Field evaluation of degree-day based equations for predicting sprouting ofhydrilla (Hydrilla verticillata) turions and tubers

The ability to predict sprouting of aquatic macrophyte vegetative propagule s is an important step in understanding their temporal distribution and abu ndance and in developing long-range management strategies. We examined the ability of degree-day based equations to predict monoecious hydrilla, (Hydr illa verticillata L. f. Royle) tuber and turion sprouting in Clear Lake, Ca lifornia using sediment and water temperatures measured in the lake. Sedime nt temperature data were used to calculate accumulated degree-days. Sprouti ng of turions and tubers was estimated using previously developed equations relating sprouting to accumulated degree-days. There was good agreement be tween sprouting predictions and field data on the presence of hydrilla in w eed rake samples. Small differences among water temperatures at the five si tes and strong relationships between water and sediment temperatures indica te that sprouting should be similar in hydrilla beds found along the wester n and southern shores of upper Clear Lake. These results can be used to est imate optimal timing for surveys of hydrilla abundance and the application of hydrilla management techniques.