Studies were conducted to examine the effect of treating Zinnia elegans Jacq. with soluble silicon on the performance of the green peach aphid, Myzus persicae (Sulzer). Z. elegans plants were irrigated every 2 d throughout the duration of the experiment with a nutrient solution amended with potassium silicate (K₂SiO₂), or a nutrient solution without K₂SiO₂. Length of the prereproductive period and survivorship of M. persicae were not affected by K₂SiO₂ treatment, but total cumulative fecundity and the intrinsic rate of increase (r_m) were slightly reduced on Z. elegans plants receiving soluble silicon. Quantification of silicon contentin leaf tissues using inductively coupled plasmaoptical emission spectroscopy (ICP-OES) confirmed significantly higher silicon concentrations in plants treated with K₂SiO₂ compared with control plants. High performance liquid chromatography-mass spectrometry (HPLC-MS) analysis was used to identify and quantify phenolic acids and flavonols in leaf tissue of z. elegans. Compared with untreated control plants, significant elevations in 5-caf-feoylquinic acid, p-coumaroylquinic acid, and rutin were detected in leaves of Z. elegans plants treated with K₂SiO₂, but none of seven other phenolics were significantly affected. Similarly, a slight elevation in guaiacol peroxidase activity was detected in plants treated with K₂SiO₂ Overall, these results indicate treatment of Z. elegans with soluble silicon provides a modest increase in resistance levels to M. persicae, which may be caused in part by defense-related compounds.