Transforming Rudbeckia into a Fall-flowering Cut Flower

Rudbeckia hirta is a colorful perennial species that produces yellow to red-brown flowers, just the right cut item for fall markets. Unfortunately, under the short daylengths of late summer and fall, these plants fail to flower, or produce flowers with very short stems. Past work has shown that Rudbeckia requires long days for flowering. Would it be possible to install solar-powered landscape lights in the production field to simulate long days? To answer that, we need to know how much light is needed, and if landscape lights can produce that light intensity. Thanks to funding from the ASCFG Research Foundation, we set out to answer both questions.

In a preliminary experiment in the greenhouse we planted rudbeckia ‘Prairie Sun’ in pots at right angles to a row of fluorescent ‘button’ lights, as used to light under counters in the kitchen (Fig. 1). The lights extended the daylength to 16 hours, after they had gotten 12 hours of light the rest of the day. The light intensity decreased for plants farther from the lights, and we could determine the distance at which the plants still had adequate stem length, and relate it to the light intensity of the light extension.

The light intensity of the extended daylength source varied from 4 umoles/m2/sec close to the light bar, to 0.5 umoles/m2/sec at the farthest pot. Plant height showed a similar pattern, with greatest stem length at about 3 umoles/m2/sec light intensity (Fig. 2).

Thus for optimum stem extension of Rudbeckia in late-season trials, a light intensity of 3 umoles/m2/sec needs to be achieved.

We then planted ‘Prairie Sun’ and ‘Cherry Brandy’ in a high tunnel on July 28 for fall flowering and exposed them to three treatments: (1) natural light only (2) light extension with solar-powered landscape lights having 6 LEDs (Fig. 3) and (3) light extension using mains-powered button lights.

As the daylength became shorter in late summer, stem growth in the unlighted plants stayed short, whereas those exposed to 16-hr daylength of mains lights started to elongate normally.

Plants under solar lights were intermediate in reaction (Table 1). By the end of the harvest season, stem length and yield were greatest under the mains “kitchen style” lights, and the plants under solar lamps were a distant second. Measurement of the light intensity in the plots revealed the reason: the solar lights produced a bright, concentrated beam of light in one spot, with virtually no coverage over the rest of the plants. The mains lights had a more even distribution. This indicates that currently available solar landscape lights are not suited for use of stimulating the daylength response of rudbeckia, but that a 9-watt fluorescent fixture every 6 ft. is sufficient to stimulate stem extension. ‘Cherry Brandy’ was not as productive as ‘Prairie Sun’, and was also later to come to flower. We will continue our search for suitable solar lights: Christmas tree lights, one per plant, are next. Stay tuned!

Chris Wien

Professor

Chris Wien is recently retired Professor of Horticulture at Cornell University. Contact him at [email protected]