Look for a high first number(Nitrogen, N) for Veg, and a high middle number(Phosphate, P2O5) for flower. Some people like the 20-20-20 (Peter's brand) because of that high first number (nitrogen).

In answer to your direct question though, Potash (K2O) is important for the formation of plant tissue and flowers, and it helps strengthen stem and leaf growth. Potassium acts as a general tonic and imparts increased vigour and disease resistance to plants.

Dboy