2018 ASHS Annual Conference

Supplemental lighting in greenhouse industry is often needed from late fall through early spring and can account for up to 30% of the value of crops produced. Reduction of this energy cost can be beneficial for profitable greenhouse crop production. Our study focused on quantifying the effect of daily light integral (DLI) on seedling production of bedding plants. We used an adaptive light-emitting diode (LED) control system to precisely control supplemental lighting by taking advantage of the dimmability of LED grow lights. The power of the LEDs was adjusted to provide only enough supplemental photosynthetic photon flux density (PPFD) to reach the threshold PPFD underneath the light bars. The threshold PPFD was recalculated every 2 seconds to assure that the crop received a specific DLI by the end of the 14-hour supplemental lighting period. This high-precision lighting control system provides supplemental light when plants can use it most efficiently, i.e., when there is little sunlight available. Therefore, we hypothesized that our adaptive LED control system stimulates plant growth more than ordinary lighting systems. We compared three adaptive lighting control treatments to achieve DLIs of 8, 12, and 16 mol‧m^-2‧d^-1 in 14 hours of supplemental lighting (10 am to midnight) to a treatment that supplied a PPFD of ~100 μmol‧m^-2‧s^-1 of supplemental light during the same 14 hours (average DLI was 8.6 mol‧m^-2‧d^-1), and a sunlight-only control treatment (average DLI was 5.4 mol‧m^-2‧d^-1). We used impatiens (Impatiens walleriana) 'Accent Premium Violet F1' and vinca (Catharanthus roseus) 'Jams ‘n Jellies Blackberry' for the study. Seedlings were harvested 40 days after seeding. The number of leaves, shoot fresh and dry weight, root dry weight, and plant compactness (shoot dry weight/ plant height) of both species were greatest in the treatment receiving a DLI of 16 mol‧m^-2‧d^-1, followed by the treatment receiving a DLI of 12 mol‧m^-2‧d^-1. Seedlings in the treatment that received a supplemental PPFD of ~100 μmol‧m^-2‧s^-1 had similar growth as those in the treatment with a DLI of 8 mol‧m^-2‧d^-1. The sunlight-only control treatment had the slowest seedling growth. These results suggest that our adaptive LED control system is capable of controlling LED's precisely while stimulating the plant growth more at higher DLIs. However, growers should determine whether the better growth from more supplemental lighting is worth the cost of providing that light.