Production of organic potted herbs with LED supplementary lighting, organic biostimulants and silicon

LED technology in greenhouse horticulture offers the possibility of using a blend of monochromatic light wavelengths that can stimulate or inhibit specific morphological and physiological responses and thus act as growth regulators. This is particularly interesting for organic herbs and ornamentals as few products are available to control plant morphology. Beneficial soil microorganisms or compounds such as silicon, used as biostimulants, may also play an important role in maintaining soil and plant health of organic herbs and ornamentals. Our study hypothesis was that LED supplemental lighting (SL) and biostimulants are useful tools for organic greenhouse ornamental growers to increase plant growth and product quality. By using randomized complete block designs with three to five replicates, two light treatments (with and without LED SL) were compared for 24 species and cultivars (a total of five experiments). Trials were carried out in 2016 and 2017 at Laval University (Quebec, Canada) and Les Serres Frank Zyromski (Rivière-Rouge, Canada). The photosynthetic photon flux density (PPFD) of the LED SL at the plant level was 193 µmol m(-2) s(-1) for a 12-h photoperiod (6 am to 6 pm). In experiment 4, five biostimulant treatments were also compared for four plant species or cultivars (lavender, oregano and basil ‘Harlekin’ and ‘Ajaka’): 1) Glomus intraradices applied as granules, 2) G. intraradices applied as a drench, 3) Bacillus subtilis + Bacillus amyloliquefaciens, 4) Streptomyces griseoviridis, and 5) silicon (wollastonite, CaSiO(3)). Our results showed that the shoot dry biomass (DM) of all species increased by 1.10-1.92 times under LED SL. However, no significant effect of SL was observed for plant height and width of all species/cultivars, except for one species mix (Tagetes lucida, Artemisia abrotanum, Thymus citriodorus; 17% decrease in plant height). The visual quality of plants grown under LED SL was improved for two species (lavender and basil ‘Harlekin’). All species grown in a medium enriched with wollastonite (2 g CaSiO(3) L(-1) of peat-based soil) showed 33-45% higher concentration of leaf Si when compared with control plants, although the studied species were poor Si accumulators (0.05-0.52% of leaf Si content). Notably, application of granular G. intraradices in the organic growing media of lavender and oregano increased leaf Si content by 2.88 (0.58 vs. 0.20%) and 6.59 (1.05 vs. 0.16%) times, respectively, compared with their control plants without any biostimulant. For oregano plants, S. griseoviridis treatments increased leaf Si content compared with control plants (0.35 vs. 0.16%). No significant effect of Si (at 0.05 significance level) on shoot DM was observed for any species. G. intraradices applied as granules increased DM of lavender by 15%. In conclusion, LED SL and biostimulants influenced plant growth, and for some species increased product quality of organic-grown ornamentals and herbs.