Vernalization and growing degree-day (GDD) requirements of Thalictrum delavayi Franch. `Hewitt's Double' were investigated by exposing crowns to cold storage for 0, 3, 6, 9, 12, or 15 weeks at 8 °C, and subsequently planting in a heated greenhouse under long-day conditions. Cumulative vernalization of crowns was complete after 6 weeks of cold storage at 8 °C. The time to flower, including time at 8 °C, was 3338 GDD (base temperature of 0 °C) without vernalization and 2802 GDD after complete vernalization. Commercial recommendations for rapid and predictable flowering of T. delavayi `Hewitt's Double' should include cold storage of crowns for a minimum of 6 weeks at 8 °C as part of the 2802 GDD during vernalization and forcing.
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