Silicon (Si) is a nonessential element that has proven to be a beneficial supplement to agricultural crops. In floriculture greenhouse production, soilless substrates have limited Si content and supplements may improve plant quality. The objective of this study was to determine Si sources, rates, and application methods to improve plant quality. Zinnia elegans `Oklahoma Formula Mix', Helianthus annuus `Ring of Fire', and Gerbera `Acapella' were provided potassium silicate (KSiO3) as a media incorporated flakes or weekly drench, sodium silicate (NaSiO3) as weekly foliar spray or ashed rice hulls. Zinnia and Helianthus Si levels were highest in leaf (0.5% to 1.7%), followed by flower (0.3-0.5%) and stem (0.2-0.4%) tissues. Gerbera accumulated lower amounts of Si compared to Zinnia and Helianthus with similar leaf and flower content values ranging from 0.4% to 0.6% with stem values 0.4% Si. Depending on source and rate, several horticultural traits were improved. Zinnia benefits included stem thickness, increase in flower diameter and stem erectness. Helianthus Si supplementation resulted in increased stem thickeness and flower diameter. However, phytotoxicity problems occurred with Si rates above 200 mg·L–1 (SiO2 applied as weekly potassium silicate drench). Gerbera stems thickened with KSiO3 and NaSiO3 applications, but NaSiO3 foliar sprays increased stem length, flower diameter and resulted in earlier flowering.
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