2019 ASHS Annual Conference

A large fraction of far-red light can signal plants that they are shaded by other plants and can induce a shade-avoidance response, such as increased elongation or larger leaves. Larger leaves increase light interception, which may increase photosynthesis and growth. In addition, far-red light can increase the efficiency with which plants use light of shorter wavelengths for photosynthesis. We thus hypothesized that supplemental far-red light can increase the growth of seedlings grown under white LED light, potentially shortening the production time, and increasing profits for growers. Digitalis purpurea ‘Dalmatian Peach’ and Rudbeckia fulgida ‘Goldsturm’ seedlings were grown in a 54 m³ walk-in cooler converted into a walk-in growth chamber. The chamber contained three shelving units each divided into six 1.2 m × 0.6 m growing sections. Each section was equipped with two white LED light bars. Fifteen sections received anywhere from 7.9 to 68.8 μmol m^-2 s^-1 of supplemental far-red light, while three sections received no supplemental far-red. Eighteen 72-cell trays were sown with Rudbeckia and eighteen sown with Digitalis. One tray of each species was placed in each growing section. Plants were grown at 21.5 ± 0.2 °C, with a 16-hr photoperiod, photosynthetic photon flux density of 186 ± 6.4 μmol m^-2 s^-1, daily light integral of 10.7 ± 0.4 mol m^-2 d^-1, mean vapor pressure deficit of 1.3 ± 0.1 kPa, and CO₂ concentration of 800 ± 15 ppm (mean ± standard deviation). The plants were regularly fertigated using an ebb-and-flow system. Data were analyzed using linear regression. The addition of far-red light increased Digitalis shoot dry weight 38% (P=0.004), root dry weight 22% (P=0.029), and shoot length 38% (P = 0.025) in a dose-dependent manner. The root fraction of Digitalis decreased 21% (P = 0.034) with increasing far-red. The Rudbeckia seeds were slow to germinate and germination was not uniform. However, we were able to harvest ten plants per treatment. As far-red light levels increased, the specific leaf area of Rudbeckia decreased (P= 0.004), not a typical shade response and not what we expected. Overall, we found a positive correlation between seedling growth and the intensity of supplemental far-red light. These findings may lead to better use of supplemental light to optimize production of perennial seedlings.