We present a symbolic method for p-semiflow computation, based on zero-suppressed decision diagrams. Both the traditional explicit methods and our new symbolic method rely on Farkas' algorithm, and compute a generator set from which any p-semiflow for the Petri net can be derived through a linear combination. We demonstrate the effectiveness of four variants of our algorithm by applying them on a suite of Petri net models, showing that our symbolic approach can produce results in cases where the explicit approach is infeasible.