.  The GDD for each day are added together, giving a cumulative GDD measurement.  This cumulative measurement is the measurement of accumulated heat for the growing season.