In general, bitterness is caused by lack of water. Other environmental factors that cause stress to the plant could be involved too. To prevent bitterness this year, provide your parsley (Petroselinum) with the growing conditions it prefers. It grows best in rich, moist, well-drained soil that is slightly acidic to neutral in pH. Although parsley tolerates partial shade, it prefers full sun. Water your plant if rainfall is insufficient to keep the soil uniformly moist. Parsley can be attacked by aster yellows, a disease spread by leafhoppers. Infected plants usually develop yellowing foliage, and flavor could be affected. Protect plants from leafhoppers with a floating row cover. Remove and destroy infected plants. Keep in mind that parsley, if it overwinters, will have more bitterness the second year. Start new plants each year to maintain mild-flavored foliage.