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Supplemental light can increase growth and accelerate production of greenhouse crops, but it can be expensive if not provided in a way that promotes efficient use of the light. Dimmable light-emitting diode (LED) fixtures have the potential to reduce lighting costs because the output can be precisely controlled to meet crop needs. Because light is used more efficiently to drive photosynthesis at lower photosynthetic photon flux densities (PPFDs), we hypothesized that providing Rudbeckia fulgida var. sullivantii ‘Goldsturm’ seedlings with the same daily light integral (DLI), spread out over a longer photoperiod and at lower PPFDs, should improve growth. A DLI of 12 mol·m⁻²·d⁻¹ was provided in a greenhouse over 12, 15, 18, or 21-hour photoperiods from a combination of sunlight and supplemental light from LEDs, using adaptive lighting control. Plants grown without supplemental light had an ≈12-hour photoperiod and received an average DLI of 5 mol·m⁻²·d⁻¹, ≈58% less light than the four lighting treatments. Lengthening the photoperiod from 12 to 21 hours increased shoot dry mass (30%), root dry mass (24%), plant height (14%), leaf area (16%), and chlorophyll content index (48%), and decreased specific leaf area (26%). There was no significant effect of photoperiod on root mass fraction or compactness. Growth parameters of plants without supplemental light were 26% to 90% smaller compared with those in the 12-hour photoperiod treatment. Treatment effects on canopy size, seen as early as 2 weeks into the study, were correlated with final shoot dry mass. Longer photoperiods did not induce a shade-avoidance response, based on specific leaf area and compactness data. The 24% increase in root dry mass for the plants in the 21-hour photoperiod suggests that cropping cycles can be shortened by 1 to 2 weeks compared with the 12-hour photoperiod. This could result in more crop turns per year and increased profits. In addition, fewer lights would be needed for adequate growth, reducing the capital cost of the lighting system.

Seedlings may be grown indoors where environmental conditions can be precisely controlled to ensure consistent and reliable production. The optimal spectrum for production under sole-source lighting is currently unknown. Far-red light (λ = 700–800 nm) typically is not a significant part of the spectrum of light-emitting diode (LED) grow lights. However, far-red light is photosynthetically active and can enhance leaf elongation, which may result in larger leaves and increased light interception. We hypothesized that adding far-red light to sole-source lighting would increase the growth of ‘Dalmatian Peach’ foxglove (Digitalis purpurea) seedlings grown under white LED lights, potentially shortening production times. Our objective was to evaluate the effect of far-red light intensities, ranging from 4.0 to 68.8 µmol·m⁻²·s⁻¹, on the growth and morphology of foxglove seedlings. Foxglove seedlings were grown in a growth chamber with a photosynthetic photon flux density (PPFD) of 186 ± 6.4 μmol·m⁻²·s⁻¹ and supplemental far-red light intensities ranging from 4.0 to 68.8 µmol·m⁻²·s⁻¹. As far-red light increased, shoot dry weight, root dry weight, plant height, and plant height/number of leaves increased by 38% (P = 0.004), 20% (P = 0.029), 38% (P = 0.025), and 34% (P = 0.024), respectively, while root weight fraction decreased 16% (P = 0.034). Although we expected supplemental far-red light to induce leaf and/or stem expansion, specific leaf area and compactness (two measures of morphology) were unaffected. Because a 37% increase in total photon flux density (PPFD plus far-red light) resulted in a 34.5% increase in total plant dry weight, the increased growth likely was due to increased photosynthesis rather than a shade-acclimation response. The growth response was linear across the 4.0 to 68.8 µmol·m⁻²·s⁻¹ range of far-fed light tested, so we were unable to determine a saturating far-red photon flux density.