It is possible to program the flowering of Sunflowers (Helianthus annus) in the greenhouse and in the open. In the first weeks after sowing, sunflowers respond to short days for flower induction while long days postone flowering. Continuous application of short days resulted in flowering plants with 50cm stems whereas in continuous long days stem lengths of up to 3m resulted. A marketable length should be somewhere in between. Daylength less than 12 hours is a short day, and all daylengths over 14 hours constitute long days. This is true both for young seedlings (up to three to four weeks from sowing) and for adult plants. The response to photoperiod during the three week seedling stage, was used to program plants by light/dark periods before planting in the field. Plants subsequently flowered in the field on marketable length stems, despite long-day conditions. T. Blacquiere, N. Straver, D. van den Berg. ISHS Acta Horticulturae 580: IV International Symposium on Artificial Lighting www.actahort.org