Modelling the timing of early season plant phases, particularly in the agricultural
sector, relies heavily on variants of accumulated heat units or growing degree days.
Well-known examples in agriculture are ‘Ontario units’ in maize production (e.g.
Easson & Fearnehough, 2003) and ‘TSum200’ in grass production. Most of these
models accumulate daily mean or maximum and minimum temperatures above a
threshold (which may be zero) from a starting date to predict a particular plant phase.
The choice of the starting date and the threshold should be selected to optimise
the relationship with the plant phase, i.e. to minimise the year-to-year variability
in the accumulated heat units. In practice, the starting date may be selected for
convenience and consistency between species (e.g. January 1) as may the threshold
temperature (e.g. 0◦C or 5◦C).