Isaak, C. K., Petkau, J. C., O, K., Ominski, K., Rodriguez-Lecompte, J. C, and Siow, Y. L. 2013. Seasonal variations in phenolic compounds and antioxidant capacity ofCornus stoloniferaplant material: Applications in agriculture. Can. J. Plant Sci. 93: 725-734. The present study was carried out to establish the antioxidant capacity of plant material of Cornusstolonifera (syn. Cornus sericea, Red Osier Dogwood) grown in Manitoba, by measuring total phenolic content and oxygen radical absorbance capacity (ORAC) over a 3-yr period. The presence and concentrations of flavonoids that might account for antioxidant abilities were investigated using ultra-performance liquid chromatography and a total anthocyanins assay. Both ORAC levels and phenolic content increased during the spring, peaked during the summer months (1631.65±166.30 µmol trolox equivalents g^-1 DW in July 2011, 220.38±2.29 mg gallic acid equivalents g^-1 DW in August 2012), and then declined during the fall in all 3 yr. Analysis of individual flavonoids in 2010 samples revealed that high levels of rutin, a glycoside of quercetin, were present ranging from 7.46±0.09 (July 2010) to 18.77±0.23 mg g^-1 dried sample (October 2010). Anthocyanin content was high in the spring, very low in the summer months, and increased in the fall over all 3 yr. Although polyphenolic and antioxidant content in Cornus spp. plants have been largely attributed to their berries, results from this study demonstrate that other plant material may also be an abundant source of these compounds.