Why is my spinach bitter?

Spinach bitterness increases sharply once the plant bolts (starts to flower). This happens rapidly above 25°C in India. Harvest leaves before any flowering stem appears. Also, very mature outer leaves are naturally more bitter than young inner leaves — always harvest young. Water stress and nitrogen deficiency also increase bitterness. For sweeter spinach, harvest in the morning, water consistently, and feed with balanced nitrogen fertiliser every 2 weeks.