Answer: Oriental poppies like deep, rich, moist soils. Lean soils, compacted soils, or dry soils (or drought) can result in yellowing leaves (particularly the older outside leaves). Make sure the drainage is good (no standing water) and they get plenty of deep soakings in spring. You may want to have a soil test done to determine if there are any serious nutrient deficiencies. Poppies are also summer-dormant plants, dying down in summer as the heat and drier conditions come on. If there's an early summer, the plants will begin the dormancy process (yellowing leaves) earlier. Also, you didn't tell me where you live but in warmer climates with minimal winter chill, poppies are short-lived and aren't good repeat performers.
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