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In many plants and animals, spring heat accumu-
lation drives the timing of events such as leaf-out, flower-
ing, egg hatch, and adult emergence. For numerous trees
and tree pests, the specific amount of heat required to
trigger a par ticular event has been documented, making it
possible to anticipate the timing of the event at a location
based on local temperature measurements.
Accumulated heat is often measured in cumulative growing
degree days (GDDs), defined as the number of degrees the
mean daily temperature exceeds a base temperature, below
which an organism will remain developmentally inactive.
The USA National Phenology Network (USA-NPN) offers
daily maps of accumulated GDDs using a January 1 star t
date and two commonly used base temperatures, 32°F
(0°C) and 50°F (10°C). These maps, available at 2.5 km
spatial resolution, are updated daily and are available up to
six days into the future.
Growing Degree Day (GDD) Maps Pinpoint
Management Windows for Tree Pests
By Theresa Crimmins • Images Courtesy USA-NPN
When combined with known growing degree day thresh-
olds, these USA-NPN accumulated growing degree day
maps can be a powerful planning tool.
Theresa Crimmins is the Assistant Director of the
USA National Phenology Network's National
Coordinating Office.
