Superintendents have commonly used plants as indicators for certain biological events. For example, Forsythia bloom is associated with preemergence herbicide applications and dogwood bloom associated with adult Annual Bluegrass Weevil emergence.
While these plant indicators have been valuable tools, heat accumulation is a more quantifiable and accurate means to accurately time the biological events associated with plants and insects.
Growing degree days (GDD) are models which accumulate and track accumulated heat units throughout a year. Growing degree days are calculated daily by measuring the heat units above a threshold for one day. These daily values are then accumulated over time to calculate the threshold is referred to as the base temperature. In turf, base temperatures of 22, 32, and 50F are commonly used.