Using Hydrothermal Time to Predict Spring Germination Period of Smooth Crabgrass (Digitaria ischaemum) and Annual Bluegrass (Poa annua L.) in Michigan.

Accurate prediction of spring germination of annual grasses is critical to maximizing performance of preemergence herbicides. Soil temperature optima for germination of smooth crabgrass and annual bluegrass have been reported in various regions of North America. Growing degree-days (GDD) have been closely linked with periods of germination for smooth crabgrass. However, soil moisture, not included in GDD, is a critical component of conditions conducive for germination. Hydrothermal time accounts for soil temperature and soil water potential to more accurately account for the factors influencing germination. This study was conducted to determine whether hydrothermal time predicts annual grass germination more accurately than GDD or soil temps alone. The spring germination period of smooth crabgrass and annual bluegrass was monitored for three years at two locations. Germination results were correlated with soil temperatures at two depths (0-5 cm, 0-10 cm), GDD accumulation at two base temperatures (0 C, 13 C) and hydrothermal time.