Calculating heat units for citrus


The growth rate of both plants and insects depends on the amount of heat they receive.

For each species there is an optimum temperature range for growth, if no other factors (such as water) are limiting.

For citrus, the optimum temperature range for growth is commonly considered to be 13-35°C.

The hours of heat within this range are referred to as heat units or growing degree days (GDD).

Heat units can be used in citrus to assess the suitability of a region for growing citrus, for estimating the length of phenological (growth) stages and for predicting fruit maturity times.

This Primefact provides the following information:

  • Example Heat Unit calculations for both winter and summer temperatures
  • Phenological (growth) stages of citrus
  • Diagrammatic representation of the citrus phenological cycle
  • Map depicting various heat unit categories in australia


Primefact 749 First Edition

Published: Dec 2007