An experiment was conducted at a private new reclaimed farm, Wadi El-Natrun, El-Beheira Governorate, Egypt, during 2020 and 2021 seasons, to examine the effects of foliar spraying selenium at 5 and 10 g/L and varying rates of magnetite application (200 and 300 kg/fed) on the growth, yield, oil production, and chemical composition of the Solidago canadensis L. plant. The application of magnetite at a rate of 300 kg/fed and selenium as foliar application at 5 g/L produced the best results for the same parameters. The best results from all the tested parameters were: plant height (cm), number of inflorescences/plant, inflorescence length/plant (cm), leaf length (cm), herb fresh and dry weight (g), fresh herb yield (ton fed−1), essential oil percentage, and essential oil yield/fed. Moreover, the application of magnetite and selenium combined to improve the chemical constituents (N, P, and K levels), oil production, and vegetative development. The application of the second level of magnetite (300 kg/fed) and the spraying of selenium at a concentration of 5 g/L resulted in the greatest significant rise in all metrics. After this treatment, the main constituent of the essential oil was α-Pinene (32.75%), which was followed by Germacrene D (27.49%). Conversely, employing magnetite and selenium resulted in a decrease in the solidago canadensis plant's Na, Cl, and proline levels.