Johnsongrass (Sorghum halepense) is a common weed species in a varieties of regions. The Johnsongrass growing degree day model developed by researchers, uses heat units and time as a method to estimate plant phenological stages through the growing season. This can be helpful to predict rhizome sprout / plant emergence.

Calculating Growing Degree Days

For this model, a lower temperature threshold of 15C is used to calculate growing degree day accumulation. For the Northern Hemisphere, April 1st, should be used as a biofix, the date when to start growing degree day accumulation. Growing degree days can be calculated from daily high and low temperatures, or via the Pest Prophet app, which uses hourly temperature data for a given field. 25% Rhizome sprout emergence will occur at 190 GDD (C), 50% rhizome sprout emergence at 350 GDD (C) and 75% rhizome sprout emergence at 580 GDD (C).

Learn More:

Satorre, E.H., C.M. Ghersa, and A.M. Pataro. 1985. Prediction of Sorghum halepense (L.) Pers. rhizome sprout emergence in relation to air temperature. Weed Research 25:103-109.