Spinach Growth Under Photoperiod Extension at Several-day Intervals in Growth Cabinets

Extension of photoperiod to 24 h a day at several-day intervals was tested for ability to promote growth of spinach (Spinacia oleracea) without causing early bolting. We grew medium-early-bolting cultivar ‘Alrite’ and late-bolting cultivar ‘Summer Focus’ in growth cabinets with 10-h lighting by fluorescence lamps. Photoperiod was extended every Monday, Wednesday, and Friday (thrice a week); every Monday and Thursday or Tuesday and Friday (twice a week); every Monday (once a week); every day; or not at all. In both cultivars, plant growth increased with more frequent photoperiod extension. However, ‘Alrite’ bolted early with flower stalks taller than 4 cm under the thrice-weekly extension and both cultivars bolted early under the daily extension. These results show that photoperiod extension at 2-or 3-day intervals was most effective among treatments in this study for promoting growth without causing early bolting.