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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.

Supplemental Far-red Light-emitting Diode Light Increases Growth of Foxglove Seedlings Under Sole-source Lighting