Extending Plant-Perceived Daylength Using Solar-Powered Lights On Rudbeckia hirta

Rudbeckia hirta is a showy perennial widely used as a cut flower, whose yellow, orange and red flowers are suggestive of fall conditions.  Unfortunately, an absolute requirement for long days for flower stem extension limits their production potential in the fall.  In a greenhouse light gradient experiment we established that the daylength light response requires an irradiance of about 3 umoles/m²/sec.  To establish this, pots of ‘Indian Summer’ rudbeckia were planted at right angles to a row of incandescent bulbs on a greenhouse bench.   Black-out curtains maintained a 12-hr. photoperiod on the bench, while the lights were on a 16 hr. timer.  To determine if rudbeckia could be produced with long stems in a late summer planting in a high tunnel, we compared daylength extension methods using mains-powered electric lights and solar-powered lamps with unlit controls.  In the 2009 season, stem length was greatest with mains lights, whereas the solar lamps produced a light beam too narrowly focused to light large areas.  The experiment was repeated in 2010 using solar-powered ‘Christmas lights’ draped over the leaves of the plants, and mains-powered lamps.  Although in this case light distribution of the solar lamps was better, light output from the tiny lights was insufficient to stimulate the daylength response.  Currently, inexpensive solar-powered lights are not suitable for stimulating daylength response in rudbeckia.