Fall Applications of Nitrogen and Potassium and Their Effect on Winter Hardiness of Annual Bluegrass (Poa annua L.).

Fall fertilization can have a significant impact on the ability of plants to acclimate in the fall.  Recommendations on fall fertilization practices vary widely, and appear to be species specific.   A granular fertility research project focused on the response of annual bluegrass to fall applied nitrogen (N) and potassium (K) was implemented to determine the effects of N and K fertilization on cold hardiness, and to determine if there is a relationship between plant cold hardiness, soil nutrient status, and tissue nutrient content.  Nitrogen and K were applied at 0, 0.113, 0.227, and 0.454 kg / 100m² in a complete factorial design for a total of 16 fertilizer rates applied biweekly from 15 Aug through 1 Oct in 2014 and 2015.  Ammonium sulfate and potassium sulfate were the sources for N and K respectively.  Cold tolerance was evaluated 4 times during the acclimation process; however the final testing date in November displayed the highest cold tolerance levels and statistical differences.  Results showed that a balanced rate of 0.125kg/100m² for both nutrients provided the highest level of cold tolerance.  Optimal cold tolerance levels correlated with tissue tests levels between 2.5 - 3% N and 2.25 - 2.75% K.  Excessive amounts of both N and K reduced the cold tolerance of annual bluegrass.