To grow well, Broccoli needs about 1-1.5 inches of water per week. Water the plant at the base, not the central heads, to avoid rot and pests. Keep the soil moist but not wet, and decrease watering as the heads approach maturity. Excessive moisture can stunt or kill Broccoli, so measure soil moisture regularly and mulch with straw or grass clippings to keep it even. Aim to give Broccoli 1 to 2 inches of water each week and add aged compost to the planting bed for best results.