Extracts of Solidago virgaurea L. (European goldenrod), Lavandula angustifolia Mill. (lavender) and Arnica chamissonis Less. (arnica) were tested as potential additives for methane fermentation in bioreactors, as it was hypothesized that their antioxidant properties may improve biogas production efficiency. Hence, methane fermentation of maize silage with the addition of tested extracts was performed in eudiometers, and both the biogas volume and methane content in biogas were measured. In addition, antioxidant properties, such as reducing power and 1,1-diphenyl-2-picrylhydrazyl free radical-scavenging activity, were determined spectrophotometrically. The results showed that only the addition of a goldenrod crude extract caused a significant increase in biogas yield, despite the fact that it presented the lowest antioxidant activity of the three tested species. No significant differences in biogas yield were found between the other two additives and the control. Biogas production efficiency was not correlated with the antioxidant activity of the crude extracts. Hence, the increase in methane fermentation efficiency in the case of goldenrod supplementation might have resulted from some other non-antioxidant compounds occurring in this species.